What is the best way to install basement insulation with existing framing?


What is the best way to install basement insulation with existing framing?

Nick Trotta Sept. 20, 2021Last updated: Oct. 1, 2021

We have to redo our finished basement after groundwater made it in following the biggest storm in our area's history. We were "dry" to the naked eye for ~10 years. The walls were faced fiberglass insulation stapled to 2x4 studs that are 1.5" away from the concrete block that was coated with Drylok. When we remediated the water, we noticed enough evidence of block discoloration & possible fungus to reconsider how the basement will be insulated in the future. My question is: can we use spray closed cell foam insulation in the current wall framing followed by drywall to achieve proper vapor barrier against the block and insulation? We'd rather not remove the 2x4's (and metal tracking) to get 2" of rigid foam board against the block and have to reframe the walls. Thank you in advance for your help.  

Responses (7)

Emmanuel Cosgrove
Emmanuel Cosgrove Oct. 6, 2021, 12:38 p.m.

If your fiberglass batts were touching that would certainly reduce air flow and evaporation of any moisture on walls, so I tend to agree with your conclusions.

French drains around the interior perimeter are a great idea. If you can pull a 6-mil poly membrane up behind the studs and attach it up high (as high as you can get it, but wherever you get to is better than not doing it) then any water run down into your French drain. Make sure the membrane goes under the new bottom plate you install.

Another wise idea is putting the new bottom plate up on foam blocks so in the event there is flooding or a burst pipes in the future, the sill plate will be off the ground. You can see what I mean in the main image on this page about how to prevent mold when renovating basements, which will be a good guide for you to follow anyway.  /guides/2270/basement-renovations-how-to-prevent-mold-when-renovating-finishing-basements/

Nick Trotta Oct. 6, 2021, 9:56 p.m.

Thank you very much. It looks like we have some work to do but it's a solid plan. One more question: would it be a good idea to still use the mineral wool batts between the studs after I get the poly membrane up? Oh, and also, it is cool that the laundry room block remains uncovered, correct i.e. no poly, since it's not a fully finished par of the basement?

Mike Reynolds Oct. 7, 2021, 8:48 a.m.

The rockwool batts are a bit more money and its something of an insurance policy in case there are moisture issues again in the future, but the poly is the real safeguard here, so if you don't have room for mineral wool batts in the budget don't sweat it, you'll be way better protected than you were before anyway. As for the laundry room, don't think of this as an 'all or nothing' approach, if you miss one section you are still way better off. If it were me I'd probably leave the laundry room if it seems okay and wait till it shows signs of a problem - smell, mold on drywall etc. If you suspect it's bad you could cut a hole in the drywall and poly to inspect it and go from there if needed.

And an additional offer Nick, this could be good content to show other readers, so if you want to take some pics and email them to [email protected] I can help trouble shoot along the way and maybe do a story on this. Hope it goes well!