Insulation: Adding foam insulation to existing finished exterior walls

[djhawk42]

I have a garden-level basement. The interior walls are framed up to a height of 42". The contractor did not install rigid foam to the concrete walls...just framed and added R-13 batt insulation prior to dry walling.

Have any of you tried to add foam insulation in behind the
batt insulation in order to fill the gap between the batts and
the concrete walls?
The cavity between the 2" x 4" framing and the concrete
wall is about 2" which I am contemplating filling with closed
-cell foam.
...... Is this plan do-able?
...... What would be the potential for the expanding foam
to press back on the existing fiberglass batt insulation,
and thereby compressing it and lower its existing
R-Value. Which in turn would some what negate the
R-value gained by adding the foam insulation?

.....Would my attempt to retrofit the exterior walls using
this process be worthwhile ???????

Any feasible suggestions would be welcomed. ( Not to
include tearing off the drywall and starting over...)

Thanks in advance

[EatenByLimestone]

Just open the wall and do it right. Drywall is cheap
So is tape and mud.

[EricU]

I thought of all sorts of crazy plans, but I'm just removing and replacing the whole wall. Of course, my situation is a little different since I had a plumbing leak that caused water to wick up inside the wall and grow mold.

foam will compress the fiberglass, no question about that. So the R value will be compromised. And you want to check to see about condensation between two layers of insulation

[Phil Thien]

EatenByLimestone said:

---

Just open the wall and do it right. Drywall is cheap
So is tape and mud.

---

This, and the fact that the walls are framed to 42", means mudding would be pretty straightforward.

OTOH, lots of other options like paneling and other finishes, too.

[fredhargis]

Without knowing the technical stuff about foaming over glass insulation, I can tell you there is at least one company (Dayton, OH) that does just that. At least they show in their ads. They just stick the hose in and fill it up, it does seem (in the ads) that the foam does compress the glass, and I'm guessing the foam has a higher R value. Don't know if that's worth anything, but thought I would mention it.

[crokett™]

At the risk of sounding like CC, I know a guy. I asked him. He said don't do it. The spray foam doesn't stick to the fiberglass very well, eventually there will be separation and moisture issues. A better fit if you don't want to take all the sheetrock off is to spray in cellulose insulation but that isn't the best fit in walls because it will eventually settle. His recommendation is pull off the sheetrock and either add more fiberglass or remove what's there and use spray foam.

[Shovel Man]

Do you need more insulation? If you have 3 1/2" of fiberglass in good shape the added benefit will be small. Of course it depends on what the outdoor conditions are, windy side of the house, family room inside etc.

[Woodenfish]

The air gap between the interior of the foundation wall and the batts of insulation is another layer of insulation. Filling that gap with foam board isn't going to solve anything. What is the problem?

[jteneyck]

You will reduce heat losses by adding the foam because the overall insulation will be thicker and foam has a higher R-value than fiberglass. Yes, the foam will compress the fiberglass and it's R-value will go down, but the foam that displaces it has a higher R-value. If you use closed cell foam your air infiltration will decrease as well. But only you can decide whether the effort and expense is worth the energy savings and improved comfort.

In my house the walls are insulated with 1" of fiberglass batt - hanging in a 3-1/2" stud wall! I guess that's what you did in 1961. The exterior is brick veneer, a 3/4" air gap, and 1" of beaver board nailed to the outside of the studs. Air whistles through my exterior electrical outlets. If I could find a foam system with low expansion force that didn't cost a fortune I would drill holes through the interior plaster walls and shoot it behind the fiberglass. That would eliminate my air infiltration and boost the R-value of the insulation. Removing the plaster walls to replace the insulation is not an option except in the rooms I've remodeled, and that's already done and there will be no more. Not to hijack your thread, but if anyone has any recommendations I'd appreciate it.

John

[Shovel Man]

It's out there a ways but it's free, can you make holes high and pour in packing peanuts? If you ask people to collect them for you it shouldn't take long.