Mid-Mod Roof & Clerestory Insulation Details
Hello,
In Zone 3 (Seattle) and working on a home addition to a mid-century modern residence (1955). The current roof assembly is T&G car decking, 4x rafters with sheathing and TPO. The addition assembly must be rated R-49 by code and will include 2 layers of polyiso (approximately 9 inches in depth). I’m trying to keep the low profile roof assembly at the eaves and transition to the deeper roof assembly above the addition space and I’m not sure the best way to detail. To make it more complicated, I have some clerestory windows over a portion of the roof. Would be great to have any feedback. Some questions I have includes: Does the assembly shown in the details make sense, did I miss anything critical? Any thoughts on how to improve the transition? Better approaches to the clerestory window? Please see attachments for wall sections/details. Thank you!
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Since you are looking at an assembly with continuous exterior insulation most codes allow for U factor based compliance. This generally means you can go for less than r49 rigid, you can even use the bit of R value the T&G deck and sheathing gives. Once you know what U factor (1/U factor = assembly R value) you need to hit, you can use this calculator to figure out how much exterior rigid you need:
https://www.ekotrope.com/r-value-calculator/
As for details, my quick red flag is the valley you are creating above the clearstory where there is the step change is in the roof height. This is a hard detail to get right and most likely cause issues. I would figure out how to avoid it either by carrying the rigid all the way out to the edge or tapering it so it still drains to the main roof.
Generally conditioned, sealed and insulated crawlspaces are more efficient and cheaper to build. Depending on code, you might only need a couple of inches or rigid on the stem walls and you can skip all the insulation under the floor. This also means you can put your ducting and HVAC in there as it is part of conditioned space. In zone 3 it doesn't matter as much but here in snow country, sealed/conditioned crawl also means warmer floors. You drawing is not showing poly under the crawl slab, this is a must no matter where your insulation is.
If you are in radon country, make sure there is a layer of gravel under the slab. Even if no radon, it is still a good idea.
One detail that I've done for my clearstory window is to put the post inside instead of between the windows. I ran wider top and bottom plates that hang past the posts to support the window. The windows can then be one single 5 element module and easier to trim and flash as you are not dealing with separate windows. Bonus is slightly larger glass area.
Thank you for the helpful input! I made some modifications based on u-factor and then checked them with the assembly calculator. I'm estimating the T&G 2x= R2, two layer of 3" polyiso = R39, and 1/2" of plywood =R-0.62. U=1/R2+R39+R0.62 is a U-factor of about 0.25, code required would be about 0.26. It definitely helps to clean up the assembly and reduces the thickness, which I don't mind having 9" fascia board, it was just starting to look weird when it was over a foot. Would you continue to polyiso over the soffits? Reattached revised details.
Those are great notes on the crawl space, I hadn't been paying enough attention to insulation strategies, but have been reading about vented vs. unvented. I revised the detail to one similar to a detail found on the GBA site. Is this the direction you were thinking? The current mechanical is under an existing crawl space, so this approach makes a lot of sense. I should probably consider insulating the existing crawl space as well.
For the clerestory windows, I like the idea of simplifying the assembly. Are there window companies that you've worked with that have 5 element modules? I was trying to find a window recommended for clerestory applications, but haven't found anything yet.
Thank you again!
Looks much simpler overall and the crawlspace detail is much better. There is also a about an R1 you get from the interior and exterior air films, won't change the calculation much though.
I've attached a shot of the windows. The columns are a 3 ply 2x4 mudded and painted. The top and bottom plate are 2x10 which gives enough overhang to support the windows. These are standard windows, normally you would join 2 or 3 for a regular opening, you are just joining more. This would have to be done on site but pretty simple to do. The one thing to watch is you need very accurate measurements otherwise the columns and windows won't line up perfectly, not much room for error.
One lesson leared is that I went with very narrow window frames which created issues when the IGUs failed. I could not remove the glass as the columns were in the way, ended up removing some of the columns to get the glass out. I would recommend to go with either profiles that have the glass retainers on the outside (ie fixed windows for sliders) or wider frames.
All the gaps between the T&G are a big air leak path. Generally the best option is to get a solid air barrier (peel and sick works the best) over the T&G. This also gets the place water tight before the rigid and roof is installed. Make sure to use peel and stick with acrylic or butyl adhesive as the standard one with modified SBS (basically fancy tar) can have an odor and can sometimes react with the sap in the wood and bleed.
T&G also leaks horizontally along the grooves between the boards, so the best is to stop the T&G half way across the wall plates and carry the peel and stick down the wall sheathing for air barrier continuity. This also lets you flip the direction of the T&G at the overhangs which sometimes makes the look cleaner. Another option is to use flexible adhesive at the wall plates and between the boards when they are installed.
I think unless the roof structure is reevaluated, the T&G needs to be continuous.
I know I have posted before that when we stripped the roof off of my 50 year old 'Deck' house I pondered this, and ended up drilling a 1/4 hole at every seam/groove
above the exterior wall and injecting canned spray foam.
This is a 'as good as you can get' retro solution, but is not a solution for a modern low leakage house.
Having a manufacturing background I am always thinking about arriving at a solution that will get done, and get done properly, and not cost a fortune.
Having lived under this roof system in 3 houses over 30 years, but never actually having installed one, I do not have a bullet proof suggestion. One that does not involve the homeowner coming to the site at the end of the day and finding his roof deck installed and an unopened box of butyl caulk or pile of unused tubes of Sikaflex on the ground
A few weeks ago I was researching the T&G air leakage issue and came across a Miller Hull Loom House project that included a sketch of the attached detail, link to entire entry found here: https://millerhull.com/2018/loom-house-renovating-a-mid-century-modern-gem/ Is this essentially what you came up with for your house? Do you think this is not a solution because of cost and effort (assuming a contractor won't drill a hole at every groove and run a continuous bead)? My house is fairly modest, but was curious if this is something to even consider doing. Thank you!
This is really helpful, thank you for sharing your clerestory and for the lessons learned. I'm thinking through all the windows and soffit details and there is a lot to consider. Connecting the windows makes a lot of sense, I was trying to match some of the aesthetics of the existing clerestory windows I have, but they are the original single pane with simple wood frames, so it doesn't really equate to today's assemblies. By connecting them, they will appear less bulky I think. It's nice to hear how others have worked through these issues.
I've also been trying to figure out how to detail around the T&G to avoid air leakage, thanks for the suggestions. As you can imagine, the current house is not very efficient.
I'm much happier with the U-factor assembly for the roof, though I learned the downside is I'll have to do a full plan review, since only prescriptive allows for a Subject-to-field-inspection review. Still worth it, I think, especially as something else could trigger the full review.
Thanks again!
Akos brought up 2 of the 3 items that jumped out at me, using the U-factor, and the drainage issue. I also note that you have a fairly sizeable thermal bridge that can easily be mitigated.
Check out the attached sketch.
According to my references Seattle is in IECC Zone 4-C (4 Marine). So you may want to double check that. I'm currently designing a mid-century modern inspired home in Zone 5, with that similar kind of narrow profile roof. Using the U-value calculation I've determined that I can do R-20 polyiso (apx 4") above the roof deck and an R-25 fiberglass batt (8" thick) compressed into a 2x8 rafter depth, yielding R-24. This works for both the U-value and the ratio of impermeable to permeable insulation. This is per the IRC 2021. I believe Seattle is still on the 2018 code, which is less restrictive. Required U-value = 0.026. Continuous rigid insulation of R-38 would get you there. You would probably actually use R-40 since rigid sheets usually come in round number R values.
You are correct, Seattle is Zone-4, I checked too quickly before messaging. I modified my detail based on these recommendations and am now showing 2 layers of 3" polyiso to get the R-38/40 you noted. This profile would be approximately 9 inches, with this I'm much more comfortable extending to the fascia board and it would look a lot cleaner than my original detail. Attached a revised detail. Thank you for your help!
Your welcome! Best of luck with your project and please do share a few photos back here once it's all done.
Dunno if the T&G can self support, but if it can, why not forgo all the wood and just finish it with foam tapered down to the fascia height? So much simpler. Actually doesn't look like any of that wood is all that structural anyway
More familiar with EPDM which one would glue right to the foam. Pretty sure the compliance helps with lifespan. Not sure on TPO install, but with EPDM the plywood is superfluous or worse.
I think the T&G is considered structural and I think you're right that it could just be the foam, that always seems strange to me, but the plywood is probably not required. I looked into EPDM vs. TPO at some point and I can't remember all the pros and cons, but I think both can work. TPO may have a lighter roof color, so less likely to absorb heat, where I think EPDM is darker. The lighter is more reflective, but I think there is was a drawback with potentially not enough heat gain, which may help with moisture control or something, I can't remember exactly, but will try to revisit before finalizing. Thank you for your thoughtful feedback!
EPDM is typically almost black. But Carlisle makes a white product. There may be others.
https://www.carlislesyntec.com/en/Roofing-Products/Membranes/EPDM/Sure-White
"I" like EPDM because you can buy the stuff at any building center now, and I understand how it works. That is not saying that the 'new' stuff is not worth doing.
I am always suspicious of things that require special tools to install.
But my personal bias is no reason to plan a house.....
I 'think' the white EPDM is layered with black underneath.
I wonder if I ask my builder about EPDM vs. TPO they would prefer the EPDM for this reason.
Thanks for sharing! Good to know that EPDM can be white, I think I may need to dig a little deeper into the TPO vs. EPDM debate.