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 (17)

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! 

Nick Trotta Feb. 1, 2022, 9:55 a.m.

Mike & team, we just had our basement finally "waterproofed" (we literally had to wait until now because the contractor was so booked): french drains around the exterior of the entire basement connected to two sump pumps leading to an exterior drain that currently runs undergrond to a town drain. The pumps run on electricity but the backup is water pressure. The walls were brushed, treated with a mold encapsulating coating and covered with a sealed polyurethane barrier. Next is to refinish. A few things that came up after the waterproofing company finished are: the poly barrier was sealed but not what I would consider 100% i.e. the top of the barrier around the window wells were not adhered to the block, while the rest of the barrier was. The covers on the sump pumps were not 100% sealed. Is there anything of concern based on what I've shared? I am having the contractor seal the barrier to the block under the window wells, but I supposed my question is, how sealed is the right amount of sealed for a poly barrier in this example?

Elissa Suhr
Elissa Suhr Feb. 5, 2022, 12:34 p.m.

So many people with these same questions - I'm glad I found your thread! We are about to do the same thing and have a polyurethane barrier put on our walls (already have french drain, sump pump, brushed/treated walls). My concern now is that there seems to be no straightforward answer for our contractor about how to do framing/insulation to finish our basement as now we can't put XRS rigid foam boards directly onto the concrete since we have the poly barrier tucked into the french drain flange. What is your contractor going to do?

Mike Reynolds Feb. 5, 2022, 11:43 a.m.

Having a poly  membrane against a foudation wall is more of an moisture barrier than an air barrier, unless there are so many cracks in the blocks that is it leaking air. As a moisture barrier it does not need to be 'extremely' well-sealed as there is not really any air pressure forcing moist air through. I don't think its a problem at all. As for the sump pump lid, I don't see a reason for concern unless you have high radon gas and a radon mitigation strategy in place, then it would be better sealed. But other than that I think it's fine. 

Elissa Suhr
Elissa Suhr Feb. 5, 2022, 12:35 p.m.

How do you recommend a contractor finishes a basement when it has a french drain and a poly membrane on the wall that is tucked into the french drain flange - as then you can't put XRS rigid foam boards directly onto the concrete anymore, but still need them. Frame a few inches out and slide the XRS boards snug into the framing, then drywall?

Nick Trotta Feb. 8, 2022, 9:38 a.m.

I'll second this question as I now have the poly barrier up against the slab over the stars. Before the barrier, we had furring strips fastened to concrete block and the sheet rock over them. Now that the poly barrier is there, I can't glue anything to the wall so should I use furring strips again? They were fastened with Hilti fasteners. 

Mike Reynolds Feb. 10, 2022, 2:07 p.m.

 Nick - are you saying you currently have wood attached to concrete and you're installing foam over it? I'm a bit unsure by your wording. But I"m going to go up and answer Elissa's question and maybe it will answer yours and if not please clarify a bit. 

Mike Reynolds Feb. 10, 2022, 2:12 p.m.

Hi Elissa - If you have a concrete wall with a poly barrier that lets moisture drain into a french drain, that's a great start. From where you're at it sounds like you have maybe two options - if your plan is to install rigid foam then drywall over top, you will need a solid surface to attach the drywall to, so fasten the foam panels using furring stips with concrete screws. Or press the foam against the poly barrier which can be held in place by a framed 2x4 or 2x3 wall, which can be fastened to the floor joists above and fastened to the floor with either an adhesive or concrete screws.

Nick Trotta Feb. 28, 2022, 10:26 a.m.

Thanks Mike, I got my answer from your reply to Elissa. However, I would like to know if you recommend the rigid foam board, now that we have the poly barrier in place or if you would recommend something like rock wool in between the studs? If the rockwool, does that simply stay in place with friction?

Mike Reynolds March 1, 2022, 10:16 a.m.

If you have a poly membrane against the concrete then you could safely put any type of rigid insuation board against the foundtaion wall depending on your preferences and budget. We like mineral wool, but the carbon footprint of EPS is not terrible in the grand scheme of things. And yes, mineral wool batts fit into stud cavities very well, even better than fiberglass as they are more rigid yet can still be compressed somewhat to fit. 

Nick Trotta April 27, 2022, 2:21 p.m.

Hi Mike, I finally got a contractor to look at the work necessary to put our basement back together. He suggested fiberglass R13 batts vs. foam board. Since we now have the poly barrier, is that an acceptable option?