Overthinking basement wall insulation?
Zone 4.5. After spending countless hours reading tons of article here, there & everywhere, I came up with a plan. Walkout basement with 12 ft high concrete walls in front, stepping down to all 2×6 stick frame in back. This 3500 sq ft lower level will be about half finished space, half utility & storage (or maybe 60/40). Includes about 1500 sq ft of concrete bunker under a suspended garage, half of which will be a home theater.
My builder’s standard solution where the wall steps down on the sides is to add a second interior wall of 2×4 with R-13 batts and a gap of a few inches to the concrete wall as a solution to moldy batts and aesthetically having a smooth, flat interior wall. In the attached drawing & photo, this would happen at Bedroom 4 and the Home Theater. One change to the drawing is the back-side under the garage has changed from concrete to steel/wood.
The whole house will be 2×6, likely with R-19 batts or blown-in fiberglass, but no final decision yet. While the builder’s plan would solve some issues, it doesn’t seem like a good insulation plan. My plan was to do every exterior facing concrete wall with 2 inches/R-10 of XPS on the inside to have an envelope of sorts. Also putting the same under the slab. Of course, right after I told the builder this is what I want, I realized can’t put foamboard in the unfinished areas without drywall fire protection.
Maybe I missed it in the pro/con articles about putting foamboard inside or outside the foundation, but I decided on inside. Of course, a few days after the outside was backfilled, I realize I should have done outside foamboard.
Adding tons of drywall in storage space doesn’t sound like a good option. In all the articles I’ve read, I don’t think I saw my situation of mixing finished with unfinished space covered. What should I do? Thanks.
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R19 batts aren't very air retardent, and only perform at R18 when compressed to 5.5" in a 2x6 framing bay. This is almost the worst possible option, and does not meet IRC code minimums for climate zone 4 or 5. R20 mid-density batts would make it, but blown fiberglass or cellulose would be superior due to the near-perfect fit.
R10 XPS is really R9 (the manufacturers' warranty), but will likely perform lower than that after 50 years of service. The higher labeled R is due to the climate damaging HFC blowing agents. As the HFCs diffuse out slowly over decades performance drops toward the fully-depleted value of about R8.4 at 2".
A continuous layer of foil faced polyiso at 2" would meet current code in climate zone 4 over it's full lifecycle, but it would take 2.5" to meet code without other insulation in zone 5. There are fire-rated versions of polyiso that would not need an interior side ignition barrier.
The R13 batts in the 2x4 wall needs both interior and exterior side air barriers to meet it's rated performance. With air barriers both sides it would meet code in zone 4, but not zone 5. In zone 5 using 1" of foil faced polyiso as the exterior side air barrier would make it. The vertical hidden cavity would need fireblocking at the top as a minimum and in some jurisdictions would also need fireblocking mid-way on a 12' tall wall.
Thanks for the polyiso fire-resistance tip. Looks like 2" R-13 runs about $55-65 or so per 4x8 sheet. That's more than double what I was going to pay for 2" R-10 brand-name XPS. With the foil layer, not sure about drying, but guess it doesn't matter. If I remember correctly, somebody said concrete is always wet.
So, do you think I should cover all my basement walls (60 sheets), finished and unfinished areas, with this stuff? Any other options I haven't thought of. Thanks.
Bruce,
I don't think you are overthinking things. But if you want to insulate basement walls on the interior, you should follow the advice in this article: "How to Insulate a Basement Wall."
As Dana said, fiberglass alone won't work -- especially if there is an air space between the fiberglass and the concrete wall. (Fiberglass won't work unless it is encapsulated on all six sides with an air barrier. If there is an air space between the fiberglass batts and the concrete wall, you'll have possible convection currents and hidden air flow paths -- and very poor thermal performance.)
Interior rigid foam is the way to go. If you don't want to protect the rigid foam with 1/2-inch drywall, use Thermax or Rmax Thermasheath XP.
I'd read the article before, but upon re-reading now, I obviously forgot some stuff. For fire-resistant, you can also add JM CI Max.
I'll probably have to order the fire-polyiso and the slab may get poured before the arrival. If I put the 2" XPS under the slab, should I then put 4" of XPS vertically on the edge up the wall to flush with the floor top? That would be some non-fire rated 2" exposed around the slab. Not to mention it might look a bit rough around the edges. Or maybe do 3" and then cover it with 1" concrete around the edge? Obviously better to have the polyiso walls installed before pouring the slab, but . . . .
One other possibility. The fire-polyiso may be close to $2/sq ft + installation. Looking at about 1900 sq ft or so. How would R-10 open or closed cell foam compare cost-wise? Would it look too ugly even for storage/utility rooms uncovered?
Bruce,
Spray foam needs to be protected by a thermal barrier (i.e., 1/2-inch gypsum drywall), just like most types of rigid insulation (with the exceptions of Thermax, Rmax Thermasheath XP, and JM CI Max).
I'm a bit confused. Here I am on the cold edge of Zone 4, which the 2012 energy code calls for R-10 on the concrete foundation walls and under the slab. My city has amended out of that code, but I want to comply. If you look at my architectural drawing and photos above, you see something like 70-80% of the concrete wall is in unfinished space. Only the home theater and Bedroom 4 have an outside concrete wall.
I'm willing to spend $25 for a sheet of 2" R-10 brand-name XPS for the finished walls and under slab. I'm hesitant to spend $50-60 per sheet for 60 sheets of fire-resistant Polyiso to cover all the concrete exterior facing walls unless I'm sure it's warranted. We'll be there for the long-term.
As I look at model homes in my area, many have no insulation in unfinished parts of basements. Of those that do have insulation, none have any Polyiso or any rigid foamboard. Please see photos of three models in my area. In the first, this is similar to some of my basement, partially finished under a suspended garage. There is no insulation anywhere. In the 2nd, 3rd & 4th (3 & 4 are the same house), I don't know what the white material is covering most of the concrete walls. I'm guessing it covers fiberglass or some other fluff. It was one of the very few with an ERV. In the 5th, the material is similar to the previous house.
I have to tell the builder tomorrow what I want to do with the slab. Do I want 110 sheets of 2" R-10 XPS under the whole slab, even though almost half will be unfinished? I assume it's all or nothing?
Guess I have more time on the walls, but I don't know what to do. Suggestions welcome along with identification of the white walls in the photos. This process is very frustrating. My builder will do what I want but says don't spend money on things the next buyer can't see. Thanks.
Bruce,
You're right -- code requirements for basement insulation often go unenforced. That doesn't mean that code violations are good.
If you have any doubt of the local code requirements, call up your local code enforcement official.
Vinyl-faced fiberglass insulation is still common in some areas, even though its use has been associated with moisture problems. The way I see it, installing this type of insulation is like rolling the dice. Sometimes you get lucky and everything is fine. Other times, you get mold and dampness.
My advice to you is to insulate your basement properly. We've referred you to all of the relevant GBA articles, and I think you understand the advice given on GBA. If you can't afford the insulation, and your local code official doesn't care, I suppose you can leave your walls uninsulated. However, there is an energy penalty to that path.
"My builder will do what I want but says don't spend money on things the next buyer can't see."
I hate hearing this line, which is far too common. It would be relevant if you were building a spec house, but he is building YOUR house. Spend YOUR money on things YOU want.
Thanks Martin. One more if I may. BSC: BSD-103: Understanding Basements says, "The foam insulation layer should generally be vapor semi impermeable (greater than 0.1 perm), vapor semi permeable (greater than 1.0 perm) or vapor permeable (greater than 10 perm) (Lstiburek, 2004)." The Thermasheath-XP is <.03 perm; the JM CI Max is .02 perm; Thermax <.03. It seems that these products will not allow inward drying. Wouldn't this cause a problem?
Bruce,
As I reported in 2012, Joe Lstiburek had admitted that his advice was wrong.
Lstiburek said, “I made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesn’t matter. It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.”
For the full story, see "Joe Lstiburek Discusses Basement Insulation and Vapor Retarders."
"I'm a bit confused. Here I am on the cold edge of Zone 4, which the 2012 energy code calls for R-10 on the concrete foundation walls and under the slab"
Yes you ARE confused. Code only calls for R10 on the walls, and veritical slab-edge insulation down to 2' below grade for slabs that are at grade (not basement slabs that are already 2' or more below grade.) There is no code requirement for foam UNDER the slab.
Yes, fire rated foam is more expensive than unrated foam (though you should be comparing 1.5" fire rated foam to R10 XPS, not 2" fire rated foam.) But you're saving on not having to add a timed thermal barrier between the foam and the room. So use it only in the areas that won't be finished.
Since a 2x4 wall with 1/2" gypsum board meets the thermal barrier requirement, you can use cheaper polyiso in those areas. At 1.5" most polyiso still hits R9.3-R9.5, and in a sub-grade application it will outperform that number. A foil facer next to an empty 3.5" stud bay adds a legitimate R0.5 to the performance due to it's heat reflecting low-E characteristics.
Foam under the slab is still a good idea, since it keeps the slab warmer in SUMMER, spending less time below the average outdoor dew point temperature. That makes anything stored on that slab (say, a cardboard box) less prone to mold, and the whole basement less prone to "musty basement smell." At the zone 4/5 boundary even 1" of EPS is enough to avoid the musty basement problem, though it might still be financially rational to go with 1.5" of EPS.
I'm even more confused than I realized. Thanks Dana, I'm having to make decisions today for Thursday slab installation. I'd like to do EPS but it's two-week special order, no returns on extra. I can get brand name XPS as follows:
1" - 15 PSI - $14.71
1.5" - 15 PSI - $22.78
2.0" - 25 PSI - $24.69
Is 15 PSI OK for 4" slab? I'm looking at about 110 sheets to do the whole slab. Should I still do the edges?
One more thing. If you go back to my foundation photo in the original post, you can see the back 8 ft or so is a 6 ft deep buried wall so the back 8 ft of front-back depth of the house will be on grade. I thought the 2" under slab would satisfy the 2 ft vertical requirement. Does it, or am I confused again? Not to mention, that part has already been back-filled.
15 psi is fine for residential slabs.
At the shallow end of the basement using 2" foam under the slab for the 2 feet nearest the edge in conjunction with 2" on the edge itself would be roughly the thermal equivalent of going 2 feet below grade. But it' doesn't have to be 2" thick for the full field of the slab area.
At 1.5" over the full field of slab are it's probably still financially rational on the XPS on a lifecycle basis, but at 1" it's a no-brainer for anybody staying there long term (or anyone looking to avoid the stinky-basement syndrome.)
If you're installing 1.5" polyiso on the foundation walls above grade, installing 1.5" of XPS at the slab edge of the deeper part of the basement still makes sense. If installing 2" polysio on the walls, install 2" slab edge XPS. The slab edge foam does NOT need to extend 2 feet below grade except where the slab itself is less than 2 feet below grade (at the end of the walk-out basement). The slab foam, can be butted up against the slab edge foam, and the edge foam should extend a few inches above the intended top level of the slab for the pour to be trimmed flush later if necessary.
For the parts of the wall that will have exposed, fire-rated polyiso the XPS would later be trimmed flush with the top of slab. Where there going to be a 2x4 wall oar furring + half-inch wallboard in front of the foam it's fine to just leave it, and go up with (cheaper, not fire rated) polyiso from there.
In termite zones it's worth trimming all the XPS flush with the slab surface and taping it to both the slab & wall with foil tape. Termites can and will tunnel through foam, but not metal. Foil tape creates an unavoidable thermal bridge at that point, but it's not as severe a bridge as metal flashing. In heavy infestation areas it's worth using copper-clad sill gasket instead of aluminum tape. Copper is toxic to the gut flora that wood boring insects need to digest wood.
The building inspector said the city itself used some kind of fire-resistant spray paint and suggested that for the exposed slab edge. The high-volume flatwork contractor said he's never put foamboard under the whole slab. He suggested the 2" for the 2' ft closest to the back wall as also suggested by Dana. He also feels confident that using 15-mil Stego Wrap Vapor Barrier will prevent the moisture/condensation issue.
Bruce,
I can understand the argument in favor of saving money by omitting the continuous horizontal layer of rigid foam under the slab (even though I generally favor continuous horizontal rigid foam under a slab). In Climate Zones 4 and 5, the continuous rigid foam is less important than in Climate Zone 6.
That said, your contractor is wrong to tell you that a 15-mil vapor barrier under the slab will prevent condensation. It won't. Condensation occurs when moisture in the air above the slab collects on the slab. It is a function of slab temperature, not the presence or absence of a vapor barrier under the slab. If you are concerted about condensation -- and I'm not saying you should be -- you need insulation under the slab, not Stego Wrap.