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joconnor
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# Posted: 26 Mar 2025 10:35am
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Hi all. Here's what I've got. A 14'x42' building (typical wood framed shed). Floor construction = T&G plywood subfloor supported by 2"x6" floor joists set on five 4"x4" runners. The building will be placed on a monolithic slab with vapor barrier. Thus, there will be approximately 3.5" of air space under the building. Location is northern Virginia (ie, humid). No utilities. Power will be via solar while in use. Heat will be via woodstove while in use. Will only be used occasional weekends.
Building is premanufactured and no flooring insulation options are available from the manufacturer. I plan to insulate the joist cavities with mineral wool batts (from underneath...I'll raise the building off the slab for this). My question is what is the best material to use under the joist assembly to "encapsulate" the insulation? I want something under the joists to help hold in the insulation, deter pests from getting up inside the joist cavities, and to mitigate air movement inside the joist cavities. I am concerned about mold. I cannot decide whether to use a Tyvek style house wrap or pressure treated plywood to accomplish this. Your thoughts?
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travellerw
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# Posted: 26 Mar 2025 11:06am - Edited by: travellerw
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Do a search.. Your question has been asked many times.
The TLDR: Spray foam is the answer. Any fibrous insulation (fiberglass, mineral wool, ect) WILL attract pests and you are unlikely to keep them out.
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joconnor
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# Posted: 26 Mar 2025 11:23am
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I did search and know both house wrap and plywood have been used by others in similar situations. But I don't know which would be better, or worse, in my situation.
Spray foam is overkill for this purpose.
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gcrank1
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# Posted: 26 Mar 2025 11:43am
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Working underneath will be time consuming and Miserable, btdt. Put sheets of styro down on top of the floor and another layer of subfloor.
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travellerw
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# Posted: 26 Mar 2025 12:23pm - Edited by: travellerw
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Quoting: joconnor But I don't know which would be better, or worse, in my situation. Spray foam is overkill for this purpose
Yup, they have both been used and many people had to pull it all back out once the critters got in. An absolute PIA and waste of money. Impossible in some situations and then you are left sharing your cabin with those unwelcome freeloaders.
Not sure what you mean about "overkill". Maybe in costs. Certainly not in work or insulating value.
Our cabin is built like Gcrank describes. Foam board on the inside with a second subfloor overtop. Its an excellent affordable solution, but requires doors, windows, outlets, switches to be all moved up the height of the insulation. If you can do that, you won't regret it as pests can't get in and you will have a pretty warm floor (our dogs sleep on it at -30C once the cabin is warmed up). You also get a SUPER solid floor as its a laminated sandwich.
P.S. Foam board can often be found on Marketplace or Craigslist for a significant discount.
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joconnor
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# Posted: 26 Mar 2025 12:35pm - Edited by: joconnor
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Foam board between layers of subfloor is an idea I'd thought about. Building has yet to be constructed so planning for that would be doable. However, it would be better for finishing the interior to work from underneath the structure. I'll give this more thought though. Thanks.
In my area, critters are not likely to chew through PT plywood to get into this structure. There will be nothing compelling them to do so...no heat, no food, etc. I'm not nearly as worried about critters as I am with trapping moisture in the cavity. I know house wrap will allow vapor to pass as needed. But I'm not so sure about plywood.
Agree laying on my back working from underside would be a PITA, but I'm used to doing such things.
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travellerw
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# Posted: 26 Mar 2025 01:29pm
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Quoting: joconnor In my area, critters are not likely to chew through PT plywood to get into this structure. There will be nothing compelling them to do so
What is compelling is the insulation. They will make a home in there as its insulated and will retain their heat. They are attracted to both rockwool and fiberglass. Personally, I have not been anywhere where mice did not do this. And anywhere I have been, they will chew through plywood (pressure treated or not) to get in.
Maybe its different where you are, but pretty sure Virginia has mice or squirrels (even worse). There is also the issue of flies and bees, them buggers will ABSOLUTELY get in. Anyway, based on the experiences here, I suggest you don't create an insulated cavity as it has caused issues for many here. In your scenario I think it would be a real PIA as once the cabin is on the slab you wouldn't be able to access those cavities.
As to moisture. Plywood is porous and allows moisture out, albeit slowly. You could make vents on the rim joists at each end of the joist bays. A 3" vent on each side would probably be sufficient. They sell screened round vents here for soffits, would work fine for that application too.
Quoting: joconnor However, it would be better for finishing the interior to work from underneath the structure. I'll give this more thought though. Thanks.
I'm confused by this? The way I did it, didn't make it any harder to finish the interior (other than installing things higher). It was done as the interior was wide open. Interior walls were then built right on top of the second subfloor. I can post some pics of how things were done in my cabin!
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gcrank1
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# Posted: 26 Mar 2025 02:03pm
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For the floor Id insulate the interior area, not under where the wall plates go. That would let the pre-build go ahead and you could fairly easily do the floor after.
For the build what mostly changes is the door sill/height accommodating what will be the 'higher' floor once the insulation and top subfloor go down.
The 'higher' floor also means the positioning of the window sill and any electrical boxes may need to be adjusted upward too.
Oh, dont forget what thickness your actual flooring needs to be factored in.
Im not sure what thickness the top subfloor needs to be over the styro, I think Ive heard of folks using thinner than the structural decking beneath?
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joconnor
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# Posted: 26 Mar 2025 02:03pm - Edited by: joconnor
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I plan to install a wood stove and assume the weight of such would be more than should be placed on foam board (of course I'd be adding plywood on top of this first). And I'd need to add spacers to the stud bays in order to give me a nailing surface (for wall board) above the new floor height. Little things like that is what I'd meant. Not deal breakers.
Did you use XPS or EPS board? And what PSI and thickness did you use?
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joconnor
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# Posted: 26 Mar 2025 02:07pm
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Quoting: gcrank1 Im not sure what thickness the top subfloor needs to be over the styro
XPS manufacturer says two layers of staggered 7/16" or one layer of 3/4" T&G on top of their foam board.
If I went this route, I'd do as you suggest and have the building built as engineered - and I'd add the new subfloor after installed.
Raising the doors isn't a big deal. I'd try to have the manufacturer do this during construction but if they wouldn't, I'd just redo their work to bring the doors up to height once the building was in place.
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rpe
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# Posted: 26 Mar 2025 02:14pm
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We used Rockwool between the floor joists, with Tyvek stapled underneath, and 1/4" galvanized metal mesh held on with roofing nails to keep the critters out. Zero issues so far - 6 yrs in. Others in my area have used 1/4" or similar plywood nailed under with success as well. Spray foam seems to be the gold standard, but there are other options.
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travellerw
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# Posted: 26 Mar 2025 02:29pm - Edited by: travellerw
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Quoting: joconnor I plan to install a wood stove and assume the weight of such would be more than should be placed on foam board (of course I'd be adding plywood on top of this first)
We have a BIG woodstove and it hasn't been an issue. The stove has to weigh over 200 pounds as it take 2 people to just drag it. Actually, the woodstove is sitting on tile that is installed directly on the subfloor, no cracks in grout or tiles (3 years).
I sourced our insulation on Marketplace so I can't exactly remember the details. However, it was pink XPS certified under concrete. We used 2" plus 3/4 OSB as the second sub floor. Interior walls installed directly on top. Many basements are built that way up here to insulate over the concrete.
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travellerw
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# Posted: 26 Mar 2025 02:45pm
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Here are a few pics.. Looks like it was pink Foamular 400 2" R10
272940802_1015814113.jpg
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317256064_1015864687.jpg
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PXL_20231009_0115100.jpg
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joconnor
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# Posted: 26 Mar 2025 02:45pm
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Quoting: travellerw We have a BIG woodstove and it hasn't been an issue.
Thanks for this information. Is your second subfloor floating or did you screw it down through the foam?
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joconnor
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# Posted: 26 Mar 2025 02:48pm
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Quoting: rpe there are other options
Thanks for this information. I wasn't considering the wire mesh, but installing it seems a good idea.
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joconnor
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# Posted: 26 Mar 2025 02:49pm
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Quoting: travellerw Foamular 400
Thanks. That's the 40psi stuff. Special order product.
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travellerw
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# Posted: 26 Mar 2025 03:02pm
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Quoting: joconnor Thanks for this information. Is your second subfloor floating or did you screw it down through the foam?
Its screwed with long screws that go into the joists. Lots of screws. I was going to glue it as well to make a real composite sandwich, like a boat. However, we did that in the winter and I couldn't source a glue that would work on the cold surface.
If you can't source the foam at big box or marketplace, then I suggest contractor building supply. At least around here is super common and used under almost all concrete. Although its not super cheap if bought new.
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DRP
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# Posted: 26 Mar 2025 05:35pm - Edited by: DRP
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Quick math... 12"x12"=144 square inches in a square foot. Assume 20 psi foam. 20psi x 144square inches per square foot=2,880 lbs capacity per square foot. We typically assume undisturbed good soil to have a 2,000 psf allowable capacity.
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Grizzlyman
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# Posted: 26 Mar 2025 08:06pm - Edited by: Grizzlyman
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If you’re concerned about the weight of the stove… just skip that area with the foam board. Build it up with plywood to the same height in that particular spot.
As I’ve mentioned on this board before I spray foamed underneath. It really isn’t difficult and took me all of an hour to do the whole cabin-600 sq ft… though you do need some room to work. It’s expensive-but so is all the plywood and foam board when you add it all up. It still will cost more probably but For the extra cost it really wasn’t marginally all that much more… and it was quick. I’ve noticed a huge difference in the floor being insulated.
I used the froth pack available at most big box stores. 650square ft (board- ft Technically) i believe for about $900. R-6 at an inch thickness which isn’t the best but you’re in Virginia as well so that’s probably ok. It’s made for a nice warm floor in N MN so it’ll probably work for you just fine. Also no worries about mice or other pests with the spray foam (that was my primary motivation to use the spray foam)
Can you just have the manufacturer insulate it for you so you don’t have to do that afterward?
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Brettny
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# Posted: 27 Mar 2025 09:07am
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I wouldnt skip any are with the foam. It may seam like it will be fine or just a cold spot but it can build condensation and mold under the floor.
Dont forget your stove also needs some type of hearth to sit on..further spreading out the load to a bigger area.
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joconnor
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# Posted: 27 Mar 2025 09:07am
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Quoting: Grizzlyman Can you just have the manufacturer insulate it for you so you don’t have to do that afterward?
The manufacturer only offers a foil-film (Prodex) as flooring insulation. That's a product I'm not interested in.
Thanks for your input on the spray foam. I haven't checked prices in a while, but $900 for 650sqft is about half the cost of alternatives. Though less R-value too.
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spencerin
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# Posted: 29 Mar 2025 08:31am - Edited by: spencerin
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Obviously just an opinion here, but I would disregard the extra cost of spray foam and go with it. It's very effective, probably quicker to install, is its own vapor barrier, and typically doesn't need to be enclosed because mice and insects don't find it attractive. This is supposed to be a one-and-done job, so with no recurring expense associated with it, the extra $ could definitely be worth it.
But, if you decide not to go with spray foam, I'd just enclose it with treated plywood. You described the building as a pre-fab-ed structure that'll be 3.5" off a monolithic slab, so I'm guessing it'll be a pre-built "shed" on 4 x 4 skids. That's enough ventilation underneath, so any moisture that in theory would pass through the plywood (which is very, very little anyway) would already be ventilated away.....
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