Please help me insulate cold shed floor

November 2, 2023 1:43 PM

A family member's new shed/home office was installed over a gravel base; there is no room to get under and spray foam insulation. My idea was to install an insulating underlayment directly on the interior OSB floor, then put more OSB, or plywood, over that. But in researching the past several days, I can't find any definitive information stating this will work, or what materials to use.

He cannot build a sleeper floor. We need to find something that can go right over the interior OSB floor. If anyone can direct me to websites, or can provide their own experience of how to do this, it would be greatly appreciated! Also - he has put insulation on the walls and gambrel roof, then covered all that with house wrap; he doesn't want drywall. The house wrap is simply to keep the fiberglass insulation fibers from floating around and getting breathed in.

posted by racersix6 to Home & Garden (12 answers total)

Best answer: You can put down EPS or XPS insulation (foam board), then subfloor OSB again on top of that. You can use adhesive for foam board insulation to attach the layers or long screws through the insulation. EPS is not as good of an insulator as XPS, but XPS is a climate disaster due to the very high global warming potential of the blowing agents used to make it, so EPS is the better option.

You might also want to consider a plywood subfloor rather than OSB, since it is more durable if you aren't putting anything on top of it. Either way, you want the real subfloor tongue and groove sheets, not just any old OSB or plywood, which may not be stiff enough on top of the relatively soft insulation.

There are also basement floor insulation systems that you can buy at the big box stores, which are easier to install, but more expensive and usually not as much insulation. But for a small space, it might not matter that much.
posted by ssg at 2:01 PM on November 2, 2023 [5 favorites]

What ssg recommends is more or less how my tiny house on a trailer's floor is put together--the trailer (which didn't have a deck) structure becomes the subfloor. Sorry if this is obvious, but I would also add you want to put 2x4 boards down on edge as joists to support the new subfloor. You can then cut the foam board to fit in the spaces between--just stack sheets to fit the 3.5 inches. A masonry/abrasive cutting blade for a circular or table saw will cut foam board sheets very neatly. (Do it in a well ventilated space or with a respirator.)
posted by sevenless at 2:22 PM on November 2, 2023 [2 favorites]

My garage had a bare concrete floor; sucked the heat right out of you. I put down a waterproofing compound, then laid a specialized LVT-style plank flooring made of stone (not wood, so it's durable enough to drive on) which had an integral layer of insulation/cushioning.

Made a huuuuge difference with minimal change in thickness. If the existing OSB is sufficiently well-supported (that is, it's not flexing when walked on) you could try a similar plank-style product. Went in super-quick, and the insulation/cushioning made, as I indicated above, just a gigantic difference in comfort. Much more than I initially expected.
posted by aramaic at 2:27 PM on November 2, 2023 [4 favorites]

Sorry if this is obvious, but I would also add you want to put 2x4 boards down on edge as joists to support the new subfloor.

For a shed, used as an office, foam board with tongue & groove subfloor on top is an OK solution without the 2x4s (this would not be true with 1/2" OSB, it needs to be T&G subfloor to distribute the load). Doesn't hurt to put sleepers in, but unless it needs to stand up to heavy loads, the foam alone can bear the weight.
posted by ssg at 2:47 PM on November 2, 2023 [3 favorites]

Agree with others that you don't need sleepers. Most new concrete slabs that you put under houses have a layer of foam under them- foam has pretty impressive strength! Is this in an actual cold climate, e.g. well below freezing? If so, I would recommend as thick of a layer of foam as practical. Since there is a high ceiling with the gambrel roof, I would recommend going for at least four inches of foam, with staggered taped seams. You can then do a floating floor of two layer of tongue and groove plywood at 90 degree angles to each other. Having two layers that you then screw together will allow you to have a floating floor rather than screwing down through the foam to the OSB/joists beneath. It will also spread the load out more from a wheelie chair.

If you are actually in South Carolina, then winter isn't real, and you'll be fine with an inch or two of foam.
posted by rockindata at 5:12 PM on November 2, 2023

Also, the 2x4 sleepers are less than ideal because of material and installation time/cost, plus the significant thermal bridging they incur.
posted by seanmpuckett at 5:13 PM on November 2, 2023 [1 favorite]

Response by poster: We are located north of Charlotte NC (sooo cold!)
The building is 10' x 20' with an existing LP Legacy OSB floor
Thanks to everyone for the information! I really appreciate it
posted by racersix6 at 5:38 PM on November 2, 2023

but XPS is a climate disaster due to the very high global warming potential of the blowing agents used to make it

This is no longer true. It's higher than polyiso and EPS, but far improved.

I would put a single layer of poly iso, with tape across the seams. I would glue it down on top and bottom, and put a sheet product on top (OSB or plywood,) seams staggered across both (so they shouldn't align with each other or with the original OSB.) I would screw the upper sheet into the lower at 12" oc near edges and 24" in the field. Then any floating floor is good, or just paint it if you're not too particular.
posted by flimflam at 6:29 PM on November 2, 2023 [2 favorites]

Dunno if I’m stating the obvious here but if you’re looking at laying down a whole new OSB floor is there a reason you don’t just pull off the existing flooring, lay down proper insulation and a new floor?
posted by bitdamaged at 8:39 AM on November 3, 2023

The way these sheds (and for that matter, pretty much any stick-framed house) are built you would need to cut the OSB all around, and there isn’t actually enough space to properly insulate under there anyway. The actual way to do this “right” would be to jack the shed up and build a new floor deck on piers, that could then be insulated with your fluffy insulation of choice, and then drop the shed skids on top of that and air seal. Alternatively, you could pour an insulated slab.

It’s actually pretty labor-intensive to take a garden shed that wasn’t built to be conditioned space and make it a conditioned space. There are a lot more things that I would want to do to this person’s shed, but it’s outside the scope of the question.
posted by rockindata at 9:12 AM on November 3, 2023

I would not underestimate the impact of having no vapor barrier between the floor and the gravel. Direct contact with the ground is cold, but moist cold is really cold, even in the Carolinas. Now, moisture is why houses have ventilation under them, so you will be shortening the lifetime of the floor by by sealing it out of the living space, but a shed isn't expected to have the same lifetime as a house. Personally, I would put it on top of the existing floor if you have the headroom, better insulation that way and no demolition.

As for the insulation, the fibers and coatings used in fiberglas can really irritate your lungs. I would also make sure that's sealed up, with duct tape or something to hold down the tyvek.
posted by wnissen at 9:20 AM on November 3, 2023 [1 favorite]

Don’t use duct tape to seal the house wrap, use the specialized branded tape specifically designed to work with that house wrap.
posted by rockindata at 9:24 AM on November 3, 2023 [1 favorite]