Replies

1. Expert Member
   Dana Dorsett | Nov 15, 2013 12:19pm | #1

   A 2x6 with studs 24" o.c. with 4" of exterior polyiso and R20 fiber (R3.6/inch) between the studs comes in at about R40 after thermal bridging (with allowances for sheathing & siding R)

   A 2x8 wall 24" o.c.with R 4/inch dense packed BIBS fiberglass and 2" of exterior polyiso comes in at about R33.

   Using staggered-stud 2x4 16" o.c. on the exterior course, finger jointed (for flatness) 2x3 24" o.c.on the interior course on 2x8 top & bottom plates with 2" of exterior foam would get you pretty close to the same performance though. (I'm not sure if that's any closer to their comfort zone. At that foam/fiber R ratio you'll need to use a "smart" vapor retarder on the interior in a Peterborough Ontario climate (comparable to US zone 6), whereas with the 2x6 w/4" foam you would have huge dew-point margin, no interior vapor retarder necessary.

2. GBA Editor
   Martin Holladay | Nov 15, 2013 12:25pm | #2

   Bob,
   I'm going to guess that your climate zone is similar to U.S. Climate Zone 6. In your climate zone, the recommended ratio between minimum rigid foam R-value and between-the-studs fluffy insulation R-value is 36% for rigid foam R-value, and 64% for fluffy insulation R-value.

   The center-of-stud cavity R-value for your 2x8 stud cavities will be about R-26, so you'll need a rigid foam R-value of about R-14 to make the wall work.

   So 2 inches of exterior foam won't quite work. I'd say you need 3 inches of XPS, or 2 1/2 inches of polyiso, to make the wall work.

3. 
   mackstann | Nov 15, 2013 12:27pm | #3

   What kind of foam are they using? If it's EPS then you may not lose any R-value. If it's XPS then you would lose about R-2, and with polyiso you'd lose about R-4.

4. 
   bobhol | Nov 15, 2013 12:42pm | #4

   I think I will try to stick to my original 2 by 6 with 4 inches of exterior foam ...the arguments they present are probably not valid ...they just dont feel comfortable doing it or they have enough business and are unwilling to try something new. ...so frustrating ..aaaagh!!!

5. 
   user-1115477 | Nov 15, 2013 08:44pm | #5

   Bob,
   Go for your plan with 2x6 and 4 in. of exterior foam. It's just that you might have to be patient and work a little harder to find good help. You won't be sorry when it's done. I took my house from the original construction of 2x4 at 16" o.c.(fiberglass batts) with one inch of exterior eps (that had shrunk at the seams considerably) and added 2 layers of 2 in. polyiso all around on top of the existing eps, sealing everything as I went. I did the whole house myself. My walls are now about R40 (nominal) and it's great. I have Passiv House performance with respect to heat load, even though I only got the infiltration down to about 4 ach50 (and counting) because of it being a retrofit. My biggest point is this: I am in Maryland (Climate Zone 4) and I consider my wall R 40 about right with HDD of about 4500. If I were designing in Ontario, I would use your 2x6 with 4 in. of exterior polyiso as a minimum, but would probably go even more wall R value. I would guess your HDD is somewhere between 5500 and 6500. Just be sure to manage the wall interior condensation profile, as others have mentioned.

6. 
   bobhol | Nov 16, 2013 08:00am | #6

   Thanks guys for all your answers ...I will stick to my guns and talk to more contractors if necessary...regards,Bob