Basement insulation for walls and floors

[Geran]

Hi all,

I am currently in the process of finishing my basement. I've put up the 2x4 walls (they have 1" xps as a capillary break)...now it is time for the flooring and insulation. This is where I need some help.

For insulation purposes, I live in Maryland (climate zone 4).

Walls
My idea is to use 1" of closed-cell foam and batt insulation for the rest to give me the R-13 for basement walls. I wanted to go with 2" of closed-cell foam but my budget was minimized by the wife so I figured the Flash and Batt method was the next best thing.

Flooring
Since regrading around the house, redirecting the gutters and installing an interior drain...I haven't had any issue with water in the basement. The headroom low point in my basement is under my HVAC which is 7'6" and the highest being 8'5". Based on this information, my plan is to use 1" XPS and 1/2" plywood on top screwed to the floor with Tapcons. The bathroom and laundry room would have 1/2" cement board on the floors instead of plywood.

Any help is greatly appreciated.

 

[Dana]

Is the 1" XPS capillary break between the bottom plate & slab, or is it between the stud edges and foundation wall?

Any closed cell foam between studs is a waste of (environmentally and cash) expensive foam, since the R6+/inch foam's potential performance is ruined by the R1.2/inch framing fraction. The difference in "whole wall" performance between filling a 2x4 wall with R13 foam and R20 closed cell foam is about R1, after factoring in the heat conducted through the framing.

If there's nothing but air between the foundation wall and stud edges, filling applying enough HFO blown closed cell foam to where it just touches the stud edges is the greenest, highest performance solution. HFO blown foams run about R7/inch, but the HFO1234ze blowing agent is three orders of magnitude less damaging than the industry standard HFC245fa. Demilec HeatLok High Lift, Lapolla Foam-Lok 4G 2000 are two examples of HFO blown foam, but there are others.

XPS is an even less-green product, due to it's primarily HFC134a blowing agent (about 30% higher global warming potential than HFC245fa). Worse, with XPS the blowing agent diffuses out over a handful of decades causing the foam to lose thermal performance, eventually settling in a the same R-value as EPS of similar density (R4.2/inch instead of the labeled R5/inch). EPS is blown with comparatively benign HFCs (usually pentane), most of which leaves the foam at the factory, where it is recaptured and often burned for process heat. It's R-value is stable over the decades. So, wherever possible (including here), EPS is the cheaper-greener material to use.

Most of the cheap EPS at box stores is 1lb per cubic foot Type-I EPS with facers, which can work here, but it's a bit crummy. If you can find a source of 1.25lb (Type VIII) or 1.5 lbs (Type-II) it'll be a bit less fussy about where the seams are placed. The compressive loading capacity of Type I EPS is sufficeint for this application, but if the Tapcons are applied close to the edge of the sub-flooring too close to a seam between sheets of Type-I EPS it can compress it a bit, and will have to be backed off a bit.

Be sure to vacuum the grit out of the TapCon holes before inserting the screw to avoid bottoming them out (or worse, snapping them off.)

 

[Sponsor]

 

[Geran]

Is the 1" XPS capillary break between the bottom plate & slab, or is it between the stud edges and foundation wall?

The 1" XPS break is between the bottom plate & slab.

Any closed cell foam between studs is a waste of (environmentally and cash) expensive foam, since the R6+/inch foam's potential performance is ruined by the R1.2/inch framing fraction. The difference in "whole wall" performance between filling a 2x4 wall with R13 foam and R20 closed cell foam is about R1, after factoring in the heat conducted through the framing.

If there's nothing but air between the foundation wall and stud edges, filling applying enough HFO blown closed cell foam to where it just touches the stud edges is the greenest, highest performance solution. HFO blown foams run about R7/inch, but the HFO1234ze blowing agent is three orders of magnitude less damaging than the industry standard HFC245fa. Demilec HeatLok High Lift, Lapolla Foam-Lok 4G 2000 are two examples of HFO blown foam, but there are others.

Based on your statements here, I understand the flash and batt is the best use case here. Enough cSPF to touch the back of the studs and fill the rest with batt insulation. I'll look into the two you mentioned as I would like to do as much as I can on my part.

XPS is an even less-green product, due to it's primarily HFC134a blowing agent (about 30% higher global warming potential than HFC245fa). Worse, with XPS the blowing agent diffuses out over a handful of decades causing the foam to lose thermal performance, eventually settling in a the same R-value as EPS of similar density (R4.2/inch instead of the labeled R5/inch). EPS is blown with comparatively benign HFCs (usually pentane), most of which leaves the foam at the factory, where it is recaptured and often burned for process heat. It's R-value is stable over the decades. So, wherever possible (including here), EPS is the cheaper-greener material to use.

I'll see if I can find some EPS...wish I had a Menards near me.

Most of the cheap EPS at box stores is 1lb per cubic foot Type-I EPS with facers, which can work here, but it's a bit crummy. If you can find a source of 1.25lb (Type VIII) or 1.5 lbs (Type-II) it'll be a bit less fussy about where the seams are placed. The compressive loading capacity of Type I EPS is sufficeint for this application, but if the Tapcons are applied close to the edge of the sub-flooring too close to a seam between sheets of Type-I EPS it can compress it a bit, and will have to be backed off a bit.

Thank you for the suggestion, I'll do my best to find some Type VIII or Type-II EPS and I have to tell my stairs guys that the floor will be 2" higher now instead of 1.5".

Be sure to vacuum the grit out of the TapCon holes before inserting the screw to avoid bottoming them out (or worse, snapping them off.)

Will do

 

[Dana]

The blue & orange box stores carry Type-I EPS with facers.

Foam reclaimers will often have (usually Type-VIII) roofing EPS. Try searching your local craigslist materials section for rigid+insulation, eg:

https://worcester.craigslist.org/search/sss?query=rigid+insulation

Many foam reclaimers advertise through those channels, usually with several square miles of polyiso in stock. You want EPS or XPS- contacting them and asking for you want doesn't hurt, even though they may be advertizing something else doesn't mean they don't have it.

Reclaimed XPS has a lower footprint than virgin-stock EPS, since no new polymer or blowing agent was created. Re-using foam is simply loading on to the benefit side of the cost^benefit balance point on an environmental hit already take.

If the studwalls weren't already up it would be pretty cheap to buy 2" or 3" polyiso and mount it to the wall with furring through-screwed to the foundation with masonry screws. The performance of 2" roofing iso would be about R11.5, which exceeds current IRC code minimum of R10 continouos insulation for US climate zone 4, which is all of MD except Garrett county, which is zone 5, and needs R15. A flash-inch of closed cell plus a 2x4/R13 wall comes in at about R17 "whole assembly" after factoring in the thermal bridging of the studs, but so does 3" of fiber faced roofing polyiso.

 

[Geran]

So what would you recommend for building up my subfloor? I have no water infiltration and the highest I can go is 1.5". We are planning on carpet in the basement once it is done.

 

[Dana]

An inch of foam and half-inch t & g CDX glued to the foam with foam-board adheseve and TapConned to the floor should do it. Stagger the seams of the CDX & foam by at least a foot, and take care to not over-drive the TapCons near the seams in the CDX creating unevenness. (The foam can easily handle the live loads of the floor and room, but the pressures of the TapCons are potentially orders of magnitude higher.)

 

[Geran]

Dana,

Thank you for the reply. More questions...

1. Would it have been better to put the subfloor under the studded walls instead of a leaving a gap between the wall and subfloor? If so, I can redo my walls since they are nailed in place yet and just friction fitted in for now.
2. Finding 1" foam EPS/XPS is easy but finding half-inch CDX T&G plywood is pretty difficult. Would going with three-quarter inch EPS/XPS and 23/32" T&G OSB like this work as well?
3. Is 6 mil poly needed for my climate zone (zone 4) between the foam board and concrete? My basement is a walk-up basement with the ground sloping away from the home.

As for the seams/joints, I plan on laying the plywood/OSB perpendicular to the foam board so the seams shouldn't be an issue hopefully.

 

[Geran]

Any issues with the last question?

 

[Dana]

Dana,

Thank you for the reply. More questions...

1. Would it have been better to put the subfloor under the studded walls instead of a leaving a gap between the wall and subfloor? If so, I can redo my walls since they are nailed in place yet and just friction fitted in for now.
2. Finding 1" foam EPS/XPS is easy but finding half-inch CDX T&G plywood is pretty difficult. Would going with three-quarter inch EPS/XPS and 23/32" T&G OSB like this work as well?
3. Is 6 mil poly needed for my climate zone (zone 4) between the foam board and concrete? My basement is a walk-up basement with the ground sloping away from the home.

As for the seams/joints, I plan on laying the plywood/OSB perpendicular to the foam board so the seams shouldn't be an issue hopefully.

1: Putting the bottom plate of the framing on top of the subfloor gives you more nailing area for fastening the kick boards.

2: Dropping the foam thickness to R3 and using heavier subfloor is also fine, but going any thinner than that increases the mold risk to the subfloor.

3: If you don't know for sure whether there is a vapor barrier under the slab, it's saver to install 6 mil (or thicker) poly, but put it between the foam & subfloor, not under the foam, and lap it up the side of your wall foam by at least 6", taping it in place. (It looks like you'll be pulling the studwall anyway.) The foam is highly tolerant of moisture, the subfloor isn't. If the polyethylene is under the foam any variations in the concrete can create small reservoir puddles/pockets where moisture can could be trapped above the vapor barrier, taking forever to dry through the foam & subfloor, keeping the subfloor damp near the seams in the foam. Putting the sheet polyethylene on the top side of the foam cuts that risk by quite a bit- the only moisture that can reach the top side of the sheet has to come through the subfloor (say, an overflowing washing machine, or way too many spilled beers in the man-cave ), and not from groundwater sources or bulk water leakage in the foundation walls.

The walls don't need or want sheet polyethylene. If the foundation wall is prone to leaking copious water it's sometimes prudent to install a dimple mat for drainage purposes between the wall foam & foundation, as well as a perimeter drain for the dimple mat to drain into. But it doesn't sound like that's your situation given the surface drainage slope, etc.