Smart insulation with smart money
There’s a lot of great info on proper detailing for insulation, but I’m trying to dial into my specific situation. I’ll try to be brief, and greatly appreciate your thoughts and input. Also, I’m learning as I go, so please forgive any misuse of terms, etc.
Overview:
– Building an 800sqft “cabin” in the western mountains of Maine.
– Framing will be 2×8 24OC (this is set after many rounds with my engineer)
– My assembly will have interior fluff or batts, and exterior rigid.
– My understanding is a 65/35 ratio of interior and exterior to reduce chance of condensation on the inside of my sheathing (zone 6)
– Sheathing CDX with smart WRB (HydroGap SA likely)
– Interior smart vapor membrane (Intello Plus likely)
– Uninsulated 2x service channel
– Latex painted drywall
– We are on a tight DIY budget, but want to be as smart and energy efficient as possible with our limited resources. For this question, there’s no labor cost to factor in.
So with smart money savings in mind, I’m considering three options:
– 1) R30 Rockwool interior w/ 3″ Polyiso exterior (r20.4 before drift and degradation). This comes out to roughly $6600.
– 2) Two layers of R15 Rockwool interior w/ 3″ Polyiso exterior (r20.4 before drift and degradation). This comes out to roughly $6200. (not really a big enough savings to justify the extra labor)
– 3) R30 unfaced fiberglass insulation packed down to 7.25″ (effective R of 24-25ish I think?) w/ 3″ polyiso exterior. This comes out to roughly $4500.
$2000 is a decent savings in our world. But I want to be smart and know that mineral wool has a lot of advantages like bug and pest resistance, hydrophobic, etc. And correct me if I’m wrong but wouldn’t lower R value interior move my dew point further into the exterior rigid and possibly be better for mitigating condensation inside the cavity?
And I’m sure there’s 1000 other options that I may not have considered.
Thank you in advance for your thoughts and advice!!
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Replies
I like and prefer mineral wool due to it being easier to install well, and it's a little better in terms of critter resistance. If you're tight on money though, swapping from mineral wool to fiberglass is usually a pretty easy way to save some money with very little downside, especially if you're willing to put in the time to do a good installation with the fiberglass.
You are correct that lower "fluffy stuff" R value while keeping the rigid foam exterior R value the same does push the point at which the temperature drops to the dew point further out into the rigid foam layer. That does make the assembly a bit more robust in terms of moisture resistance.
With your assembly and the fiberglass, you have about R19 for the 3" polyiso (it's common to derate that to around R15 for very cold Northern climate zones), for a total R value for the assembly of about R39-R44 using your R value numbers for the compressed fiberglass. That's excellent.
Another possibility you may consider is a double stud wall and dense pack cellulose. Using 2x10 top and bottom plates and 2x4s or 2x6s as needed for the structural requirements as the studs, you could get about R33 this way in a 9.25" deep wall. Using 2x12 top and bottom plates you'd be at a bit over R40 with an 11.25" deep wall, which would be in the same R value range as your originally proposed assembly but 1" deeper overall. The double stud walls and cellulose may save you some money over the polyiso and fiberglass/mineral wool wall.
Bill
Thank you so much for your reply Bill. On the double stud assembly and dense packed cellulose route — that was actually my original plan. However, one part of the DIY I'm not doing in the majority of the framing. I've had two smaller outfits quote the framing (labor only) for the double stud and both were in the $30K range, where the 2x8 is $20K range. Materials are slightly less too. The DPC insulation was about 12" thick and came it at $4600, so that full assembly was about $34,600. The 2x8, with even the more expensive option, is closer to $27K. This is all to say, I had considered this as my first choice, but sweat equity on the insulation part is going to win. Very slow, well thought out, sweat equity. Cheers ~ SB
Now I'm wondering if there's any reason I can't just use an insulation intended for a 2x6 wall in my 2x8 wall assembly? I would still maintain a 35% outside rigid ratio. Like an John Mansville R19 fiberglass, with an R13.6 foil faced Polyiso exterior (assuming it'll degrade to R11 or so). I would still be at a respectable ~R30. Building will be airtight, properly detailed and flashed, and with smart membranes in and out. What's the downside? Thanks!!
SB,
The problem is the 2" air-space in the wall cavities where you will develop convective loops. With batt insulation, especially fiberglass, voids degrade performance and can cause moisture issues.
Even completely filling the cavities your ratios are fine - and the ratios can be decreased based on what type of internal vapour control is used. How that is calculated is something that I confess to having not paid enough attention to. Hopefully Akos, or someone else more conversant with these assemblies will weigh in.
Hi Malcolm.
The convection loop makes sense. Assuming this isn't a thing with an uninsulated service cavity since there's a smart membrane between?
Still going with concept of 20 + 10ci on this:
IF SO, I had another crazy idea: what if I essentially turn the 2x8 stud into a 2x6. Let me try to explain this idea:
OUTSIDE > IN
Conventional section:
- cladding
- 1x strapping
- R13.6 polyiso (effective of R10ish in winter?)
- SA WRB
- 1/2" CDX sheathing
CONCEPT section:
- 2x8 stud wall, with 5.5" of fiberglass insulation (R19) flush against CDX sheathing
- at the 5.5" mark, using something like Siga Twinet (double sided tape) create vertical lines on the insides of the studs.
- install smart membrane like Siga Majrex along face of insulation, back out along 1.75" exposed end of stud, around end, and back into the next cavity and continue.
- tape membrane seams as normal.
- now there is a 1.75" service cavity in each stud bay, but the stud is fully enclosed in the membrane keeping any moisture from inside the house from potentially infultrating the wood stud and traveling behind the membrane.
- drill holes for wiring in the service cavity, and use tape rolled inside out to go through each hole -- again, sealing the membrane completely.
- latex painted drywall finish.
Possible issues and concerns:
- time intensive (not really a concern for me)
- thermal bridging?
- other?
Maybe I'm really just overthinking this. I appreciate your thoughts.
If you want budget and you are doing a service wall, I would skip the exterior rigid and insulate like this:
https://www.greenbuildingadvisor.com/article/furred-continuous-insulation-wall-assembly-to-meet-2021-codes
If you bump up the strapping to 2x3 on edge, it is about the same R value as the assembly you are proposing without having to deal with foam, window bucks and long screws.
You would still want a rain screen though but that can now be nailed up directly to sheathing.
As for your original proposed assembly, the 3" iso is just about good enough for condensation control so you can skip the warm side smart vapor barrier especially if you go for fiberglass batts (since slightly lower R value).
On the outside, you don't need any fancy membranes. You do want to detail the sheathing as your main air barrier, that can be done with a quality tape. Over that you can use regular house wrap either under or over the foam depending on where your windows are located. Full peel and stick over the sheathing also works if you don't want to deal with tape and house wrap.
Thank you’ll for this Akos. I love this approach of not having to deal with exterior rigid, as I’ll be the one actually doing all the detailing with little more than GBA, YouTube and some chats with a local consultant as my guide. It’s the original reason I like the double stud.
But, does this mean that just using R30 Rockwool ($3600) in the 2x8 cavity / smart membrane / uninsulated 2x service cavity / drywall will work too?
Or is this better: (to confirm the FCI concept)
- latex painted drywall
- horizontal furring
- R6 batts between furring (recommendations?)
- Smart vapor membrane (intello plus?)
- R30 unfaced fiberglass insulation 10" compressed into the 7.25” cavity (effective R25ish) (saves about $1800 over R30 rockwool)
- CDX plywood sheathing (not taped)
- SA WRB (recommendations?)
- Rain Screen (thinking something like BO Slicker)
- Wood cladding
Thanks again for everyone’s input. This group is simply amazing.
"But, does this mean that just using R30 Rockwool ($3600) in the 2x8 cavity / smart membrane / uninsulated 2x service cavity / drywall will work too?"
Yes. The smart vapor retarder or faced batt is needed in this case for this assembly. Without insulation in the service cavity it would get you a bit above R20 assembly. Not great, but not terrible either.
The rest of your stack up is fine. Not too many options for R6 so that will have to be fiberglass.
If you go up to 2x3 strapping, you can go for rockwool safe and sound (3" but can squish a bit) which should be around R10. Than again, cheap 2x4 batts would also squish to 2.5" and cost a lot less.
Compressing 10" batts might be a bit of a stretch, but the 8.25" should be no problem. Those are also R30.
Slicker is great for something like cedar shingles. For regular lap wood siding, vertical 1x3 strapping is much cheaper and simpler install.
Any locally available SA WRB works as long as it is permeable. Lot of places only stock stuff for foundations which are near zero perm, that won't work on walls.