How to insulate the exterior walls under a 3 season room.

[Rughead]

Hi there. In Scarsdale, NY we've recently had our backyard deck, which was attached to the rear of the house, removed and 3 season "Florida" room built there. They did a good job and we're pleased with the new room, lots of windows, cathedral ceiling and well insulated. We'd now like to use the space underneath this room as a storage area for bicycles, gas grill, etc. but want to keep it as dry as possible. At present there is only a lattice type facade around the 3 sides which we'd like to keep for the look. Can I simply attach some XPS to this, from the floor of the room above to the gravel covered ground below, and then face it inside with plywood leaving a gap at the bottom so there's no contact with the gravel? The space will not be heated and we plan to leave the gravel floor as is. Look forward to any advice. Cheers and best regards, Rughead.

[Dana]

XPS is rated for ground contact, but must be covered with an interior-side thermal-barrier such as half-inch wallboard or plywood to meet fire codes.

The other issue will be ground moisture. Burying a a sheet of 6-mil poly in the dirt under the gravel works (it must be protected from puncture to STAY working. Sloping it away from the foundation is also a good idea, should it ever get bulk-incursions of water. If you don't put in ground vapor retarders when you seal & insulate the walls, in Westchester the mold & mildew season in that space would be 8-9 months out of the year!

To do it at least half-right would involve a poly vapor retarder under a rat-slab (non-structural thin concrete slab that keeps varmints from burrowing in to the now-cozy & dry shed-room), with at least 2" of XPS on the exterior behind the lattice with at least 1/2" ply on the interior through-screwed to the structural members. Use duct-mastic to seal the vapor retarder to the XPS before putting up the plywood,and at where it meets the exterior if the house. Air-seal with spray foam or cobble'n'cut XPS foam-sealed at the edges & seams any of the gaps at the top of the wall to block thermal bypasses of air through the joists, and use a reasonably weatherstripped door or hatch as access. Rat slabs don't take a lot of concrete- an inch or stwo will do, and it'll protect the ground vapor retarder from puncture. For small rooms this is an afternoon of hand-mixing the concrete in a wheelbarrow, slopping it in there, and floating it roughly level. If it's over a hundred square feet do the math, and after you've already placed & sealed the XPS & ground vapor retarder order up the necessary cubic yardage for a 2" slab and have it delivered.

[Rughead]

Thanks Dana. Am glad you replied as I've been following this forum for some time and you seem quite knowledgeable when it comes to insulation and vapor barriers, etc. Just a bit of clarity if you don't mind. From your above, I should put the 2" XPS directly on the lattice, then a 6-mil vapor retarder (sealed), then the plywood through screwed to the vertical structural members, correct? I like the "rat-slab" idea too. Should this be done before or after the walls? I could embed the XPS into the perimenter of the rat-slab couldn't I ? At present the "floor" under the room is dug out about 3 or 4 inches, then geo-textile fabric and then 3 or 4 inches of gravel. From your advice I assume I should remove the gravel and geo-tex, then put down the new 6 mil poly vapor retarder and then the 2" concrete rat-slab. Can I re-use some of the gravel with the concrete or would it be over-kill? BTW the floor space is 12'x 14', the size of the floor of the Florida room. Looks like a few days of hard work over my summer home leave. Again thanks for your advice and I look forward to anything further. Cheers and best regards, Rughead.

[Dana]

Thanks Dana. Am glad you replied as I've been following this forum for some time and you seem quite knowledgeable when it comes to insulation and vapor barriers, etc. Just a bit of clarity if you don't mind. From your above, I should put the 2" XPS directly on the lattice, then a 6-mil vapor retarder (sealed), then the plywood through screwed to the vertical structural members, correct? I like the "rat-slab" idea too. Should this be done before or after the walls? I could embed the XPS into the perimenter of the rat-slab couldn't I ? At present the "floor" under the room is dug out about 3 or 4 inches, then geo-textile fabric and then 3 or 4 inches of gravel. From your advice I assume I should remove the gravel and geo-tex, then put down the new 6 mil poly vapor retarder and then the 2" concrete rat-slab. Can I re-use some of the gravel with the concrete or would it be over-kill? BTW the floor space is 12'x 14', the size of the floor of the Florida room. Looks like a few days of hard work over my summer home leave. Again thanks for your advice and I look forward to anything further. Cheers and best regards, Rughead.

The XPS is best applied to something with structure to it- the lattice itself may not be rigid enough- it may need at least a header & footer board to glue & screw it to, as well as whatever posts are holding up the lattice. Dual or triple layers of 1" 4x8' or 4x9' XPS sheathing willl be better able to span between structural elements than the 2" 2x8' or 2x10' t&g stuff, so you may find a way to make it reasonably easy to span between structural elements on the exterior layer. Lapping the layers & bonding them with -sized blobs of foam board cement on an 18" grid between the layers, will make the foam much less flexy for when you put the plywood up. Then, walnut-sized blobs on a similar grid between the plywood & foam gives the final assembly substantial structural rigidity. The plywood still needs to be through-screwed to the structural framing to make it code-legal as a thermal barrier though.

You don't need a poly vapor retarder on the XPS, but DO seal the vapor retarder on the ground to the XPS: Apply a bead of foam-board adhesive or caulk applied to the XPS, then staple the poly to the XPS with 1/2" or longer staples, then apply 1/8-1/4" of duct mastic (paintbrush or putty knife over that to seal the edge completely. (You don't want ground moisture finding it's way up to rot the plywood at the transition line.)

Yes, pull out the gravel, but you can put the poly on top of the geo-tex, then seal it to the XPS before putting up the plywood. The rat slab can poured after the plywood goes in, and should come up to at least an inch below the plywood unless you're planning to put up wooden kick board after the rat slab is set.

If you're mixing it yourself you can re-use the gravel as aggretate in the concrete. If it's crushed rock rather than rifer-rock/screenings it has some potential for penetrating the poly during installation, but the risk isn't high. A higher ratio of aggregate to portland cement also weakens the concrete, but it's a non-strucural slab- it's OK for it not to meet any floor-loading standard- it just has to hang together enough to keep rats, squirrels raccoons, skunks, possums, etc from digging their way in.

[jadnashua]

While the ply on the inside may be sufficient for fire codes, the lattice on the outside might not be. You may need some sheathing on each side.

[Dana]

While the ply on the inside may be sufficient for fire codes, the lattice on the outside might not be. You may need some sheathing on each side.

That's a reasonble thought, but SFAIK there's nothing in the IBC about exterior thermal barriers, and as a point of common-practice vinyl siding, etc is applied over foam sheathing without any form of thermal barrier all the time. (Then there's the foam-insulated vinyl clad siding, etc.) Were it me, I'd put something up there to keep the pink or blue foam from shining through the lattice, but it need not be a fire-rated thermal barrier. If it's a place where it might get a lot of rainsplash some sort or rain-proofing like 15# felt or housewrap wouldn't hurt, but it's not critical.