Ugg, I am going to have to add cooling

[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.

20210721_215205[1].jpg

[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