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Exterior insulation board

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Aug 20, 2013, 05:15 PM
Exterior insulation board
I have a 100+ year old rowhouse with a small rear addition that sticks out above an open-air crawlspace. Half of the addition is an old back porch with a concrete floor. The other half only has cement backer board, underneath our dining room floor. Neither seems to have any insulation, though the gaps and edges around both halves are sealed well with spray foam to keep out rodents.

Before winter, could I put a layer of rigid foamboard insulation on over the backerboard, over the concrete, or both? I see that extruded polystyrene or polyisocyanurate boards are available in a range of R-values and costs. Would it be best to use concrete anchors, glue, or some other adhesive?
Aug 20, 2013, 09:25 PM
Maybe I don't fully understand what you want to do here but if you want to add a layer of foam on top of the subfloor then the important thing is what your finished floor is.

It is possible to have a layer of foam, then two layers of plywood (to make a floating subfloor) and then you could have a floating finished floor, vinyl or carpet o top of that. But that's a lot of layers that will also raise the floor surface.

Have you considered some form of carpet for the winter?

Aug 21, 2013, 09:42 AM
My idea was to put foam board *over* the exterior, ie- attach it to the bottom of the outside of the house where insulation is lacking. Is there any reason not to do so?

We definitely aren't going to make any changes to the floor from the inside, especially cover our restored 105 year old parquet hardwood floors with carpet!
Aug 21, 2013, 05:55 PM
Ah......Yup, can't hurt. Any insulation will help but unfortunately, it's more important to have it first in the ceilings, then walls and finally under the floor.

Sep 24, 2013, 10:07 AM
The thin board doesn't offer much in the way of R-value. IMHO it is mostly hype. If you went with a 1" poly iso you'd raise the R-value of the wall by r-8, however this will cause trim out problems around windows and doors. The thin stuff most siding contractors install is much thinner and might add an R-2 to the wall.

If the home was under insulated and I was going to replace the siding I'd look to take the opportunity to beef up the insulation in the wall cavity before I put the new siding on the house. That would likely be a blown in cellulose insulation. I'm not sure you can get a foam into the cavity without tearing off the siding and sheathing to expose the wall cavities. But I'm not a contractor and I don't play one on TV.
Sep 24, 2013, 12:04 PM
I've seen posts on here that claim the local authorities require at least an R-4 exterior insulation if you reside a house with 2x4 walls. that is a local issue and not national.

but you do lose your windowsills in the process, and then rain can't drip off an inch or so beyond the siding. that has implications for infiltration.

so the waterproofing before the siding becomes critically important, in my non-certified opinion as a plain old DIYer who hasn't faced the issue. if I got into that quandry, I'd look to have the sashes and sills flashed out, I think, by somebody who knows what they're doing. a siding contractor would probably also do that work, perhaps with seamless extrusions or hand braking the roll metal for a custom fit. somehow they should include a drip edge so water doesn't creep around the bottom of the sill inward, so an extrusion machine might be a better choice. might be able to fake it with a seamless bead of silicone under the sill.

and at that point, there is no substitute for properly installed Tyvek housewrap.

This message has been edited. Last edited by: swschrad,

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