Jump to content

Insulating a floor platform on a building on piers


Recommended Posts

I have a colleague who is trying to determine the best way to insulate a floor platform on a new building (think hunting cabin), 2x8 floor joists with OSB subfloor. There are 3 beams that consist of 2 treated 2x10s bolted to either side of 6x6 treated posts. The whole affair is approximately 24" above bare earth, and will likely be skirted like a double wide in a park.

The owner would like to spray foam it from underneath. I think this is a bad plan because we would either have to encapsulate the joists and leave the foam exposed (edit: this would be approx. 2" ccSPF), or fill the voids (edit: approx. 6" ocSPF) and try to deal with vapor diffusion rotting the spruce joists from the middle out. Plus, I don't think my applicator will fit under the building with his PPE, let alone have room to wield the gun.

I am thinking he needs to insulate it like a doublewide with a mat under the joists, or with rigid foam from above.

Any other ideas?

Link to post
Share on other sites

If it was me, I would stuff unbacked fiberglass batts between the joists and staple up Typar or Tyvek housewrap to hold it and to form a wind barrier.

Then I would skirt the place, with rat-proof metal flashing around the base of the skirting. I've used perforated metal soffit covers split in half for that.

Link to post
Share on other sites

That's the way doublewides are done but the access sucks, ranging from 11" to not quite 24". I would think for future serviceability the poor bastard going under this place would appreciate the extra 7 1/4 inches open joists would afford. If that ends up as our recommendation it won't be fiberglass though, we sell Roxul.

Rat proof flashing? Are Canadian rats as amicable as Canadians? American rats are as gluttonous as Americans, flashing won't keep them out. I keep baits outside my place right near the tunnel they dug under the crawl space stem wall.

I still think treating this exercise in poor planning like a cold slab and insulating it from above would be the cheapest solution. The labor savings are going to be substantial.

Link to post
Share on other sites

I can just get my chest in thru an 11" slot. That would be for an attic access. Under a house, claustrophobia lurks there on the left edge of the brain, and sheer panic is just around the corner. [:)]

Insulate the skirting and call it a conditioned crawl? You could heat it with a couple of light bulbs when the generator is running.

Yeah the rats are mild mannered here, Tom. The real mean-assed strains have all moved south to the US where they can openly carry weapons. [:)]

Link to post
Share on other sites

24-inches is plenty of room to work. I'd just stuff the joist bays with unfaced fiberglass and then secure 1/4-inch galvanized mesh to the underside of the floor platform. That will keep out rodents and allow unimpeded vapor diffusion. It will be plenty warm enough for a hunting cabin with a skirt around it. Just make sure he installs a tight vapor barrier over the earth, wraps his pipes with foam insulation and installs some crawlspace vents.

If he's hung up on saving heat, install some foam against the underside of the floor as Kurt has suggested, seal the edges with exanding foam, then install your unfaced fiberglass and mesh. Be sure to frame out removable access openings with furring attached to the mesh under toilets and other components you'll need access to and where the mesh is cut for a pipe to pass through you sandwich the mesh between a couple of metal collars with galvanized screws.

It will keep the cabin warm. The insulation will be protected so critters can't pull it down onto the floor and can't nest above it. Do it right and it will look artful - yeah, you're right, it's a friggin' hunting cabin, Mike, nobody cares.

By the way, is there a code guy sneeking around? 'Cuz i'm pretty sure that method - securing beams to both sides of a post for support under the floor platform using bolts is now out-of-favor and disallowed by the codes. Could be wrong though - often am.

ONE TEAM - ONE FIGHT!!!

Mike

Link to post
Share on other sites

I'd be thinking rigid foam panels.

2" of polyiso would give him R-20 and leave more than 5" of joist exposed. Is that enough to avoid vapor drive rot issues?

So I need a guy cutting 4' strips on a table saw and a guy, maybe two, under the house with a can of one part foam for gaps and a mallet for tight pieces. Sounds like a blast.

Spraying up into joist cavities with barely enough clearance to move......sounds like a mess.

It does, and it's not just trying to move. There is a gun and hose to manipulate, plus supplied air, and a hose tender to keep the applicator from hogtying himself. And the heat. We're using 'room temperature' foam, but the stuff kicks at over 200 degrees.

Not sure how I'd do that yet. From above makes some sense...maybe a lot of sense.

From the pics I've seen the place is only dried in, and it's little, maybe 20 x 30. If there aren't too many partitions up my crew could blanket the floor in rigid foam and plywood, and raise the two entry doors faster than we could set up and tear down the foam rig.

Link to post
Share on other sites

By the way, is there a code guy sneeking around? 'Cuz i'm pretty sure that method - securing beams to both sides of a post for support under the floor platform using bolts is now out-of-favor and disallowed by the codes. Could be wrong though - often am.

ONE TEAM - ONE FIGHT!!!

Mike

There is a permit in the window, but it's out in the sticks. It's the kind of place where a picture of your tape measure in the hole counts as a foundation inspection.

Link to post
Share on other sites

We did a project like this last year... joists over post/pier, no skirting and none desired. We installed 3/8" CDX under the joists with medium-crown staples, drilled a 4" hole into each bay, and dense-packed it with cellulose. We put 6" square plywood covers over the blow holes. The attached photo shows one area ready to blow.

This is a property that is rarely used, and the owner is not there to notice whether rodents are moving in, so I wanted something that was fairly impenetrable (unlike batts and housewrap) and not subject to wind damage. Dense-pack does a good job of air-sealing compared to batts.

It was expensive, it worked well, and it is permanent. The main issue was cleaning up cellulose afterwards.

Click to Enlarge
tn_201322517222_10_01_12%20004%20RS.jpg

42.4?KB

Link to post
Share on other sites

We didn't paint it, maybe we should. I thought long and hard about this one before doing it. The entire underside of the place was bone dry. No signs of any splash-back, and in spite of a windy location, no rain blows under there. It's waterfront facing southwest, up on a bluff a bit, out in the open with a few trees around it. Most of it is well clear of the ground, but it's tight in the back. It's all slope so there will never be standing water.

There's one hole per joist bay in the shorter sections, one at each end in the longer sections. All the holes are covered with 6" plywood squares. We thought about running 1x over rows of holes, but then we'd have to drill them in a line, which was not conducive in the tighter spots.

I have a long-term involvement with the place so will be watching how this ages. Housewrap would get ripped to shreds by animals and wind under there. I do have a client with batts and housewrap under his joists, but he is in a protected location and he is there every day to notice if anything starts flapping.

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

×
×
  • Create New...