Replies

1. 
   Jason S.|Jul 26, 2020 09:14am|#1

   1. Minimal risk, based on your proposed R values.
   2. Sure, if it's airtight.
   3.Yes, if the concrete is smooth and straight. If not, you'll still need furring.

   Lots of ways to skin the cat, though the exclusion of foam narrows your options. Consider how to finish the comfortboard where it's exposed between grade and siding. Detail for continuity of control layers. Lots of good articles on this site. Don't forget the capillary break between footing and foundation wall.

2. 
   Jon R|Jul 26, 2020 09:55am|#2

   > do i risk moisture condensing on the inside face of the concrete wall?

   Better to ask if it's excessive. While you can design for no condensation/sorption, this is rare and unnecessary. It's a question of how much accumulation, how well it is contained and how well it can dry later.

   With your wall (3" exterior insulation), the amount of moisture accumulation is minimal and it will dry quickly when it warms up. You meet the guidelinehere.

3. Expert Member
   Dana Dorsett|Jul 26, 2020 10:14am|#3

   >"2×4 wall studs @15″ o/c
   R-14 comfortbatt mineral wool between studs
   1/2″ air gap between studs/concrete
   10″ thick concrete wall"

   A couple of comments:

   1> The batts need an exterior side air barrier to hit their tested performance level, not a half inch of air.

   2> Even with an exterior air barrier on the batts, with the thermal bridging of the batts and R0-values of the concrete & wallboard & air films factored in the "whole-wall" thermal performance is only about R11, maybe R12 if the stud spacing is opened up to 24", which is barely more than half of what the IRC 2018 levels spell out for zone 7.

   IRC code min would be R15 continuous insulation, not thermally bridged by framing, or a U-factor of less than 0.050, which is the same a "whole-wall R" of R20, counting the thermal performance of the concrete + gypsum + interior air film.) See TABLE N1102.1.2 and TABLE N11o2.1.4:

   https://codes.iccsafe.org/content/IRC2018/chapter-11-re-energy-efficiency.

1. 
   Eric M|Jul 26, 2020 04:31pm|#5

   Thanks all for the comments.

   Dana,
   Per comment 1: would using a layer of tyvek house wrap between studs and concrete wall fix the mineral wool batt performance issue and still let moisture dry inward? Adding this would be very easy and cheap.
   Per comment 2: Am I not meeting my minimum R20 whole wall value when accounting for exterior layer of 3" mineral wool comfortboard, adding R12 to the interior R11, for a total of R23?

   Thanks!

1. Expert Member
   Zephyr7|Jul 27, 2020 10:16am|#7

   Yes, you can use tyvek as a vapor permeable air barrier here, but you’d need to have it directly against the back of the batts — you don’t want a gap where convection currents can circulate between the batts and the air barrier.

   I would use something less vapor permeable against the concrete though, probably EPS in your assembly. EPS will be cheaper than rigid mineral wool too.

   Bill

1. 
   Eric M|Jul 27, 2020 02:40pm|#8

   Thanks Bill.

   Yes I would install the tyvek to the stud wall (exterior facing side), then tip it up into place and keep the 1/2" gap between tyvek and concrete. I think that should fix the air movement issue you and Dana noted.

   I'm not really a fan of any foam, even though I know EPS is better regarding blowing agents used in XPS. It still not great for the environment and pretty terrible if you ever have a fire. Batts between the studs aren't perfect i know, but with the exterior having continuous insulation I think its an alright compromise if I can make it work.

4. 
   删除d|Jul 26, 2020 12:28pm|#4

   删除d

5. Expert Member
   Akos|Jul 26, 2020 10:00pm|#6

   As long as you have 45% of your R value as exterior continuous insulation it should work. This does mean running the rigid mineral wool all the way up to your rim joist to cover your entire foundation.

   If you want to skip the interior studs, you would more exterior insulation to meet code. I would install the drywall over 2x strapping on flat though as it will make electrical much easier.

   Fastening strapping to concrete is a fair bit more work than building stud walls, so unless you need the extra space, I would just build a stud wall.

1. 
   Eric M|Jul 27, 2020 02:46pm|#9

   Thanks Akos,

   Fantastic idea about doing the studs on flat. More labour perhaps, but I will probably be doing the basement wall framing work myself, so thats fine. Obviously Ide need to adjust my insulation if I do that, but I like the idea of giving myself a bit more space in the basement.