Opinion on a wall system.
Salibonz
| Posted in General Questions on
Asking for opinions on a a wall system. I’m looking to frame with 2×4 16 o.c., zip sheathing as wrb and air barrier, 2 inch of halo exterra , rain screen and siding. On the inside r15 rock wool 1 inch of halo interra taped and sealed acting as a vapor barrier, and then a service cavity and drywall. Would this wall system have any issues? How would it perform? The r value would be r30 in the cavity and about r18 in the framing. So insulation factor is good. Just wondering on any possible issues, thanks for any advice
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Replies
2" exterior rigid is plenty for condensation control for a 2x4 wall everywhere less than zone 7. This means you don't need an interior vapor retarder other than drywall.
If you want more R value, bump up the thickness of the exterior rigid or go to 2x6 studs.
Having foam on both sides creates more issues than it solves.
I forgot to mention I'm in zone 6a. Ok so insulation on the inside isn't necessary? Halo gps foam is said to be vapor permeable, but if you think it isn't worth it I appreciate the advice. I guess I'm worried about the humid summers where the warm outside air wants to reach the cold inside air. But I guess I'm over engineering it. Thanks for the advice.
Permeable insulation makes for a more robust wall but it is not required. So if the cost is about the same, go for permeable, if not, don't worry about it.
This wall (2" or more exterior rigid, 2x4, no poly, drywall) will work in any climate including down south with longer cooling season. Even with very heavy cooling, the condensing plane will be somewhere inside the rigid. Since air can't get there, there will be no condensation, thus zero risk.
In zone 6, if you keep the 2" rigid but go up to 2x6 you will need faced batts or interior vapor retarder though. This wall would still have no issues with cooling.
Ok so in a performance perspective it would be good in increasing the r value and reducing thermal bridging, but not necessary. OK sounds good. If I get the cost down on the product, I might go through with it. They are giving a better price on bulk and I figure it'll beef up performance, but it has to work cost wise.
Fiber faced polyiso might be a cheaper alternative here. I'm not sure how vapor open that Halo product is, so I can't really offer a comparison. With polyiso, the facer is what defines the permeability (for the most part), so with a fiber facer, you end up with a vapor permeable insulating material instead of the vapor barrier you have if you use the foil faced polyiso.
Bill
Hey Bill the perm rating on the exterra is 1.78. I guess this would be considered type 2 vapor barrier. Also I can get the 1 inch for $22 a sheet. I'm not sure how much polyiso with a fiber facer would be. So the inside is 1.78 and the exterior perm rating is 1.2 for 2 inch Is that a decent perm rating to allow for any condensation to dry from both inside and outside?
Here's maybe a novel approach if you want to still have a service cavity & a thinner wall
2X4 naked tstud w/ 2" polyiso foam zipr (R9) sheathing
spray 2" closed cell foam (R12) in such a way that it leaves enough of the stud bays open horizontally to do your electrical and plumbing "service cavity"
finish service cavity after wiring & plumbing by installing w/a 3.5" Rockwool comfort batt (cut in half - 1.75") so an additional (R6)
There is also a 2" FG batt you can use (off the shelf) rather than cutting the Rockwool - about the same rvalue
R27 wall that has a 2X6 profile (before rainscreen/siding)
That's a spendy assembly!
I see no practical advantage to using a Tstud when you also have exterior rigid foam. Having continuous insulation on the exterior drastically reduces the relative improvement you get with the additional thermal break provided by the Tstud.
I think you'd have a very difficult time getting an even layer of spray foam in there like this, and you'd have the cost of the spray foam too. I'm not a big fan of service cavities in walls, but if the OP really wants one, I would consider using Safe'n'Sound (thinner mineral wool batts) here, which would leave a 1/2" or so space behind the interior drywall, which would give you a place you could easily fish future cables if needed. If you want a "real" service cavity, delineated by a rigid material, I'd use my trusty 1/4" waferboard over the studs, then 2x2 furring strips, then drywall. This would give you a 1.5" service cavity with hard surfaces, and would allow for regular batts between the waferboard and exterior sheathing.
BTW, I would not want to deal with the extra labor from splitting an entire wall's worth of batts in half.
Bill
Bill,
I agree. Service cavities rarely make sense, and only when you are able to separate them from the rest of the exterior wall by an air barrier. Otherwise why not insulate around the services?
I think people overestimate the liklihood of them changing those services around in the future too. There are only a few places that might need more cables someday. The one that immediately comes to mind is behind a wall-mounted TV. The easy way to deal with this isn't to build a fancy service cavity over the entirety of the wall, the easy way is to run a 1.25" conduit to a purpose-built connection box behind the TV. 1.25" is big enough to fit all the normal size connectors used these days -- even HDMI connectors -- so you can swap out cables as needed down the road. If you think you might add a three way switch one day, run 1/2" or 3/4" conduit to that box from somewhere accessible. This gives you a path for future cables without the need to build any unusual structural stuff.
BTW, for anyone thinking about HDMI cables -- run a cat6A cable instead if you think you'll need some future proofing. There are all kinds of "extend [fancy signal] over cat5/6/etc cable" devices on the market for relatively little money. cat6A is the highest current standard for twisted pair cable, and can easily handly 4k extenders today, and should be safe for a while into the future.
In my case, I just fish wires, but I have decades of experience doing that and can reliably get a new wire just about anywhere, even in an insulated wall cavity. A few secrets:
1- There is a company called "Labor Saving Devices" that makes all kinds of nifty wire fishing tools. Their CreepZit is especially useful, as is the Grabbit.
2- You can do all kinds of fancy wire fishing with little more than some cheap nylon twine, a weight (I usually fold over some leftover copper ground wire 4 times), and a wire coathanger with a small hook formed into one end. I have fished miles of wire in all kinds of awkward places using only those three things.
Service cavities are more trouble than they're worth, IMHO, outside of unusual circumstances.
Bill
Hey Malcolm, that is the only reason to do a service cavity. It's so my halo exterra with a perm rating with 1.78 will act as a back up vapor and air barrier without any penetrations. Plus I would have an extra r 5 behind the electrical boxes. Is this not worth the cost?
You can always sneak a piece of 1/2" to 1" or so polyiso between electrical boxes. I usually just stuff a little scrap mineral wool behind them myself. They represent such a small amount of relative area on the wall, the reduced R value behind them doesn't represent much loss in terms of absolute BTUs. Air sealing the boxes is much more important than insulating behind them.
In my own case, I like using exterior rigid foam everywhere, so that reduces the issue with the "behind the electrical boxes" reduced R value anyway.
Bill
Salibonz.
Unlike air-barriers, vapour-barriers (or retarders) work as a percentage of the area they cover, so small punctures or gaps don't affect their performance. I like the idea of protecting the primary air-barrier by burying it if possible, but it seems like a lot of effort for a secondary one - especially as you already will have the drywall performing that task.
I'm not against service cavities per se, but before deciding if they make sense on any wall I think you need to map out what will be in them. Most exterior walls just have a couple of electrical boxes on them. Some may have none. In almost all cases they are a disproportionate response to a small easily remediated problem of sealing a few penetrations - and if those penetrations aren't in your primary air-barrier, they don't even really need sealing.
I don't think they make 2x4 Tstuds.
My bad - you're correct, they don't ...
Maybe they should!
T studs are way out of my price range. Reason for going 2×4 is because with a 2×6 I would need more exterior insulation to deal with the dew point. Anything more than 2 inches becomes difficult to work with. I want simple but effective, I guess
You could also use 2x4 steel studs in the assembly I proposed - which are predrilled for the service cavity ...
begreener,
What advantages to steel studs give you over wood ones which have a much higher R-value?
Hi Malcom-
Just quicker to create a service cavity with the knockouts already there ...
begreener,
Apart from the thermal bridging... Here anyway, metal studs are almost entirely reserved for multi-unit residential or commercial projects, so including them would mean hiring a separate trade. Drilling studs for services really takes next to no time - especially when you are already drilling plates and joists.
If you are only using the "studs" to define a service cavity, then they are really just "furring strips". You could, in that case, just use gaps and avoid the need to drill any holes. If you go back to my earlier example with 2x2s and 1/4" waferboard, you could just use, as an option, 20" 2x2 sections, leaving some gaps between them up the wall. This leaves gaps for wiring. You could staple to the "real" studs through the waferboard to support the cables. You'd have sufficient depth to avoid the need for nail plates this way.
I still think the service cavity is more effort than it's worth though.
Bill
You don't need more rigid with a 2x6. The only thing you need is faced batts or interior vapor retarder. Both are minimal cost. In zone 6 you can even bring that rigid down to about R6 with interior vapor retarder to save even more cost.
If you go with polyiso, that would be about 1" so you can even nail up some siding directly through the foam.
I thought with a 2×6 you need a minimal r 11.5? And for a 2×4 r 7.5? Won't the sheathing collect condensation? Also with zip being osb it wouldn't dry as easily as cdx? And last question, with vinyl siding I would be able to nail directly through the polyiso? I appreciate your help.
There are two different ratios. One is for walls with just interior drywall (R11.5) and one for walls with interior vapor retarder (~r6).
You can see the wall here:
http://effectiver.ca/calculator/wall.php?id=6213
Montreal is about zone 6.
With interior VR, it is better to go for somewhat permeable rigid insulation. Micro perforated GPS or permeable polyiso are worth the extra bit of cost.
The vinyl can be nailed up directly through the rigid with longer nails.
Yes, "spendy"!
but I wanted to present the "thinnest" wall assembly possible with the highest rvalue & service cavity ...