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EPS, PolyIso, or Double Stud Wall

CTSNicholas | Posted in Green Building Techniques on

I’m doing my research and having quite the controversy between these three options.
Zone 5B – SO I have to worry about hot summers and freezing winters.

In a couple of questions I posted here I learned that EPS is more of the favored child around here while XPS is considered more costly (Agreed) and poorer performance in cool temperatures (I don’t know, just going by what I heard)

I do not have the options most folks do when they live in a densely populated area. I live in a rural town so any cost savings by finding a more local or recycled product are diminished when I add on cost of hauling or shipping.

I was going to opt for 2″ of EPS outside my 2×6 Wall with R19 Fiberglass (Or R21 Mineral Wool)

Products offered in my area:

$7.99 R4 1″ x 4′ x 8′ Expanded Polystyrene Insulation
$15.19 Owens Corning FOAMULAR 250 1″ x 4′ x 8′ R-5 Rigid Foam Insulation
$30.99 JM CI Max¯ Silver 1″ x 4′ x 8′ R-6 Polyiso Rigid Foam Insulation

The EPS is extremely fragile, it breaks easily and makes a mess with the white beads. The XPS is must more rigid. The Polyiso is the most rigid and is protected with aluminum facing.

House dimension is 44′ w x 30′ d = 148 Lineal Feet @ 9′ wall to cover + gable end = roughly 1500 Square Feet to cover with foam.

This calculates out to roughly 50 sheets, or 100 sheets after I overlap the first row to get 2″ total.
EPS: $800 for total of R-8
XPS: $1,600 for total of R-10
PolyISO: $3,100 for total of R-12

I’m kind of thinking to myself that maybe I should ditch the foam idea and spend the savings on some interior 2×4’s and make my interior cavity be 10″ wide for cellulose. I would want to have my windows be outties for simplicity and because interior trim is no problem.

What am I overlooking here?

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Replies

  1. iLikeDirt | | #1

    Polyiso does worse in cold weather; EPS and XPS both do better. Over time, XPS's R-value slowly falls to barely higher than that of EPS. Not worth it long-term unless its mechanical properties are otherwise desirable.

    At those prices, EPS is a no-brainer, and you should go thicker if you have the option to. R-12 EPS would be literally half the price of the polyiso for the same on-paper R-value.

    Furring out your 2x6s with 2x4s to create 10" cavities is not a worthwhile endeavor ; the studs will still be causing thermal bridging. If you don't want to use foam, your best option is a double-stud wall with each side made out of cheap 2x4s and filled with cheap cellulose. A 10" double-stud wall full of cellulose thermally braking the studs is an inexpensive great performer.

    With outie windows, you'd apply your housewrap over the foam. The thicker the foam, the more of a problem it becomes to adequately attach the window and the housewrap. A double-stud wall simplifies this detailing; you just build a plywood box right through the whole wall nail the housewrap right over the exposed sheathing. Some people worry about cold sheathing collecting condensation with double-stud walls; I'd use plywood rather than OSB for peace of mind. And use a ventilated rainscreen gap made of vertical furring strips over the housewrapped sheathing. Fasten your cladding to that.

    Long-term, the double-stud wall may be more resistant to termites. They love to tunnel in foam, but dislike the borates in borate-treated cellulose.

  2. CTSNicholas | | #2

    Nathaniel, I wouldn't mind adding more foam if I had the expertise and time. My build time is tight to meet bank regulations. I think anything over 2" will become troublesome when hanging windows as you mentioned it becomes an issue.

    Double stud walls seem more conventional to me in terms of WRB detail. Bringing up the idea of plywood over OSB and *still* using firing strips sounds like a compromise not worth the hassle if I would already be doing that if I used rigid foam exterior, therefor not gaining much in terms of labor time.

    Do you think my assessment of "anything over 2" is troublesome" is accurate?

    Am I the only one who finds EPS Foam quite a chore to work with in terms of making sure it doesn't break...especially when using 1" sheets versus 2".

  3. Expert Member
    Dana Dorsett | | #3

    The crumbly EPS is probably Type-I (1.0 lbs per cubic foot nominal density). Type-II EPS (1.5lbs density) is much more robust to handling, and labeled R4.15 or R4.2 @ 1" rather than R3.9 or R4. It's usually 25-30% cheaper than XPS at any given R-value.

    In a zone 5B location, a layer of 1" polyiso next to the structural sheathing with 1" EPS between the polyiso and siding will give better mid-winter performance than 2" of polyiso. EPS gains performance when the average temp through the foam is lower, whereas polyiso performance falls off a cliff when the average temp through the polyiso layer is under 40F. An inch of Type-II EPS as the outermost layer would be hitting north of R4.7 during the coldest weather, whereas if it were polyiso it would be somewhere around R3. By putting the EPS on the exterior and an inch of polyiso in-between you'll get a fairly stable mid-winter performance of about R10, a performance level that will not change much over decades of time.

    A double layer of XPS will perform at about R11 during the coldest hours at least for the first few years, but as it loses it's climate damaging blowing agents over a few decades that eventually drops to about R9-9.5, about the same as EPS of equal density.

    R19 batts are from hunger- they are among the LOUSIEST fiber insulation products sold. An R19 has the same weight per square foot as an R13 fiberglass batt, and (not surprisingly) performs at R13 when compressed to 3.5". When compressed to 5.5" in 2 x 6 framing they only perform at R18, according to the manufacturers' own compression charts. (They are labled based on tested performance at their manufactured loft of about 6".) R23 rock wool is pretty good stuff, as is R21 "cathedral ceiling" high density fiberglass batts, either of which will perform at their labeled R when installed in 2x6 framing. R19s are fine for insulating under a radiant floor (with a conditioned basement below it), but a poor excuse for wall insulation.

    The "whole wall" performance of R10-ish foam on the exterior of a 2x6 fiber-filled wall comes in at about R23-R25 after factoring in the thermal bridging. A double 2x4 wall with 10" thick cellulose (An 11" thick wall from sheathing to wallboard paint, 12" from siding paint to interior wall board) will come in about R28-30, depending on stud spacing and framing details at the bottom & top plates, and window/door framing etc.

  4. CTSNicholas | | #4

    Hmm, I bet that is the reason I get funny looks when I tell people the EPS is a better product than XPS, they are use to the Type-I CRUMBLES A LOT product :) I will see if they can special order Type-II instead.

    I didn't think about Poly on inside, EPS on outside. However, if the R value of poly decreases in winter and I only have 2" of rigid exterior, I wonder if I would face the 'cold sheathing' issues caused by under insulating the exterior.

    The R23 mineral wool is something I will consider. It appears to be higher priced. It costs me $1.27 per sq ft compared to $0.59 per square foot for R21 CertainTeed Fiberglass.

    With your 'Whole Wall' evaluation you mentioned R23-R25 with thermal bridging calculated. Wouldn't the foam eliminate thermal bridging just as much as a double stud wall?

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