Choosing a High R-Value Durable Roof Assembly
Hi everyone,
Relatively new to understanding the principles of building science and I’m trying to decide between two different roof assemblies for our two-storey residential home in climate zone 7A. I’d be grateful for any advice the community has on which option would be better. The priorities are both a high r-value and ensuring the longevity of the system (ie. water penetration, moisture management, etc).
Option #1:
– Interior decking
– Strapping (for interior service cavity)
– Vapour barrier (Intello plus or poly 6mm)
– TJI roof joists 16″ deep @ 16″ OC (Rockwool batts or blow-in cellulose)
– Strapping (2×6 laid parallel to the eaves)
– Sheathing (1/2″ OSB, taped)
– WRB (Solitex Mento Plus)
– Metal roofing
Option #2:
– Interior decking
– Strapping (for interior service cavity)
– Vapour barrier (Intello plus or poly 6mm)
– TJI roof joists 16″ deep @ 16″ OC (Rockwool batts or blow-in cellulose)
– Sheathing (1/2″ OSB, taped)
– WRB (Solitex Mento Plus)
– Exterior insulation (2″ EPS)
– Strapping (2×6 laid parallel to the eaves)
– Metal roofing
When considering these two options, a couple additional questions floating around in my head:
1) For insulation within the TJIs, would blow-in cellulose (likely loose packed) be that much better than two layers of Rockwool? We have access to the equipment through a local building supply store but very few builders use cellulose in the region.
2) For strapping, I’m guessing that boards laid parallel to the eave will restrict the ventilation channels from the eave to the ridge vent, so should we do two layers of strapping (vertical below and horizontal on top) or diagonal strapping instead?
Thanks!
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Replies
I would contact the people at the Cold Climate Housing Research Center and look through their website if you are in climate zone 7A
http://cchrc.org/
Thanks, I'll take a look.
My vote is option #2. The main difference seems to be with option #1 you are putting an extra layer of strapping underneath the sheathing just to squeeze in another 1.5 inches of insulation? But keep in mind that added insulation is only about ~75% effective because of the framing factor/thermal bridging caused by your 16" o.c. rafters. The exterior insulation option is 100% effective with no thermal bridging to worry about. Also if you bump up the amount of exterior insulation (and consider switching to polyiso for higher R-value per inch) you can get away from needing an interior vapor barrier. Also try to make the gap between the exterior insulation and metal roof vented from eave to ridge to reduce the temperature under the metal. I don't know if Solitex is rated for high temp but it might need to be. I also don't know if that is a self-adhered membrane but you may want to look into those especially if this is an exposed fastener metal roof. Probably less of a concern with standing seam roofs.
I would also consider switching to 24" o.c. but keep in mind if you do, the metal roof companies still want fastening every 16" so you'll have to lay your strapping diagonally so you can still attach the roofing every 16" even though your rafters are 24". Matt Risinger has videos about this and he borrowed the detail from Peter Pfeiffer:
https://www.youtube.com/@buildshow/search?query=metal%20roof
The framing percentage of your insulated cavity is more of an issue with solid lumber. With Ijoists, it's pretty negligible.
Thanks, looks like we'll be going with 24" OC for the TJIs so that will be better. I'll look into the diagonal strapping approach as that might remove the need for two strapping layers.
Option 1 - I'm not understanding the strapping. It's perpendicular to the Ijoists? Is it providing more cavity for insulation and a ventilation cavity? If parallel to the Ijoists, it might be easier to install on the underside. I also believe you can get thicker Ijoists than 16", so you could just get 20" Ijoists. I don't know if these are common or significantly more expensive. Boise Cascade offers them at least.
Option 2 - 2" of EPS is not sufficient for your climate zone to go with 16" of fluffy. It will create a condensation issue in your sheathing.
That was my concern with option #2, as well. That said our wall system is almost exactly that, with a double stud wall => sheathing (tape) => WRB => 1.5" EPS.
For option #1, that's correct, I believe the idea was to strap between the TJIs and sheathing to provide more of a ventilation cavity and more wood for the metal tin to screw into.
That wall system will also be problematic. Also there is a lot of wasted cost. If you are doing a double stud wall, why the EPS? Just spread the two walls by another 1.5" if you want more continuous insulation.
In option 1 the horizontal 2x is not enough to vent the assembly. You need a 1.5" clear channels from soffit to ridge. With I-joists you can staple to the bottom of the top flange some sheet good (ie, card board, thin OSB/fiberboard or house wrap) to form the channel.
You do loose a bit of insulation in this case but the vent channel is needed for the roof. You can spec a taller I-joist or add an extra layer of parallel strapping directly over the I-joist flange for the extra height.
You can also swap the orientation and build the roofs with I-joist as purlins. In this case the 2x strapping runs in the correct direction so you can fill the full I-joist with fluffy. This does mean another layer of horizontal strapping or a OSB/CDX roof deck to support the metal roof.
As others have pointed out Option 2 won't work unless you go up to something like 10" or 12" of rigid.
Akos, in your experience, are taller I-joists the prefered method? The 18"+ ones seem quite a bit less common. But I see floor trusses in 24" readily available. I was just curious if there were other downsides to using floor trusses vs Ijoists as rafters. The fire concern of Ijoists is one I always see brought up, especially on the roof, it seems worth considering.
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After speaking with our local building inspector, most homes in our region with metal roofing do not have sheathing in their roof system, nor do they have an air barrier or WRB above the rafter (the only air barrier would be on the interior).
I then spoke with Westform Metals, the roofing company, and they recommended having sheathing/decking under the metal roofing. Turns out, with roofing with hidden screws, like Prolock, the seams are pressure-sealed so it's advised to have decking underneath. (The alternative is typical metal roofing with exposed screws that need to be replaced every 10 years). So now we need to include decking/sheathing back into the mix
This is what I'm leaning to now, pending a few questions:
- Interior decking (T&G)
- Service cavity (2x4 strapping)
- Vapour barrier (Intello plus, interior air barrier)
- Rafters (16" TJIs, 24" OC) (2 layers Rockwool batts, R64, 16")
- Strapping (2 layers of 2x4, first running parallel, second going perpendicular to TJI)
- Decking (1/2" plywood)
- WRB/roof underlayment (Solitex Mento Plus)
- Metal roofing (prolock 12")
* The WRB now sits between the decking and metal roofing, and the TJI insulation cavity will be open to the air/wind coming up from the soffits. Any concerns? The only air barrier in the roof system, then, will be the interior air barrier.
* In this system, does it make sense to go with a cheaper (but just as waterproof) roof underlayment rather than Solitex Mento Plus? This would help lower the cost of an already expensive roof.
* To help remove one of the strapping layers, I could also go with thinner Rockwool batts and a layer of thin EPS on top to serve as a baffle for ventilation. This would leave around an 1" of air gap at the top of the TJIs and then 1.5" with the perpendicular strapping.
Any advice or thoughts on this assembly, especially the WRB/air barrier layers would be appreciated!
Should work. Swap the 1x4 and the 2x4 strapping (or use 2x for both) as you need the big air gap going from soffit to ridge.
You only need one solid warm side air barrier for the roof. Including a 2nd exterior one doesn't hurt but I would not spend too much money or effort at it, you do need something under the metal roof to deal with night time condensation though so can't eliminate it. You can use regular house wrap (watch as some very slippery to walk on) a permeable roofing underlayment (ie Gaf deckarmor) instead of the Mento. You can also use a budget synthetic roofing underlayment if it is installed over the vent space.
Unlike trusses, larger I-joists tend to be much more expensive. Generally best to keep the size to the max you need to span the space. Going too crazy on expensive insulation is also not really worth it even in very cold climate. For example, in zone 7 say with 10000 heating degree days. Assuming 2500sqft of roof, the heat loss through your roof through the whole season would be:
10000HDD*24h*2500sqft/R64=93 Therms
If you went with code min ( two layers of MW) that goes up to:
10000HDD*24h*2500sqft/R48=125 Therms
That heating cost delta between the two roofs is pretty much noise. It would also let you use smaller I-joist which is a decent cost save around me.
Aiming for higher R value is only really worth it if you are using cheap loose fill insulation. In this case I would design with trusses and low enough slope that you can loose fill without having to deal with a dense pack roof. Without an attic space these are still not easy to fill so make sure you can figure out how that will happen.
Thanks, Akos. Good point on the heating cost delta, as mineral wool is not cheap.
The cheaper option would be two (thinner) layers of Rockwool for an R56. Also, our roof pitch is 3/12, so would blow-in loose-fill cellulose be a better approach than two layers of MW? I haven't compared the cost but assume the r-value would be similar. In this case, we would definitely need a baffle to contain the blow-in cellulose. (I could use Solitex for this but, again, adds up on the cost.)
With this roof system (and similar ones), I'm a bit surprised that the insulation cavity is directly exposed to the ventilation channel and wind washing. Would you recommend I cover the insulation layer with a baffle or anything else to avoid any wind turbulence?
For the strapping, what did you mean by, "Swap the 1x4 and the 2x4 strapping "?
*UPDATE: I priced out Rockwool batts vs blow-in cellulose and it's around $6-7k cheaper for materials to go with cellulose. Given our cathedral roof is a low slope (3/12) and the rental machine can handle dense-packing, I'm starting to lean toward cellulose given the savings.
Thought I'd share some photos of the roof, now complete. As mentioned in other posts, it was slightly dangerous and somewhat tedious putting the WRB directly on top of the roof joists but we're happy with the end result.
Atari,
Nice job. I wish you had taken Akos advice though and used 2"x materials for the strapping. You are starting out with a continuous 3/4" gap which will be reduced by the cellulose pushing out the WRB. It wouldn't meet code here, and I thought a 1" gap was the minimum required under the IRC?
Hi Malcolm, that first layer of strapping is also 2"x (it's a 2x6 ripped in half). Good call, though. I didn't think how the WRB would push up and reduce air space once the cellulose was blown in. We actually just finished dense-packing the entire roof (~3 lbs/cubic foot) with an old Krendl 325 from the local lumber yard. That was an experience :)