Using the U-Factor for roof assembly & amp; exterior foam
Wow, this community has been so helpful and just want to say thanks! This will be my 1st question, probably of many, so go easy.
We are in the midst of an entire home remodel, down to the studs. We will, for several reasons, be replacing the roof deck and roof. My plan is to turn our current vented attic into an unvented conditioned space by adding exterior ridged foam along with interior rockwool. A hybrid approach. We live in zone 6a and had planned for r-26 on the outside and r-23 on the inside for a total code compliant of r-49. I was planning to install 2 layers of 2” polyiso (r-13.1) for r-26.2. While not the most economical, it was about the only way I could see meeting code with our build and was nearly set on our plan. Until….I started reading about the U-factor alternative. By following the U-factor alternative I could use 2 layers of 2” EPS (r-7.7) for a total of r-15.4. By doing so, this would get me to a U-factor of .025, which is below the code maximum of .026 and save me probably $3000 in materials.
Sounds great. But…I’m wondering, while this will meet code and save me money, will this present problems with the roof decking since my exterior foam is not 51% of the total r-value as stated in “The Ratio Rule for Hybrid Roof Insulation”.
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Since you are not even close to the 50% ratio, the roof is asking for trouble.
The better solution is to reduce the R value of the batts and add more exterior rigid. Nothing wrong with good old R19 batts and about the same R value of rigid. Should get you pretty close to code U factor with any condensation risk and still be cheaper than the r49 roof.
You know, that r-19 could actually save me another $1000 to $1500. I guess I was just a little leery of using fiberglass batts on the exterior wall. My gut says to use some type of mineral wool, even though its about twice as much. But, maybe since I'm using above deck insulation my worries with the fiberglass are unfounded.
Your proposal wouldn't meet code because you also have to comply with the condensation control requirements of R806.5.
Most people just choose to comply with sentence 5.1.2, which requires at least R-25 of exterior foam in climate zone 6 (with the assumption that the total R-value is 49).
However, there is also the alternative in sentence 5.1.4 of using your average winter temperature to calculate the minimum ratio of exterior foam R-value to total R-value. If you are in an area of CZ 6 with warmer than average winters, then you might be close on the required ratio, but probably not there yet.
Thanks for this point! I had not been aware of R806.5.
So it seems if I can satisfy both the U-factor and condensation control requirements I could be in the clear.
After doing some research and calculations this is what I find.
Zone 6a, where I live, has an average daily temperature of 26.29 over the last 25 years for the coldest 3 months of the year. Not sure how far back they would require to go, but gives a good impression. With a described interior temperature of 68 I find the bottom of the roof sheathing would reach 50.48 based on Michael Maines comment here - https://www.greenbuildingadvisor.com/article/the-ratio-rule-for-hybrid-roof-insulation#comment-215875
Difference between outside and inside temp - 42
Above deck insulation percentage - 58%
58% x 42 = 24.36 add the outside temperature of 26.29 = 50.65
For me to achieve 58% above deck insulation I could use
2" EPS - R-7.7
2" Poly ISO - R-13.1
Deck
Rockwool R-15
This set up also achieves a U-factor of .026, which is the allowable maximum. This set up also would save me at least 15% or around $2300.
I guess my question would be with regards to the alternative in sentence 5.1.4 and how the Residential code calculates the temperature of the roof sheathing? Is there some sort of predetermined calculation my inspector would be looking for, or will I need to persuade them to accept the calculations based on the comments from Michael Maines?
You're doing the calculation correctly, but you might want to confirm that your building department is okay with your source for the weather data.
With an average winter temperature of 26°: (45° - 26°) / (68° - 26°) = 45%
So at least 45% of the R-value has to be on the exterior. For this calculation, you should use the R-value for aged polyiso, about R-11. Then you could still use an R-19 or R-21 batt.
You probably already know that some polyiso brands lose R-value in extreme cold. You can reduce that effect by making the EPS the outer layer, keeping the polyiso warmer.
Just yesterday I talked with a former client who has a similar assembly in CZ6a, but decided to ignore my detail for cost reasons and went with 38% of the R-value above the roof instead. We've had a fair amount of snow lately, which insulates at around R-1/in. That was enough to shift the dewpoint to above the roof and he was sharing his photos of large ice dams. He probably had condensation at the sheathing as well. Do not skimp on the ratio--it's a MINIMUM of 50% on the exterior.
There are two different code sections. You may well be meeting energy code requirements, but you have to refer to this section as well: https://codes.iccsafe.org/content/IRC2021P1/chapter-8-roof-ceiling-construction#IRC2021P1_Pt03_Ch08_SecR806.5. Unfortunately the R-values relate to previous code cycles' minimum required assembly R-value--i.e., in CZ6, R-49 for roofs--approximately 50%.
To expand on Frank D's comment, all closed-cell foam, including XPS and polyiso, lose R-value over time, and most brands of polyiso also lose R-value at low temperatures. When just trying to meet code thermal requirements, I have no issues with using manufacturers' idealized R-values, but when talking about long-term protection from moisture, I use conservative values. I would assume R-5.5/in for polyiso, with the outer inch at R-4.5/in. Every other type of insulation increases in R-value the colder it is, which is great, except it works against you in this situation, unless you use EPS or GPS on the exterior.