Ugg, I am going to have to add cooling

[Scott Winners]

Summers up here are just getting to be too hot. Think of me as a Newfoundland or sled type dog, just shaped like a human.

FWIW I am running a Blaze King Ashford 30 in 1200sqft, it was installed May 2014. I cycled 8 cords through that thing every winter for a while, got by on five cords each of the last two winters and might get by with four cords this winter if the weather keeps doing what it has been doing. I do have eight cords ready to go just in case, but it has been uncomfortably hot and humid for me up here this summer for six weeks already, with no end in sight; instead of the usual two weeks of summer misery that should have ended two weeks ago.

I have two fundamental questions.

1. Where do I put the vapor barrier in new construction when I am running both heat and AC?

2. At what square footage or conditioned volume do heat pumps get up on an efficiency plateau? With pellet stoves it is around 1800-2000 sqft. With cord wood stoves some of the smaller ones can do OK now in 1500 sqft, but they also really settle in around 2000 conditioned sqft and up. Typical house size and weather for the lower 48. With these minisplits some of you folks are running it sounds like I could go pretty small for an AC area.

I am kinda toying with the idea of building the retirement home with attached shop as a cold climate building with the vapor barrier directly under the drywall, heated in the winter with humidity control and fresh air flow in the hot season, and then building a separate but very nearby shed/ tiny house sized building 200-400 sqft with the vapor barrier on the outer face of the wall so I could cool it in the summer as office and sometimes sleeping space, and not go out there in the wintertime.

3. How do you keep mold out of your insulation when running both heat and AC?

Thanks for your input, I moved up here to get away from long hot summers and it isn't working.

[Rob Luter]

If you have only one, your vapor barrier should be on the interior. It will keep any humidity from exfiltrating and condensing in the wall cavity when it gets towards the cold exterior. In my neck of the woods they put one on the exterior as well, referring to that one as the WRB (water resistant barrier). You want your wall cavity to be like the walls of a thermos bottle with no path for air or moisture transmission in either direction.

[Scott Clausen]

So many choices these days between foam and fiberglass. I can offer no help but you may want to talk to a insulation contractor to get some pricing and maybe some free advice. All I do know is you want to get this vapor barrier thing right.

[William Hodge]

Air seal and insulate. Don't use plastic insulation, it pollutes in its manufacture, use, and disposal. The propellants used in making foam do so much climate damage that they have to save heat for decades before they lessen climate impact. Cellulose and rock wool pollute less, and can be disposed of easier when they are thrown out.

Air sealing the conditioned space keeps moist air out of the wall and ceiling cavities. This raises the dew point in the walls, so you won't get condensation. On the exterior, one would normally want to keep water out. Air sealing on the interior reduces chimney effects that pull moisture into the walls.

Most of all, air sealing the top of the conditioned space keeps the attic dry, and lessens suction in gaps in the building envelope. For example, if you have holes where wires perforate the ceiling, air will chimney out. Pressure is reduced in the living space. The leaky door now becomes a conduit of outside air. That air is warmed and moisturized as it goes through the living space. When it shoot into the attic, it's like a fountain of moisture and heat that creates a biosphere of mold over your head.

[Jim Becker]

For cooling, a heat pump split type setup will work for you for mid-spring through mid-fall, but do not expect to be getting heating value from it during the colder times in your geography. Even the best units don't operate with enough efficiency below 0º F to -5º F or stop working altogether as you get close to that,. Stick with your already effective heating setup for that and do a "dedicated to cooling" system for, well...cooling.

Insulation and sealing done property is important as I'm sure you know. A really well sealed structure will permit you to cool it efficiently with notably lower cost. Do keep in mind that a really well sealed building will need formal ventilation for when you are heating if your wood burning setup is physically inside the structure.

[Paul F Franklin]

I recommend you spend some time at the green building advisor site. A lot has changed in terms of best practices for air, water, and vapor management and insulation in the last 15-20 years. There are really knowledgeable folks at that site that are welcoming and helpful.

[sean meltvedt]

Scott, the guys at CCHRC at the university are very helpful. I’m also in Fairbanks and heat/cool the whole house with a GSHP. I’m using the “remote” wall system. We can meet up and I’ll show you what I’ve done if your interested.
Cheers
Sean

[Steve Rozmiarek]

Air seal and insulate. Don't use plastic insulation, it pollutes in its manufacture, use, and disposal. The propellants used in making foam do so much climate damage that they have to save heat for decades before they lessen climate impact. Cellulose and rock wool pollute less, and can be disposed of easier when they are thrown out.

Air sealing the conditioned space keeps moist air out of the wall and ceiling cavities. This raises the dew point in the walls, so you won't get condensation. On the exterior, one would normally want to keep water out. Air sealing on the interior reduces chimney effects that pull moisture into the walls.

Most of all, air sealing the top of the conditioned space keeps the attic dry, and lessens suction in gaps in the building envelope. For example, if you have holes where wires perforate the ceiling, air will chimney out. Pressure is reduced in the living space. The leaky door now becomes a conduit of outside air. That air is warmed and moisturized as it goes through the living space. When it shoot into the attic, it's like a fountain of moisture and heat that creates a biosphere of mold over your head.

No, spray foam systems use compressed air as a propellant. No, the actual manufacture of part A and part B components of spray foam do not use propellants. No, cellulose is not a great wall insulation because it absorbs water. Yes, walls are susceptible to the chimney effect, but the moisture is coming from outside generally, not inside the conditioned space. The RH outside changes radically throughout a day in most of the north america, as does the temperature. Those differentials in relation to the mostly constant conditioned space temps are what cause condensation. Air sealing exterior walls is for keeping moisture out. Assuming the exterior wall vapor barrier is working, air sealing interior walls is for keeping the cool or warm buy dry wall air from moving in the wall cavity. Ideally you want that air to not move in a wall. No, you are backwards on water damage for attics. The ceiling insulation is generally much thicker than walls (R38 vs R13 as per code), because are incredibly hot and cold, depending on outside and because heat rises, the ceiling becomes the place where insulation is needed most. Generally there is very little air movement within ceiling insulation, which makes cellulose insulation a good product for that zone. The attic space above however, is designed to breath traditionally. Usually air is allowed in through soffits, and out through gable or ridge venting. This use of the chimney effect promotes a healthy, cooler roof, which makes it all last longer. When the attic is turning over air, the moist air coming in the lower vents, needs to exit, so the top vents need constructed as high as possible to allow the warmest, wettest air out. This is why ridge vents are prevalent today.

Rock wool is not some magical insulation, it is very similar to fibreglass, but costs more and itches less. Ironically, asbestos was the previous generations rock wool. Maybe there is already an attorney working on the next round of lawsuits. I'm not an expert on that though, so take it for the value of free advice.

Closed cell spray foam insulation has changed the industry. It allows complete air movement sealing within the wall and roof cavities. This function is a great potential tool and can completely change the design of a building. I do not recommend open cell spray foam insulation for anything other than sound deadening.

I'm a custom home builder and I strongly recommend you be careful who you get this info from. Lots of people have ulterior motives in the "green movement", often not with the homeowner's best interests in mind. I do not know Fairbank's weather, and before I insulated a new home there, I'd make for sure I got intel from someone who works that area that I trusted. Don't rely internet only.

[Myles Moran]

I recommend you spend some time at the green building advisor site. A lot has changed in terms of best practices for air, water, and vapor management and insulation in the last 15-20 years. There are really knowledgeable folks at that site that are welcoming and helpful.

I learned so much from them when I air sealed my rim joist and I've seen the results already with reduced AC runtime this summer. Definitely a great place to get the best information on conditioning a building.

[Scott Winners]

I will check out green building advisor, thanks for that. Also I will be talking to pros before I break ground, but in general I find folks here know what they are talking about and don't have anything to sell me.

Sean, thanks for the invite. I will speak to my wife, maybe we can buy you a coffee or something. We are over by Chena Pump/ University West for now. The remote wall system looks good in theory, I am a bit concerned about all those 9 inch screws sagging with time.

I do appreciate the perspectives on vapor barrier placement. It does make intuitive sense the various ways y'all have explained it. Rather than a separate cabin I'll just put another layer of barrier on the outside and AC the whole thing.

I have been round and round the mountain on rim joist insulation. One thing my current home doesn't have is a water impermeable barrier between the ground and the sill plate. I pulled my rim joist insulation out because my wife was reacting to the mold in it. In my experience vapor barrier is all or nothing. If vapor barrier is done halfway, or 85% or 99 percent, there is still going to be moisture (and eventually mold) in the insulation cavities. If I could jack this house up off the continuous foundation, put in a layer of thick plastic and lower it back down I would put the rim joist insulation back in.

My current thinking, if we go with a crawlspace, is water impermeable layer on top of the foundation, with Ground Contact Pressure Treated for the sill plate and floor joists, with insulation betweeen the joists and vapor barrier under the subflooring. This is a high dollar solution. Adding AC to the mix I could put a water barrier on the foundation, then span the crawlspace with vapor barrier, and then do the floor framing with regular lumber, insulate the joists from above and put down another layer of vapor barrier before the subflooring goes down. These are the kind of steps that give framing contractors heartburn and make them see dollar signs.

FWIW I am sick (and tired) of floors on 2x10 joists on 24" centers. I will be building with 2x8 joists on 16 inch centers, subfloor will be two layers of 3/4 tongue and groove. This is one of those places where theory doesn't work in practice.

For heating, since you all are sharing with me, my current dream home with the experience in the climate will be minimum 2x6 construction with outstanding air sealing and triple or better pane glass. I am strongly tempted to do a 2x6 shell to support the roof, insulate, then blue foam on inside the 2x6 load bearing studs, then 2x4 curtain walls inside that to support the ceiling, mechanicals, more insulation, then vapor barrier and drywall. Ground snow load here is 55 pounds per ground square foot, gsf. Current wind load is 90 mph but might go up when the comittee gets around to it.

For the ceiling I want to leave all the room in the attic I can for blown in, minimum 40 inches corner to corner. Primary heat, as far as my insurance agent knows, will be a boiler on a thermostat with radiant infloor heating. I will be a "supplemental" wood stove user from the insurance perspective with pro installed chimney for low homeowner's insurance payment, and a Cold Air Intake or Fresh Air Intake installed on the wood stove so combustion air doesn't come through the HRV.

I haven't picked an HRV (Heat Recovery/ Ventilation) system yet, but I will choose one with washable rather than disposable filters. I know a few folks who feed their cord wood stoves HRV filtered air for combustion, the finished ones have a summer/winter switch on the HRV to keep household airflow balanced seasonally. Better to put the wood stove on its own air circuit and keep the HRV balanced year round I think. I am kinda looking at trying to keep all the HRV piping inside the vapor barrier to reduce penetrations of the vapor barrier, except for fresh air in and exhaust air out of course. Hopefully I can find one that will accept cooling from a heat pump to pipe through the house.

I did look at OWBs a fair bit, but to run them clean I would need a big tank of hot water for heat storage somewhere inside the envelope and a bunch of extra pipes and pumps and maintenance. It would be nice to keep all the cordwood chips and bark outdoors, but I am trying to get away from maintenance and there is nothing quite like laying down on the hearth in front of a raging stove after a long day out in the snow.

Pencil plan is 3 beds, 2 baths, 2 car garage on one level. Floor plan will depend on the lot we find. We are putting a dividing wall in the garage so my shop will no longer be in my wife's garage. We will probably come in around 2000sqft, maybe a bit under.

[Jim Becker]

For your subfloor, as an alternative to two layers of 3/4", Advantech, LP and other products are available in a thicker material that's really stout and sturdy. A pot framer that I follow on the 'Tube uses the LP product extensively for mezzanine type applications and it's...really solid.

[Kris Cook]

I live in Montana and am trying to finish up a shop building that has a garage (west) and workshop connected to the east with a second floor above the workshop. The shop is insulated and drywalled, and the garage is yet to be insulated (although the doors are insulated). I haven't gotten around to hooking up the mini-split in the shop because I am working out of town. We have seen low 100s here this summer. The shop hasn't gotten above 76 degrees. The walls are 2X6 with R19 batts with no vapor barrier. I did use a WRB over the sheathing. I also did a lot of research on vapor barriers and came to the conclusion I didn't need one. The primary consideration for me and the shop (as opposed to a home) was with one person working in there and no bathroom or kitchen there is very little moisture being generated inside the space. The upstairs office/studio will have a bathroom. My plan is to put a vapor barrier in the bathroom only, and I have a programmable exhaust fan that will be set to exhaust the humid air. In the longer term I plan to rely on the mini split for cooling and continue to use the wood stove when it gets too cold for the mini split to be effective.

I don't know if any of this helps but I guess at a minimum I would consider the vapor barrier issue differently for a dedicated shop space.

[sean meltvedt]

Scott I would certainly meet you for coffee, or at the shop. I’ve attempted to send a PM a couple of times not sure if you got them, as they don’t show in my “sent” items. Anyway let me know if you received them, otherwise I can contact you via FB.
Cheers
Sean

[Scott Winners]

For your subfloor, as an alternative to two layers of 3/4", Advantech, LP and other products are available in a thicker material that's really stout and sturdy. A pot framer that I follow on the 'Tube uses the LP product extensively for mezzanine type applications and it's...really solid.

Thanks for that idea Jim. My current home has no effective water barrier between the dirt and the subfloor at ground level. I do have plastic in the crawlspace on the dirt and open cell foam on the exterior of the poured foundation. Vertically I have dirt, one layer of plastic on the floor of the crawlspace, and untreated lumber for the framing, with the next layer of vapor barrier just under the drywall of the ceiling at the second floor. At the foundation edges I have no water barrier under the pour, open cell foam on the exterior, and no water barrier between the pour and untreted framing. 1980 build, it was state of the art at the time.

This is not ideal.

Honestly my "best" solution is to not call the fire department right way if it catches fire so I can start over with the poured foundation and place a water impermeable barrier before I start framing.

I am still wrapping my mind around placing one vapor barrier just under the flooring and drywall, with a second layer of vapor barrier on the exterior on all six sides to make a "thermos bottle" out of the walls and lower floor and upper ceiling. It makes intuitive sense, but executuion does not look like something I can hire out. It looks like to do it correctly I will have to do it myself.

I am currently thinking about doing the sill plate in GC PT and then unrolling an enormous sheet of plastic barrier under the floor joists as they are installed, 16 inches at a time, so I can insulate edge to edge as I go between the joists and then install one layer of subflooring after each third floor joist goes in. In this plan I would have, bottom to top, poured foundation, thick water and termite barrier (the way the weather is going they will be here soon), vapor barrier spanning the crawl space, untreated and insulated joists, first layer of subfloor, interior vapor barrier, second layer of subfloor.

If I do that I will have a good shot at the exterior vapor barrier. Once the wall framing is in I can just drop the exterior vapor barrier from above to the rim joists, acoustical sealant (black death) and done. But the interior vapor barrier will come out from between the two layers of subflooring, wrap around the upper layer of subflooring under the bottom plate of the walls and then come up inside the wall framing.

Besides thermal bridging issues, no way can I get that done without having a framing crew of all old guys with arthritis poking only minimal holes in the interior VB layer and taking extra months to get the job done. If I hire young dudes to get it done quick the interior vapor barrier layer is going to get torn to shreds where it comes out from between the two layers of subflooring.

I could maybe put single layer subfloor on the joists, get the walls up as above, and then put down the interior VB on the single layer subfloor, get that going up the walls and then put in the second layer of subflooring. I want to be able to put in hardwood flooring and/or ceramic tile, but grout doesn't stick to vapor barrier and I don't want a bunch of nail holes in the VB either.

This is why the idea of not using vapor barrier at all and just having higher utility bills/ cord wood usage is somewhat attactive. Vapor barrier, this far north, is either perfect or not.

I do appreciate the product lead and will investigate.

[Scott Winners]

Scott I would certainly meet you for coffee, or at the shop. I’ve attempted to send a PM a couple of times not sure if you got them, as they don’t show in my “sent” items. Anyway let me know if you received them, otherwise I can contact you via FB.
Cheers
Sean

PM'd you back a few minutes ago. Had a hunch you figured out who my wife was via FB. You are not a stalker my wife says, but my rifle battery is ready to go for anyone who is.

The one at the the head of my bed is .338 caliber on an AR10 form factor, come on over. Just cleaned and lubed it Sunday. If someone wants to meet your maker tonight I will be happy to post my street address so as to faciliate your introduction to King Jesus. Sean is invited to ring the doorbell, but I sent him my cell number.

May the Peace and Love of Christ be with you now and forever. Use my doorbell if you show up on the front porch please.

[Scott Winners]

I'm a custom home builder and I strongly recommend you be careful who you get this info from. Lots of people have ulterior motives in the "green movement", often not with the homeowner's best interests in mind. I do not know Fairbank's weather, and before I insulated a new home there, I'd make for sure I got intel from someone who works that area that I trusted. Don't rely internet only.

Appreciate the input. One question. Fairbanks should perhaps be known as "Extremebanks" often seeing +90dF in the summer and -50dF in the winter (historically, I haven't seen -50dF since the winter of 08/09). Outdoor ambent was +82dF at 2100 tonight with 'sunset' at 2341. I get civil twilight until the sun comes tomorrow (07-22) at 0414.

If I build the outer framing shell at say 12 feet to support the roof out of 2x6 and the inner shell at 8 feet (2x4 ) to support the ceiling, and then blow in one meter (40inches of cellulose) in the attic, am I pretty much stuck with placing a meter (40-48 inches) of cellulose and covering that with vapor barrier before the trusses or rafters go on so the 'exterior' vapor barrier layer is continuous?

Asking for a friend so to speak. I can't see trying to cut in the external vapor barrier around all the rafters and still get a good seal. I mean I can see it in my mind's eye; but I can't think of a human I could hire to get the job done.

[Scott Winners]

. All I do know is you want to get this vapor barrier thing right.

You are exactly correct my friend. Vapor barrier is just like surgical instruments. Long term it is either 100.00% perfect for pass, or 99.99% perfect for fail.

[Scott Winners]

Most of all, air sealing the top of the conditioned space keeps the attic dry, and lessens suction in gaps in the building envelope. For example, if you have holes where wires perforate the ceiling, air will chimney out. Pressure is reduced in the living space. The leaky door now becomes a conduit of outside air. That air is warmed and moisturized as it goes through the living space. When it shoot into the attic, it's like a fountain of moisture and heat that creates a biosphere of mold over your head.

Yup. You are exactly correct. Fixing those air/vapor leaks was literally a PITA up on a ladder with my neck cranked over, but it was worth it. The first couple years in this house I carried blue chalk in one hand while feeling for outdoor air leaks with the other. In the warm seasons I fixed all the blue chalk marks, also worth it. I started with the upstairs ceiling and worked my way down to ground level. Air leaks are huge on the utility bill and cheap to fix on the maintnenance budget. R200 is useless if you have a bunch of leaks.

[Scott Winners]

I live in Montana and ... My plan is to put a vapor barrier in the bathroom only, and I have a programmable exhaust fan that will be set to exhaust the humid air. In the longer term I plan to rely on the mini split for cooling and continue to use the wood stove when it gets too cold for the mini split to be effective.

I don't know if any of this helps but I guess at a minimum I would consider the vapor barrier issue differently for a dedicated shop space.

Well, yes and no. Short term your solution will work. If you have annual subfreezing tempertures, the building will end up with moisture and eventually mold in the insulation, possibly after you pass off your mortal coil.

My local experience is if you are going to use vapor barrier it has to be perfect - or over the course of decades don't bother. Pick your poison, mold abatement later or ongoing higher utility bills today.

Even if I do all the labor myself as owner/occupier, I know I will punch some holes in both the exertnal and interior vapor barriers during construction. I can budget some time and materials to repairing those holes, but at the end of the project I am a fallen sinner who has fallen short of the glory of God, my vapor barrier will leak and eventually, someday, there will be moisture and mold in the insulation of my hand built house as well.

I need to think about making the vapor barriers modular, and repairable around the framing.

Thanks for having put a fine point on the question.

[Steve Rozmiarek]

Appreciate the input. One question. Fairbanks should perhaps be known as "Extremebanks" often seeing +90dF in the summer and -50dF in the winter (historically, I haven't seen -50dF since the winter of 08/09). Outdoor ambent was +82dF at 2100 tonight with 'sunset' at 2341. I get civil twilight until the sun comes tomorrow (07-22) at 0414.

If I build the outer framing shell at say 12 feet to support the roof out of 2x6 and the inner shell at 8 feet (2x4 ) to support the ceiling, and then blow in one meter (40inches of cellulose) in the attic, am I pretty much stuck with placing a meter (40-48 inches) of cellulose and covering that with vapor barrier before the trusses or rafters go on so the 'exterior' vapor barrier layer is continuous?

Asking for a friend so to speak. I can't see trying to cut in the external vapor barrier around all the rafters and still get a good seal. I mean I can see it in my mind's eye; but I can't think of a human I could hire to get the job done.

Your weather is surprisingly similar to western Nebraska. I've never seen -50 here, but -40 does happen, and 100 plus days are pretty common, so I'll offer what is common practice here. You probably get more moisture there, but the monsoon season is no slouch here either, so it's probably relevant info.

Here we look for R19 in the walls, making sure the exterior sheeting is wrapped and taped before siding. I prefer Huber's ZIP sheeting system, but it is practically impossible to get right now. Regular old Tyvek house wrap is fine as long as it's taped properly. You can do just R19 kraft faced fibreglass batts, paper in, and staple the paper to the studs under the drywall. I do prefer to use a 2" closed cell sprayfoam then R13 kraft faced fiberglass batts. The hybrid system makes about R27. and a really good break/barrier in the wall system.

In the roof we're looking for R38 minimum, but I prefer R45. That's only 14" of cellulose, or R38 fibreglass plus a little. What we do here with standard bottom chord trusses (flat ceiling), is just spec a taller heel height, trying to get at least 8". Attach ceiling to lower chord with no drop, and install baffles to allow soffit air to pass over insulation at the plate. As we don't have a full 14" of room in this spot, the foam packs more R into the available space. Sprayfoam the baffles with a couple inches (R14), and blow cellulose in the attic on top of the ceiling drywall, adding however much you want. I think anything more than 15" is unnecessary here. The vapor barrier is a sheet of 6 mil plastic that staples to the bottoms of the trusses before the drywall goes up. Hope that makes sense.

[Rob Luter]

I live in Montana and am trying to finish up a shop building that has a garage (west) and workshop connected to the east with a second floor above the workshop. The shop is insulated and drywalled, and the garage is yet to be insulated (although the doors are insulated). I haven't gotten around to hooking up the mini-split in the shop because I am working out of town. We have seen low 100s here this summer. The shop hasn't gotten above 76 degrees. The walls are 2X6 with R19 batts with no vapor barrier. I did use a WRB over the sheathing. I also did a lot of research on vapor barriers and came to the conclusion I didn't need one. The primary consideration for me and the shop (as opposed to a home) was with one person working in there and no bathroom or kitchen there is very little moisture being generated inside the space. The upstairs office/studio will have a bathroom. My plan is to put a vapor barrier in the bathroom only, and I have a programmable exhaust fan that will be set to exhaust the humid air. In the longer term I plan to rely on the mini split for cooling and continue to use the wood stove when it gets too cold for the mini split to be effective.

I don't know if any of this helps but I guess at a minimum I would consider the vapor barrier issue differently for a dedicated shop space.

If I was you I'd rethink this approach. Humidity is relative. In the winter time the cold air outside doesn't hold as much water as the warm air inside. One person generates plenty of moisture through respiration and perspiration. The warm air soaks it up. As it permeates the walls seeking thermal equilibrium it will condense. Read the IRC for proper wall construction in your climate zone. It's a free view on the ICC Codes website.

[Steve Rozmiarek]

If I was you I'd rethink this approach. Humidity is relative. In the winter time the cold air outside doesn't hold as much water as the warm air inside. One person generates plenty of moisture through respiration and perspiration. The warm air soaks it up. As it permeates the walls seeking thermal equilibrium it will condense. Read the IRC for proper wall construction in your climate zone. It's a free view on the ICC Codes website.

That's true, to a certain extent, but in the real world, you will never have a problem. Because air moves around in a building and humidity throughout the building will tend be relatively constant throughout, any air exchange in the building will lower the relative humidity. Human activity in a sealed space will likely only raise the humidity to something we would consider comfortable if there was no air exchange. There is though every time someone runs a bathroom fan or opens an outside door or window. Human existence in a building alone will not create enough moisture to warrant excessive vapor barriers or hurt normal building materials.

This another of those myths that are perpetuated by the interwebs and "green building" experts, but in the real world does not exist. Kris's plan will work just fine.

[Scott Winners]

I don't know if any of this helps but I guess at a minimum I would consider the vapor barrier issue differently for a dedicated shop space.

It does. It feels like half of my brain is just processing this puzzle while I try to drive to work and talk on the phone with the half of my brain that is available to me.

One thing I kind of became aware of today is where does the water vapor inside the home come from.

Typical sized adults lying in hospital beds with maybe bathroom priveledges are going to give off about 1400mL, not quite a liter and a half, of water daily as water vapor exhaled a tiny little bit with every breath. A 24 hour day. Generally when I have had patients on strict Intake and Output measurement I know their urine output to the nearest one milliliter, IV fluid intake to the nearest one milliliter and can guess at their sweat volume from how moist the bed linens are (there is a table for that estimation). When I can compare yesterday's and today's weight on the hospital's scale knowing exactly what went in and what came out, I typically come up the patient lost another 1000 to 1500mL of water weight somewhere else, within the normal range for "insensible respiratory losses." More active people will have higher respiratory losses, and more sweat. I have no idea how much water a marathon runner loses during a race, but way more than a liter and half for sure.

Cooking not very much can release a slew of water vapor as well, but for me to add 1.5 liters or quarts of water to the stove top I would have to be running multiple batches in a water bath canner in all the same day.

Hot showers is another. It depends on how long and how hot your shower is, but certainly some water vapor production, and a few spoons full here and there after toweling that will evaporate directly from your skin, plus the towel drying on the rack waiting for tomorrow.

Deep in heating season here I often manage to pump two gallons out of the humidifier as water vapor daily. I could do more, but I would need a bigger humidifier to do it.

In heating season net interior humidity change should be negative from interior/exterior air exchanges. One reason my home is so dry in the winter is I am pulling combustion air for the wood stove out of the living space. My main incoming air leak, makeup air, comes in under the front entry door on the lower level of the house, wood stove is upstairs. If I seal that leak under the front entry door my makeup air is going to be coming in around the sliding glass door out to the upper level deck.

In cooling season, exchanging indoor/ outdoor air is going to raise the temperature of the house, and let in some water vapor too.

Where else does water vapor come from inside the envelope?

Clothes drier a little I guess. Most of it goes out my dryer vent, but some, sure.

There will be a little evaporative loss from the toilet bowl, even with the lid down, but after a two week trip I don't see that as a major source of water vapor.

Anyone? I am trying to be thorough, if you know of something I am missing please speak up.


For Kris' office/studio space I can see his rationale for only putting vapor barrier in the throne room. But if he later adds a kitchenette and a shower head and a tenant, I would want interior vapor barrier under the drywall all around and over the studio/office space.

[Scott Winners]

I hope I don't crash a server forcing a multiquote...


Humidity is relative. The warm air soaks it up. As it permeates the walls seeking thermal equilibrium it will condense.



That's true, to a certain extent, but in the real world, you will never have a problem. Human existence in a building alone will not create enough moisture to warrant excessive vapor barriers or hurt normal building materials.


I am biologist home owner, I am not a builder, I am not a bunch of things; I think you are both right. The scale I have access to is a 1980 build home I am living in now. It is time that makes the difference, from my perspective. Americans move on average, what every five to seven years? In that window Steve is right, and in some climates Steve will still be right in a 40 year old build.

Up here, in my state of the art when it was built in 1980 home, I have mold problems. Well, technically it is my wife that is allergic to mold; but we are one flesh in the eyes of God. Anytime I have opened the drywall (bigger windows to meet current fire code for instance) I find black mold in the fiberglass insulation.

What I think of as normal building materials, like the Old Ship Church or the Fairbanks House in the original Massachusetts Bay Colony, are still standing without mold problems, but by modern standards both are quite leaky and poorly insulated. Expensive to heat. But all the materials and techniques used allow adequate airflow to dry back out when they do inevitably get wet.

Pic is one of the windows I replaced. It is a ground floor bedroom, so an egress window. We left the header alone, but lowered the bottom of the window, I dunno, 8-10 inches, well well inside the current spec for egress windows. Only one penetration through the vapor barrier for the receptacle (no hats in 1980, it wasn't a thing yet). I had four or five stud cavities open. The one with the receptacle in it had the "most" mold, but all the cavities had "some" mold.

What I see is the vapor barrier on the wall isn't connected to anything vapor proof in the flooring. The easy way to get water vapor into those stud cavities was moving along the floor to the wall, under the edge of the vapor sticking out from under the drywall I can see flapping loose in the breeze, and into the stud cavity. Boom done. Might as well leave the cow pasture gate open on a Friday afternoon.

I think vapor penetration through mudded drywall with an intact vapor barrier under it is essentially zero; but just like bugs will find that one hole in the window screen, water vapor will find all the leaks in my vapor barrier.

How soon the mold in the stud cavities is a problem for the residents is going to vary wildly with a host of factors, not least among them the mold sensitivity of the people living inside the envelope. I am miserable during pollen season here, but don't have any trouble with allergies when the house is closed up like tupperware (tm) in winter. My wife is the opposite. She sails through pollen and suffers when the house is sealed.

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[Scott Winners]

So what about floors and ceilings? This is driving me bananas (qv).

When I opened this thread several of you were able to explain (using small words that I could understand) that the vapor barrier goes on the hot side of the wall; so if I am running both heating and cooling I need vapor barrier on both sides of the wall. Like a thermos bottle. Got it.

I quizzed Steve R on that directly in this thread. He is a pro home builder in a climate remarkably similar to mine. If I read his repsonse correctly he is putting vapor barrier on the underside of the ceiling framing, and then drywall on the inside, with insulation above the vapor barrier. But no vapor barrier above the insulation in the attic space, and I am not clear what Steve is doing under the living space.

My wife (who used to work at the same company as Sean M like 10 years ago) and I got to see Sean's house this evening. Sean has a strong background in math and engineering. I can do trig, but it makes my head hurt. Thankfully Sean is used to dealing with stupid people like me and I feel like I have a pretty good understanding of how his home works for vapor control.

Sean and I agree on having a vapor impenatrable layer under the entire building, under the footings, under the floor of the basement or out from under the footings and across the bare dirt of the crawl space underneath, but total vapor barrier under the building, and wrapped up the outside of the foundation to tie in to the external vapor layer under the siding. This might be a local thing that isn't needed in other environments; to me it seems like an easy and cheap thing to do while you are looking at a big hole in the ground and prohibitively expensive to do after the house is built.

At the end of the day I don't see a good executable way to build the walls "like a thermos" without putting a floor and a lid on the bottle. Consider a short piece of double wall stove pipe with no end caps.

But Sean didn't install vapor barrier above the blown in insulation is his attic either, just like Steve.

Both of them said, more or less "it doesn't matter." But I am the guy who uses a stud finder on my ceiling when installing a smoke detector. I don't want to poke uneeded holes in my vapor barrier, so putting both screws of a smoke detector into a ceiling joist makes sense to me. Less warm humid air leaking into the attic insulation that way.

I can see that access is a code issue. Why not put the attic access trap door adjacent to the wood stove chimney? You (I) can't have blown in cellulose in direct contact with a wood stove chimney anway. Why not just build four walls out of micore say 24x72 inches footprint, four feet tall. Run the chimney through it, have the trap door open into it, blow in 40 inches of cellulose in the rest of the attic. Have a place to stand when you are checking to see if the vapor barrier above the blown in is ballooing from air leaks?

Is anyone concerned about having water vapor penetrate the upper layers of the attic insulation during cooling season? I am at 38" blown in cellulose where not constrained by the roof framing. When I have stuck my head up in there in wintertime it is pretty dang cold up there above the insulation. Could it be I don't need a vapor barrier up there because the temp differential just doesn't get all the way through and as long as I have it sealed on the internal side having vapor barrier up top really just "doesn't matter?"

It just seems to me the external vapor barrier needs to be continuous on six sides, not five sided open at the top. My main reluctance is having the ceiling vapor barrier working alone. Just like the lid on a thermos. When I stash some coffee in a thermos it will eventually cool off, it seems to me most of the heat gets out through the single layer lid.

The other thing is if I do have air leaks through the ceiling internal vapor barrier, I will see it as ballooning in the external vapor barrier laying on the blown in if I go up and look. Execution to get the blown in down and exterior vapor barrier down before the trusses go on will be a cold hearted witch from Eastwick. Installing the trusses will be problematic as well.

Third and finally, I would as a bare minimum have to wrap the exterior vapor barrier over the top plate and down the inside of the wall framing to meet and seal to the interior vapor barrier to fully enclose the walls, which seems like the path Sean chose.

One thing really got my attention. My house is about 2400sqft, 1980 build. With two kids home I was using 275-280 MBTU annually just for heat and domestic hot water, not counting electric and I have no AC. Sean's home is a lot bigger than mine but with one child at home he is sitting around 125MBTU annual including electric and AC. I was sorely tempted to just lay on his floor since he is running in floor radiant cooling with a heat pump. Instead I came home and held the refrigerator door open while eating supper standing in front of it. Technology has come a LONG way in 40 years. I will be delighted to cut my energy consumption in half/ sqft with my next home.

[Steve Rozmiarek]

Isn't this fun Scott ;)

In my experience, the place you found the mold under the window is very common. It usually can be traced to water getting in from the window though. 99% of the time it's an incorrectly flashed sill plate that is allowing condensation and weep water to flow back into the house rather than out. Not sure why window flashing is so hard, but it's the most common installation error I see and eventually causes a serious problem. Water damage from a bad window install or failing window can cause damage a stud bay or two away from the window as water moves horizontally a bit as it travels down too.

This illustrates my point about water from outside being a much bigger problem than inside moisture. Once the outside water gets in, a vapor barrier actually makes things way worse because it can't dry out. That is the real world tipping point that dictates practical vapor barrier usage. A properly taped (or lapped) and installed window and house wrap system would have eliminated the water in your wall. Tar paper will even do a fine job with this which is what the old-timers used.

As I explain this to my new crews, keep the rain outside the sheeting and keep the warm air inside the drywall. Vapor barrier goes as close to interior as possible and exterior water barrier as far to the outside as possible. That's all that a vapor barrier does. Wall interiors should be dry and have no air movement. Focus 99% of your effort in keeping exterior water out of the walls and roof.

Drywall and Kraft backed batts actually offer most of the interior vapor barrier that you will ever need, add some plastic under cellulose or blown fiberglass. That's pretty much it.

You don't want a vapor barrier over insulation in the attic because you want it to be able to dry out if water gets in. I have never seen moldy insulation in a roof that wasn't caused by a roof leak.

Crawl spaces should be considered part of the conditioned air space, at the very least they should have a sealed plastic membrane over the dirt. no other barriers needed in interior floor platforms.

[Scott Winners]

Isn't this fun Scott ;)

No. Is driving me bananas. I imagine the smliey face means you were joking.

Today we have forest fire smoke in town, we popped over the 100 PPM threshold to have air quality that is unhealthy for sensitive groups.

My wife (works from home) had a headache from the smoke, so she closed all the windows and now it is 86 degrees in her home office what with the 20+ hours of sunshine today.

Usually here "uncomfortably hot" lasts for about two weeks around July 4. Started in May this year and still going strong.

I look over my exterior wall system a bit, but I don't think you would have built mine this way.

Thanks

[Bill Dufour]

Can you just pump ground water through radiators for cooling. Like a water chiller cooling system. No idea on water laws in Alaska. Are you on permafrost?
Bill D