Floor insulation in a building on skids

[Zachary Hoyt]

I am getting close to starting work (probably in 4-6 weeks) on my smaller-than-desired shop building on skids and I have the design pretty clear in my mind except I am baffled about the floor insulation. I was thinking of putting foam sheets between the joists, but that is expensive, environmentally unfriendly, and awkward. It may still be the best option, but I'm wondering. Rockwool batts are another possibility, but I am afraid of them holding moisture against the pressure treated wood and attracting varmints to make nests. Another idea which occurred to me yesterday was to run foam sheets vertically down from the bottoms of the walls and bury them in the ground a foot or so. This would be a lot less foam, and would perhaps be easier to seal well than all those strips of foam between the joists. The other thing is just to not insulate the floor at all, and live with it. I think I read somewhere that only 5-10% of heat is lost through the floor, so floor insulation is less effective than roofs or walls. If I did that I would still seal up the walls to the ground in the winter so that the wind didn't go howling through. I'll be grateful for any advice about this.

[Maurice Mcmurry]

Mold and mice can be a problem. Uninsulated floor with plenty of ventilation is how our building on skids is. The floor and floor structure are constructed with treated lumber. I have thought about putting foam on top of the treated floor and another plywood floor on top. I am uncertain about that idea.

[Frank Pratt]

As long as the rock wool is not in contact with the ground, there won't be a moisture problem. Put some steel mesh under the joists to hold the insulation up & keep out the critters.

[Jack Frederick]

Perhaps 2” of foam on the floor between 2x stringers and more plywood above. You loose 2 3/4” of so of height, but it gets the job done. Electrical can be run in it as well.

[Jim Becker]

Rockwool is actually a good material for this application as it really doesn't hold moisture nor is it friendly to critters. It's also formatted to fit in pretty snug. Frank is spot on about using a mesh barrier to prevent physical damage from below as well as account for any potential of the Rockwool to sag downward over time, but that's minimal compared to material like fiberglass because of the density and resistance to moisture.

[Zachary Hoyt]

Thank you all for the input. I'll have to look into options for mesh to go under the joists. I'd rather use Rockwool than foam for environmental reasons and because it is not as messy as cutting 4x8 sheets of foam with a table saw.

[Scott Brodersen]

Whatever you do, be sure not to trap moisture below the floor and ensure there's plenty of air circulation under the floor -- I speak from experience!

[Zachary Hoyt]

Thanks, that's a helpful thing to know too. I'm thinking now that what I may do is build it without insulation under the floor, with the idea that I can put rockwool in from below with mesh under it at some point. It would be easier to insulate the floor before putting down the plywood, but I would need several days with no rain in order to get the walls and roof built to keep the insulation dry. Rigid foam would be almost impossible to retrofit from below, but rockwool should be manageable, if not fun. I might even lift up the plywood sheets that are not under the walls and do the center of the building from above, to save some crawling.

[Jim Becker]

How much clearance off the ground are you going to have here? They type of building you are constructing are generally "on the ground" so future access from below wouldn't normally be possible.

[Zachary Hoyt]

I'm planning to have 1 4x8x16 block above ground and one in/on the ground, and maybe more in spots where the ground is low. Then there'll be 6" or 5-1/2" of skid below the joists, so I should have 10" of space. I'm fairly skinny so I should be able to squeeze in, though it would not be a lot of fun.

[Jim Becker]

If you are going to raise it up like that instead of sitting the skids on gravel as would be customary for a "portable" structure, you may want to consider pouring stable columns to below frost level to make sure your building doesn't "rock and roll". Ground heave in your area may not be pretty at all, especially with a structure of the size you are planning.

[Zachary Hoyt]

I would prefer to have columns (I wanted to pour a slab, really) but I can't put things in the ground near to the town water line. I'm planning to have concrete blocks in a 4 foot grid, so there will be 45 support points for the building. I could do gravel instead, but I think it should be better drained and ventilated on blocks, and I won't have to wait for the spring road weight restrictions to be lifted like I would if I had a dump truck come in to bring gravel. I can bring the blocks in myself. The garden tool shed here that I built in 2009 is 12x16 feet and sits on 6 large rocks, and it has not had any trouble with frost heaving. I am hoping that this new building will be okay as it will have more support points even though it will be bigger.

[Brian Gouldman]

I have a 12x20ft on typical 4x4 skids on gravel foundation I’m building out to be my shop. I’ve insulated the walls and ceiling with rockwool. I will be putting down 2” foam sheet continuous and topping with 1 or 2 layers of advantech followed by some nice 4” prefinished red oak I got for a song. The ceiling is finished out as a cathedral, so the lost height isn’t that impactful since the ridge is about 10ft. I think how you finish the ceiling will play a significant role if you build up from the first subfloor.

[Zachary Hoyt]

That seems like a good way to insulate the floor. Will you need to move the door up so you don't trip over the new higher threshold, or was it built with that in mind? I am planning on a cathedral ceiling so I can store bulky light things up there like bags of shipping peanuts and banjo cases. I was surprised to see yet again a price spike in sheet goods when I spent some time this afternoon thinking about my plan. I found a sawmill about an hour from there that sells rough cut pine and hemlock for 70-75 cents a board foot depending on length, and I am going to see if either local Amish or Amish up there are at a similar price point. I came up with a plan to put up the whole building except the skids from rough lumber, including the floor, and it looks like I would need about 4k board feet so it would cost about 3k. A board floor will dry out and have gaps, but I can put something else down over it when I "finish" the building into proper shop space. My plan is to build the shell in the spring and use it for storage and a place to run the tablesaw while I'm working on the house. Then in the fall when the wood has had some time to dry out I can insulate, cover the insides of the walls and roof with something, run electrical and so on. I just can't see spending $75 a sheet on 3/4" plywood that's not that great anyway. That's about $2.30 a square foot, so pine or hemlock at 70 cents looks a lot better.

[Brian Gouldman]

My shell was a prebuilt, delivered to site, shed I ordered just months before prices started spiking a couple years ago. So it has a vinyl double door that overlays the rim joist on the front. So I will be making my entrance a little shorter, again nbd because of vaulted ceilings, and I plan to just use while aluminum flashing and some sort of transition to make the rim joist there look nice. Currently it has the typical diamond plate angle stock at the corner transition and the bottom of the rim is just exposed.

Main downside to my building is the 7ft walls vs you can build yours as tall as you want and fit a normal single or French door with a threshold if you want. If I were to redo it I’d probably have site built with 9ft walls and a nice full lite French door.

In either case, you could frame the walls on the initial floor, pad under the door with solid wood the same thickness as the foam, put continuous foam down, new subfloor over and screw through to joists, and then just do proper weather flashing like you would any sill. Then you can make it pretty with aluminum flashing like I will, you could have your siding of choice, or just like a trim board. Whatever works for the aesthetic you are going for. In my case I need something thin because of the type of door I have already on the building. But once it’s done you’ll never know there is a double layer subfloor on the structure.

If you go with XPS you can get it in high psi so you shouldn’t need any sleepers within the floor between subfloor layers. It also has some vapor permeability which should help prevent any moisture related damage.

I’m sure someone will come in and say for the price of wood right now you could probably go concrete floor for the same price, maybe even cheaper. I wouldn’t disagree with them at all. I’m stuck with this option because I ordered a prebuilt shell. If I were doing site built I’d prob have a monolithic slab floor.

[Zachary Hoyt]

Thanks, that makes a lot of sense. My aesthetic for this job is cheap (relatively) and reasonably durable, and preferably not something that will make passing motorists cover their eyes and scream in horror. I'd prefer concrete for the floor but am not able to use it due to site constraints. The yard space I have is not ideal, but it will be good enough. I'm planning to use an ordinary 36" entry door and can always move it up later if I need to, I imagine. Not having to use sleepers would be nice as it would save a lot of cutting and fitting, and be a more effective barrier to cold, I imagine. I'm planning to make the sidewalls 8 feet or so, so headroom should be fine.

[Bill Dufour]

I will suggest the 36" door and another one maybe 12"wide. AKA french doors. Or two 24" doors.
Bill D

[Zachary Hoyt]

A wider door is an interesting idea for sure. The shop I've been in for the last 20 years is accessed by 36" doors and I have had no trouble with that to date, as I don't tend to build very large things. My business is banjos, mainly, and they are quite manageable. Even when I built a couple of canoes in 2014 and 2015 I was able to bring them out quite easily by turning them on their sides. I've built some cabinets and a little furniture, but nothing that wouldn't fit out the door. The one thing that I know is going to be too big is my Delta contractor saw. I'd like to go to a cabinet saw instead, but the right deal hasn't turned up yet. I've always kept the contractor saw in an outbuilding, but I think I can bring it into the new shop by removing the wings, or the motor, or tipping it over so the legs can wiggle around the door frame.

[Lee Schierer]

Since it appears that you are building this structure, build the floor framing and then install the insulation and the protective mesh. Don't use chicken wire, it will rapidly rust and is fragile. Use 1/2" mesh hardware cloth. Once the floor structure is insulated, build the walls and roof. If you don't get your supports below frost level youe will need to relevel your building every few years.

Be sure to check your zoning for building permits, setbacks and adding power.

[Jim Becker]

My old shop had double, outward opening, insulated steel, 36" doors with a nominal six foot wide opening when they were, um...opened. This was in addition to the standard 36" man-door. That setup was quite functional, although very tight getting my CNC machine in the door. But for material handling, getting projects out, etc., they were perfect. For my new shop I'm still debating on how best to handle one "big opening" for the same purposes...it could be an 8x8 insulated overhead door and it could be "shop made" carriage doors that provide similar to what Mike Farrington did on the 'Tube for his shop. I don't really want to use the same door type on the new shop because I do want a little wider and a little taller based on "lessons learned".

[Zachary Hoyt]

I found an online supplier of 1/4" hardware cloth that looks suitable, but I will also look for 1/3". I hadn't heard of that before. I'm a little nervous about putting the rockwool in between joists and under floorboards that are all fresh off the sawmill, but I don't know if it would really be a problem or not. I've read that it is vapor permeable, but I don't know if it would make the boards want to cup from drying faster on top than on the bottom.

I am willing to re-level the building as needed. There is some tradition in the Adirondacks of building on piers that are not frost safe, and it has been done on a large scale at Camp Santanoni and likely other places. There the main lodge is a group of 6 log buildings under one giant roof, and (if I recall) 270 feet long overall. It's been standing since the 1890s and has needed some jacking up and repair around the 100 year mark. Also around here mobile homes are commonly set on concrete blocks set on the ground, and vinyl skirting is put around them, and the heat from inside keeps the ground from freezing underneath. I'd rather have been on a slab, but that's how it goes.

I am going up on Monday to get a building permit. I'll have to find out what the building inspector will allow about types of floor insulation for this kind of building, among other things. I don't know what I'll have to do about supplying power. I would normally bury a line in conduit, but it would have to cross the town water line so maybe an overhead line will be needed instead.

I have found out what the setbacks are and will do some measuring Monday before I go to get the permit. It's 25' from the center of the pavement on the road side, and 7' from the neighbor's line. The lot is officially listed as 58' wide, but I don't know whether that is measured to the edge of the pavement or to where the edge of the road used to be in 1920 or whenever the measurements were made. There are no markers on the back side, but there are two sheds there and I am going to assume that they are at least 5 feet from the line, and aim to have the new building at least 2 feet closer to the road. I hope that will be enough to satisfy the inspector, and that there will be enough room for a 32' building length.

The inspector also told me that I will need to get the sawmill owner to sign a form stating that the structural lumber he is selling me is equivalent in strength to #2 grade of the same species. I talked with the sawyer's father who was at home and he seemed to think it would be okay, but he is going to talk to his son about it.

I am going to stick with a 36" door for now, and will plan to change it in the future if a need should arise. Building my own doors sounds interesting, but I don't want to try to do it now with everything else going on. Once the dust settles I'd like to give it a try.

[Scott Winners]

You have a legit winter there I imagine, I had to pull up a map to find the Orwell area. My only real contributions are I would:

1) Raise it up high enough for small predators to get under there. I don't know what you have for snakes in your area, and I love not having snakes up here; but I would a whole lot rather have a step to get up into the building than build an attractive snake den. If there is head room for opossum and foxes to get under there, that will minimize snake and rodent problems.

2) I would definitely insulate the floor from the get go. Even if you pour a slab. My shop is on an uninsulated slab a bit north of you, heat transfer through the floor is enormous. I keep the air in my shop at 55-60 degrees F, but everything touching the floor gets colder and colder and colder as winter wears on.

3) My tentative plan for your area would be to use ground contact rated pressure treated for the floor framing, use hardware cloth on the underside of the joists to keep rodents out. And kids. And dogs, and opposum and anything else coming along that would want to pick at the insulation. Given the size you are talking about I would get it stapled up from underneath, and then use some PT 2x2 as battens perpendicular to the joists from underneath, nailed up, to support the hardware cloth better. Then on the good days, lay in your floor insulation from above, and keep it covered with an impermeable vapor barrier as you go.

Will you be able to frame the walls adjacent to the building, or will you have to build them on the subfloor? The trick is going to be getting the subfloor down and the roof up between rainstorms on a day there is no standing water on the vapor barrier you have laid down.

I got nothing for codes back east. Having a regular man door for access and a larger door for material is a great thing. Good luck.

[Lee Schierer]

1) Raise it up high enough for small predators to get under there. I don't know what you have for snakes in your area, and I love not having snakes up here; but I would a whole lot rather have a step to get up into the building than build an attractive snake den. If there is head room for opossum and foxes to get under there, that will minimize snake and rodent problems.

Being any distance above the ground skunks, opossums, woodchucks, etc. won't hesitate to go under there and dig a den. They will also burrow down against a full foundation. Snakes, such as the corn snakes, milk snakes and black snakes keep the mice away and are harmless to people and pets.

[Zachary Hoyt]

There are snakes in NY, but the only poisonous ones are a few rattlesnakes in a limited area many miles from where I'll be living. I am always glad to have snakes around, since as Lee noted they eat mice, and they're easy to get along with. I imagine woodchucks could be a problem. I used to work for a man in Portland ME who had woodchucks always digging under his garage foundation. He would block the holes and fill them in as best he could but they would just dig a new entrance and keep using the burrows.

I think that stapling or nailing the hardware cloth to the joists should hold it up, since they are every 16". Hardware cloth is pretty stiff. Or if the inspector doesn't like rockwool under the floor I can see about foam on top. I'll have to nail down the subfloor around the edges before putting the wall frames up, and then I'll cut the rafters, etc, so it would be a bit of time between insulating the floor from the top and having it roofed over.

[Jim Becker]

Be sure you are very careful about zoning and not just setbacks...there may be other aspects that have to be addressed because of the square footage of the building, even though it's not "permanent". For example, here in my jurisdiction, a shed that's less than 200 sq ft doesn't need a zoning permit, but something larger does. It's joined at the hip with the building permit, too. It sounds like you've already been engaging with your local zoning/building person, so just make sure all your tees are dotted and eyes are crossed. :)

[Scott Winners]

I don’t pretend to know the local animals near the op. On the farm where I grew up, southern Kentucky, 90% of our snakes were copperheads. Some animal is going to want to get out of the weather under that thing for sure.

[Zachary Hoyt]

I think zoning is OK, but I have not asked specifically with that word. Before I bid on the house last summer I emailed the building inspector (who is also the tax assessor), described my business and my need for an outbuilding and asked if that would be permissible and she said she didn't see why not. I asked on the phone Tuesday about the setbacks and also if there was anything that would be a problem about my building a building of that size. My understanding is that they wouldn't mind if it was permanent and closer to the house where I had originally planned if it wasn't for the town water line, and it didn't occur to me to ask if there was a water line across the middle of the lot. I assumed the lines would follow the roads, but when the town was built the paper mill owned all the land except where the church was, so they could do whatever they felt like about the layout. If I was in a more rural area I would have to get permission from the Adirondack Park Agency, but they don't regulate as much in the hamlets except for some general rules like nothing over 40 feet tall without consultation, and no billboards and that kind of thing.

I am sure some animals will go under the building unless I use wire to make a barrier around the edges, and even then I expect they'll tunnel under if they want to.

[Jim Becker]

Given the uncertainty of where utilities are, get an 811 call in so your state's equivalent of our "PA OneCall" can document where any known utilities are on and around your property. You protect yourself that way in case there is any damage to something remaining unknown until you, um...hit it.

[Zachary Hoyt]

That's a good idea. We have an 811 system here too, though I have never used it. I'll be having a septic system installed as soon as the road weight limit is lifted and the installer has time, so it would be a good idea for that too. The highway superintendent told me where the water line is, since he was there when it was replaced nearly 30 years ago.

[Jim Becker]

The legal ramifications for not calling 811 when there is any chance of hitting a utility is, um...not fun. And AFAIK, it's at no cost. The septic folks probably have to make their own call, but you can do it for your project, too.

[Zachary Hoyt]

Yes, that makes sense. It's a town of 300 people, so one benefit is that there are folks who know everything about the history of the house and yard.

[Zachary Hoyt]

I went up today to get the permit and the measurements were good, the yard is actually slightly wider than the tax map shows, so that means I can fit a 16x32. The unsuccessful part of the day was that the building inspector called in sick today, so I left some paperwork and am hoping the plan can be approved provisionally so I can order the lumber before going back to get the permit. The building inspector is only in on Mondays, and it's 120 miles each way, so it gets a little awkward. On the whole I'd rather have enough room and be delayed getting the permit than have to make the building smaller and get the permit right away, so it should all be fine in the end, I hope.

[Jim Becker]

Interesting...my jurisdiction prefers to work all the paperwork via email and I only need to go to the Borough office to pick up the permit, etc, when it's ready...and, of course, hand them money so they give it to me. Even my zoning application was handled that way.

[Zachary Hoyt]

I guess it's different in different places. Where I currently live the building inspector has a computer and prints out the forms needed, and then fills them in in longhand and files them. Up there the application form is photocopied and they keep a stack of blank ones in the office. I don't think any of the actual form filing work is done on the computer, though the inspector does have email and will answer questions that are sent that way.

[Jim Becker]

Yea, different strokes. The forms used by the engineering firm that handles many of the local jurisdictions' building/zoning work uses PDF forms that can be filled out electronically or printed out and filled in by hand as you note. When folks submit by email, it saves time scanning, etc., so the documentation can be passed around quickly to various resources. That's less of an issue where you are because it's a very small population area and you deal directly with the inspector. We communicate with the Building/Zoning officer who in turn assigns the request to another resource (or handles it himself if that's expedient) and an inspector, who is a separate resource, is assigned for a project once the permit is issued. Even that last part may have an additional layer. I called in yesterday for inspection for our whole house generator project which is now complete. The name assigned on the paperwork subsequently called to set up an appointment today and then said it would be another individual actually coming for the inspection. Yea, it sounds complicated, but this particular firm works pretty darn efficiently, unlike back in the day when it was actual employees of the jurisdiction. This engineering firm is well honed, IMHO, and I feel fortunate about that, given how different it was back in 2007 when we were doing the addition project at our previous property. THAT was an eye opening and unpleasant experience due to the "personality" of the local inspector who was basically the back end of an equine...