Converting fiberglass+plastic basement insulation to rigid foam – worth it?
Brick/masonry house with a concrete, finished basement.
The foundation walls are insulated with R11 fiberglass, topped with plastic, and covered with drywall.
I’ve read vapor barriers below-grade are bad & that wet fiberglass is useless as an insulating material.
Am I overthinking the usefulness in tearing down the drywall in the basement, removing the fiberglass/plastic, and installing rigid foam boards instead?
There are some uninsulated foundation walls that get pretty cold & bring down the temp of the rooms right above. I was gonna stick Thermax boards on those walls but wondering if I should redo the entire basement.
Only thing is:
1) Is it a pointless, low-benefit operation
2) Is it expensive to install foam board & re-drywall a 900sqft basement, assuming I do the demolition myself & hire someone for the install process?
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If you have problems then yes do it. Check the walls by digging in and patching up later. If no problems do not worry about it unless you are bored and have extra money laying around.
Air seal first with blower door test if you want true performance boost.
Jeff,
I agree with S.E. The way to assess the performance of the existing insulation is to open up one of the walls -- preferably near the floor. If everything is dry and mold-free, leave things as they are.
If you encounter damp walls, damp fiberglass, or black mold, you get to make the decision about what you want to do next.
Understood. If my goal is to correct a poorly insulated wall (fiberglass is loose & unfaced, not even against the wall in areas, with plastic being the thing holding it up), would I not achieve better heat-retaining quality by switching to rigid board? From what I can tell with the exposed insulation areas, I don't have any damp/mold issues but I just severely doubt the insulating quality of the current setup.
75% of the interior basement perimeter is insulated this way & I initially just wanted to knock out the uninsulated 25% with foil-faced Dow Thermax boards (since it's just concrete/brick + some white paint or substance).
IF I did want to tear down the drywall, rip out the fiberglass+plastic, put up rigid board & re-drywall - how expensive is something like that since it's not a full basement finish?
If there is no air-barrier on the back side of the studwall it will undercut the performance of low-density batts like R11s or R19s by quite a bit due to convection & infiltration in the air cavity. The air gap between the fiber/studs and the foundation wall has been effective as a capillary break, and air infiltration to the outdoors through any cracks in the concrete and gaps at the foundation sill have probably been beneficial for limiting the mold potential though. The actual performance of that wall in it's currrent condition is probably about R8 "whole-wall", which is still WAY better than ~R1 of the above-grade concrete. It's the performance equivalent of about 1.5" of Thermax.
If you are in US climate zone 6 or colder it may be "worth it" in comfort, energy savings, and resilience perspectives to eventually upgrade the existing studwall, but not an energy emergency. In zones 4 or warmer it's probably never cost effective on energy savings alone. In zone 5 it's sort of in-between- the square footage of above grade foundation and fuel type may tip the balance.
If you ever re-finish any of those studwall rooms it's probably worth stripping the wallboard & plastic, and using a 2lb foam kit to insulate the foundation with an inch or so of foam (whatever it takes to bring the surface at least flush with the exterior side stud edges- even deeper if need be to bring it up to an inch), then install R13-R15 unfaced or kraft faced batts (compression fitted) with no interior polyethylene.
Dana - "In zone 5 it's sort of in-between- the square footage of above grade foundation and fuel type may tip the balance."
I am in Zone 5.
Just took some quick measurements - from the slab there's 4.75 feet of concrete of basement wall and then 3 feet of (double-wythe) brick until I hit the subfloor. The brick begins right at grade, so you could say that 38% of the wall is above grade. This is just your basic rectangular house. Fuel type is natural gas.
Would you consider these numbers significant?
Jeff,
In case you haven't seen it, here is an article you might want to read: How to Insulate a Basement Wall.