I have a walkout basement that I'm renovating. New home, no water issues, walls already studded. Two walls are below grade, one is mostly above grade and the last wall is completely above grade and is 2x6 framing. I priced cc spray foam and it's not in the budget so XPS it is. My problem is that some framing is far enough off the foundation to slide in 2" XPS behind the studs... others are not. One wall will only fit 1" XPS behind it. Can I slide in 1" and then fill the cavities with another 1" and double it up? I'm afraid of flash and batt and would like to stick with foam. Additionally, is it ok to cut and fit 2" XPS in the 2x6 above grade wall cavities? Lastly, I'm going to frame out a mechanical room that will not be studded... what do you recommend I use to insulate those walls since they will be exposed? Polyiso? Thanks in advance for your help.

I'm unclear if you want to put the XPS outside or inside? Or whether these walls are on top of the foundation or inside the foundation? Cutting and fitting foam sheets inside a stud wall is difficult due to air sealing issues. If you can put XPS or polyiso between the inside of the foundation and the stud wall, that works well to keep moisture from reaching the wall insulation, as long as it is well sealed on the top, bottom and sides. Do not place stud walls directly against the inside of the foundation.

Posted By Bob I on 19 Nov 2013 05:43 PM
I'm unclear if you want to put the XPS outside or inside? Or whether these walls are on top of the foundation or inside the foundation? Cutting and fitting foam sheets inside a stud wall is difficult due to air sealing issues. If you can put XPS or polyiso between the inside of the foundation and the stud wall, that works well to keep moisture from reaching the wall insulation, as long as it is well sealed on the top, bottom and sides. Do not place stud walls directly against the inside of the foundation.

Furthermore, putting expensive high-R foam between the studs is also a (nearly) complete waste of foam, since the thermal bridging of the framing severely undercuts the overall performance.  It really belongs between the sheathing & siding, where you get the full performance out of it, thermally breaking the bridging of the framing. (Air seal the sheathing to the framing with acoustic sealant caulk or a bead of can foam, but put the cheap stuff in the cavities.  Blown/sprayed cellulose is usually the best bang/buck.)

Posted By Dana1 on 19 Nov 2013 06:08 PM

Posted By Bob I on 19 Nov 2013 05:43 PM
I'm unclear if you want to put the XPS outside or inside? Or whether these walls are on top of the foundation or inside the foundation? Cutting and fitting foam sheets inside a stud wall is difficult due to air sealing issues. If you can put XPS or polyiso between the inside of the foundation and the stud wall, that works well to keep moisture from reaching the wall insulation, as long as it is well sealed on the top, bottom and sides. Do not place stud walls directly against the inside of the foundation.

Furthermore, putting expensive high-R foam between the studs is also a (nearly) complete waste of foam, since the thermal bridging of the framing severely undercuts the overall performance.  It really belongs between the sheathing & siding, where you get the full performance out of it, thermally breaking the bridging of the framing. (Air seal the sheathing to the framing with acoustic sealant caulk or a bead of can foam, but put the cheap stuff in the cavities.  Blown/sprayed cellulose is usually the best bang/buck.)

Sorry for the confusion. This is a basement reno. It's a poured foundation and the walls are inside not on top of the foundation. I can fit 2" XPS behind one of the studded walls, no problems. I can only fit 1" behind the second studded wall which is why I'm wondering if I can supplement that with an additional 1" of XPS in the stud bays. The third wall will not be covered with drywall which is why I'm wondering if polyiso is the right choice there. The last wall is completely above grade (walkout basement) and I'm not sure what to do with it. XPS in the stud bays? Roxul and a vapor barrier? Sorry, I'm a newbie and I want to get it right. Thanks.

Posted By dgrove12 on 19 Nov 2013 06:13 PM

Posted By Dana1 on 19 Nov 2013 06:08 PM

Posted By Bob I on 19 Nov 2013 05:43 PM
I'm unclear if you want to put the XPS outside or inside? Or whether these walls are on top of the foundation or inside the foundation? Cutting and fitting foam sheets inside a stud wall is difficult due to air sealing issues. If you can put XPS or polyiso between the inside of the foundation and the stud wall, that works well to keep moisture from reaching the wall insulation, as long as it is well sealed on the top, bottom and sides. Do not place stud walls directly against the inside of the foundation.

Furthermore, putting expensive high-R foam between the studs is also a (nearly) complete waste of foam, since the thermal bridging of the framing severely undercuts the overall performance.  It really belongs between the sheathing & siding, where you get the full performance out of it, thermally breaking the bridging of the framing. (Air seal the sheathing to the framing with acoustic sealant caulk or a bead of can foam, but put the cheap stuff in the cavities.  Blown/sprayed cellulose is usually the best bang/buck.)

Sorry for the confusion. This is a basement reno. It's a poured foundation and the walls are inside not on top of the foundation. I can fit 2" XPS behind one of the studded walls, no problems. I can only fit 1" behind the second studded wall which is why I'm wondering if I can supplement that with an additional 1" of XPS in the stud bays. The third wall will not be covered with drywall which is why I'm wondering if polyiso is the right choice there. The last wall is completely above grade (walkout basement) and I'm not sure what to do with it. XPS in the stud bays? Roxul and a vapor barrier? Sorry, I'm a newbie and I want to get it right. Thanks.

As with above-grade walls, there's no point to putting high-R/inch foam between studs. Adding the 1" cut'n'cobbled foam between the studs buys you next to nothing in thermal performance. 

For the above-grade portion of the foundation it's important to have a sufficient foam-R/fiber-R ratio to limit wintertime condensation on the foam/fiber interface, which is climate dependent. If there were moisture intolerant sheathing between the foam & fiber, at R5 (1" XPS) you would need to live in a US zone4 or lower climate to be able to use a fiber-insulated 2x6 studwall. (http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm ).  But if you are using unfaced fiberglass or rock wool it's not going to be a problem even if you're a bit shy of where you need to be.

You get ~15-20% more R/inch out of polyiso than XPS, and the environmental hit from  it's blowing agent (pentane) is dramatically less than those used for XPS (primarily HFC134a )  Polyiso usually has a more favorable $/R ratio too (about 10 cents per square foot per R as oppose to 12-14 cents for XPS.) In a wall assembly the hygroscopic aspect of polyiso is not an issue (the way it would be under slabs), as long as you have a reasonable capillary break under the cut edge at the bottom to protect it from wicking moisture through the slab.  A strip of 1" EPS (also blown with pentane) under both the bottom plate of the stud and the polyiso is enough to protect both the bottom plate and iso from taking on moisure from a slab that has no sub-slab vapor barrier/capillary break.

Do NOT use an interior side poly vapor barrier, even if your R-ratios are less than ideal for dew point control, since that has a tendency to trap moisture in the stud bay.  XPS is about 0.6 perms of vapor diffusion and the sub-grade concrete will be close to 100% saturation after you insulate, and ground moisture would find it's way into the stud wall, guaranteed.  With foil-faced polyiso the foil is a vapor barrier, but to keep moisture out would require the unattainable- a perfectly sealed assembly.  If the  foam-R is only half the IRC recommended values or less it may be worth springing for a "smart" vapor retarder like Intello Plus or Certainteed MemBrain on the interior side, which would slow the accumulation of wintertime moisture, but would allow the assembly to dry quickly if very much moisture got in.  (When the air in the cavity next to the smart vapor retarder reached a mold-inducing 70% relative humidity,  it becomes on the order of 100x more vapor open than 6 mil polyethylene, but when the air is at 30% RH or lower as it might be in the winter it's only about 10x as open as 6-mil poly,  and in the class-II vapor retardency range.)

On the portion of wall that won't be drywalled, you can only use fire-rated polyiso (as Marc Rosenbaum did on his place in MA.  See parts 2 & 3 for lots of details on how to do it.)

An extremely thorough response. Of course I can't understand half of it... lol. You definitely know the science involved. I really appreciate your responses. Based on comments here and elsewhere I think I'll squeeze in 2" of XPS where I can, 1" XPS where 2" won't fit and the above grade wall I'll just insulate with Roxul and a vapor barrier... it should dry to the outside. Not sure yet what I'll do with the exposed wall. Guess I'll try to find Thermax. Thanks again.

by code the foam has to be fire retardant. thermax (dow), the only company to get their foam approved, made certain the approval is specific to their brand. you can also use another brand if you coat it with intumescent (fire retardant) paint. Be sure to tape all the seams on all the foam.

Posted By dgrove12 on 20 Nov 2013 05:38 PM
An extremely thorough response. Of course I can't understand half of it... lol. You definitely know the science involved. I really appreciate your responses. Based on comments here and elsewhere I think I'll squeeze in 2" of XPS where I can, 1" XPS where 2" won't fit and the above grade wall I'll just insulate with Roxul and a vapor barrier... it should dry to the outside. Not sure yet what I'll do with the exposed wall. Guess I'll try to find Thermax. Thanks again.

PLEASE use polyiso (R-Max, Thermax, or Tuff-R are common brands of polyisocyanurate- there are several others) rather than XPS (pink, blue, green, gray, depending on manufacurer).  It's usually comparable, often cheaper per R than XPS,  higher R/inch, far less environmentally damaging, and it's easier to reliably air-seal.  In most of the US foil faced polyiso is already stocked at the big box store chains in 1" and 2" thicknesses or worst-case, it can be ordered online and picked up at the box store, eg:

http://www.homedepot.com/p/Super-TU...o6AfOKQM4c

http://www.lowes.com/pd_15099-46086...Id=3099161

Roxul in the non-foundation walls can work- whether a vapor retarder is needed and what type would be most appropriate depends on you wall stackup and your climate. Where are you, and what type of siding/sheathing are we dealing with?

Posted By Dana1 on 21 Nov 2013 04:57 PM

Posted By dgrove12 on 20 Nov 2013 05:38 PM
An extremely thorough response. Of course I can't understand half of it... lol. You definitely know the science involved. I really appreciate your responses. Based on comments here and elsewhere I think I'll squeeze in 2" of XPS where I can, 1" XPS where 2" won't fit and the above grade wall I'll just insulate with Roxul and a vapor barrier... it should dry to the outside. Not sure yet what I'll do with the exposed wall. Guess I'll try to find Thermax. Thanks again.

PLEASE use polyiso (R-Max, Thermax, or Tuff-R are common brands of polyisocyanurate- there are several others) rather than XPS (pink, blue, green, gray, depending on manufacurer).  It's usually comparable, often cheaper per R than XPS,  higher R/inch, far less environmentally damaging, and it's easier to reliably air-seal.  In most of the US foil faced polyiso is already stocked at the big box store chains in 1" and 2" thicknesses or worst-case, it can be ordered online and picked up at the box store, eg:

http://www.homedepot.com/p/Super-TU...o6AfOKQM4c

http://www.lowes.com/pd_15099-46086...Id=3099161

Roxul in the non-foundation walls can work- whether a vapor retarder is needed and what type would be most appropriate depends on you wall stackup and your climate. Where are you, and what type of siding/sheathing are we dealing with?

Dana, I would certainly use polyiso, even if it costs a little more, if it's a more responsible product to use. It just seems as a newbie there's TONS of DIY install help available for the XPS. Is the polyiso installed in the same fashion? That is, use PL300, glue it to the foundation, tape the seams, etc. It's foil faced... I thought that was a no-no. Which way does the foil go, in or out? Can you put Roxul or fg in the stud bays after using polyiso against the foundation? I already bought a small amount of XPS to experiment. I can use that on the rim joists I guess. As far as the make-up of the exposed wall, I'm not 100% sure. I believe it's 2x6 framing, osb sheathing, house wrap of some kind and normal vinyl siding. I'm in South Central Pennsylvania. I had an insulation contractor come and give an estimate for ccsf. He advised me just to use fg batts throughout the whole basement. Needless to say, he was kindly dismissed. As always, amazingly useful info. This really is an invaluable resource. You guys rock.

Polyiso has foil facers on both sides. You can glue it to the wall with foam-board construction adhesive, but it also needs to mechanically secured to meet code. Trapping it to the foundation with a studwall works. In the places where you're using fire-rated Thermax you can use the Hilti fasteners the way Marc Rosenbaum did it (if you can find them), or put up some 1x furring 24" o.c. through-screwed to the foundation with Tapcons 24" o.c.. Just be sure to keep the cut edge of the iso off the slab.

South Central PA is the warm edge of US climate zone 5, and with vinyl siding over house wrap over OSB it qualifies as "vented cladding" from a code perspective, which means it doesn't need an interior side vapor barrier more vapor retardent than standard latex paint. Skip the vapor barrier, just make it air-tight and it can dry well in both directions.

http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm

That said, with rock wool or fiberglass you would still need to pay extra care to air-seal both the sheathing AND the gypsum (including foaming around all the electrical boxes, foam sealing where wiring or plumbing runs through studs or sheathing, etc. ) With blown cellulose the moisture buffering capacity of the cellulose takes some of the edge off of air-transported moisture concerns, but you'd still want to caulk the studs to the sheathing, and lay a bead of caulk between doubled-up top plates of the studwall, and where the bottom plate meets the subfloor. Basically make each stud bay as air tight as possible on all 6 sides as you go, and don't leave any seams that go from the interior to the exterior unsealed.

The whole-wall R of that vinyl-sided wall with R23 Roxul is about R14 after thermal bridging, which is less than a 2x4 studwall with R15 and an inch of polyiso (and WAY less than a 2x6 studwall and 2" of polyiso.) If you ever re-side, it's worth looking into adding some rigid foam under the new siding.

But for the foundation insulation, even the long term economics of going with more than about 1.5" of polyio and a 2x4 studwall with R15 Roxul or 1" of iso and a 2x6/R23 studwall only make sense if you're heating fuel is something expensive, like oil or dollar bills. (Either is about an R20 whole-wall, after thermal bridging.) If you're heating with oil, investing some of the insulation money into buying a mini-split and heating some/all with the mini-split heat pump brings your heating costs within natural-gas heating range, at which point the ~R20 wall is about the financially rational limit for the sub-grade portion of basements in your climate.

For the basement stackup it's fine to have:

concrete | polyiso | roxul or fg | wallboard | latex paint

Just don't use fiberglass with a foil facer, or put a vapor barrier anywhere in that stackup.

Posted By Dana1 on 21 Nov 2013 06:24 PM
Skip the vapor barrier, just make it air-tight and it can dry well in both directions.

concrete | polyiso | roxul or fg | wallboard | latex paint

Just don't use fiberglass with a foil facer, or put a vapor barrier anywhere in that stackup.

This is what I'll do then. Thanks again.