Trick question: What is the R-value of this roof?
user-901114
| Posted in General Questions on
As I look at this assembly, I wonder…. If there’s only 6.5ish inches of spray foam under the sloped ceiling, can they really call this an R-60 roof?? And then I think… Does it really matter?
It looks like the peak and knee walls are R-60. What’s the loss at this sloped ceiling? Is a sloped ceiling more like a wall than a flat ceiling and thus R-40 is enough? Or maybe I’m just not interpreting their drawing correctly.
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Replies
Stephen,
The way I read about the project (here: http://www.buildingscience.com/documents/case-studies/ma-jamaica-plain-ngrid-comprehensive-der-three-family-triple-decker/), it sounds like BSC is claiming that the house has an R-60 vented attic. I don't see any claim that the insulated sloped ceiling is R-60.
I looked for any reference describing the spray foam thickness, and I couldn't find it. You report that there is 6.5 inches of spray foam (plus one inch of polyiso), and you may be right... but the thickness of the spray foam might be more.
This was a retrofit job, with all of the usual constraints of any retrofit job -- not new construction.
Martin, I'm only guessing at the thickness by looking at the pics of the 2x8 rafter and the diagram of that section... And I can also guess as to why they had to keep the existing ceiling height. What I'm wondering how this effected the project overall.
Sometimes you seem to only point out the obvious in you're postings.
Stephen, Are you working on new construction or a retrofit?
Stephen,
I'm sorry that my answer disappointed you.
It looks like the one diagram indicates that they oversprayed the rafters to a greater than 2x8 depth. If they sprayed another 3-4 inches, they would hit their claimed number.
Windows on Washington - I thought the same until I realized that was only the space behind the knee walls.
Fred,
I know that Stephen hates it when I make an obvious statement, but I still think you're wrong about BSC's "claimed number." They only claimed R-60 for the insulation on the attic floor.
Martin, You raise an interesting point. Did they really insulate under the attic floor? I didn't notice that.
Stephen,
Here is the BSC description: "R-60 (nominal) vented attic."
(http://www.buildingscience.com/documents/case-studies/ma-jamaica-plain-ngrid-comprehensive-der-three-family-triple-decker/)
Here is the BSC illustration:
(http://www.buildingscience.com/documents/enclosures-that-work/retrofit-building-profiles/ma-jamaica-plain-ngrid-comprehensive-three-family-triple-decker-building-profile/)
Martin, Is there a different code for an attic ceiling or is it considered part of the roof?
Stephen,
I'm not sure I understand your question. The prescriptive path of the IRC energy code and the IECC stipulate minimum R-values for ceilings. Some ceilings are flat (for example, under a vented attic), and some ceilings are sloped (for example, under a cathedral ceiling). Whether the ceiling is sloped or flat, the same insulation requirements apply.
Martin, The attic living space ceiling in this project has both a sloped/cathedral (r40ish with thermal bridging) and a flat ceiling (r60) section
If the above were true and if the code called for an r49 ceiling, would this assembly would be out of compliance?
Stephen,
The house is located in Climate Zone 5. According to the 2009 IRC, the prescriptive code requirement for ceiling insulation in that zone is R-38. The code requirement does not consider thermal bridging, only the R-value of the insulation.
If the roof in question has 1 inch of polyiso (R-6.5), then the code requires that the spray foam insulation provide at least R-31.5. This can be accomplished with 5 inches of closed-cell spray foam.
If you wanted to exceed code requirements by installing R-49 of insulation, you would need to install 6 1/2 inches of closed-cell spray foam -- exactly as much as you estimate was installed at the house in question.
Martin, I agree this building is within code for zone 5, but it does not have an r60 attic ceiling since the sloped attic ceiling area needs to be included in this r rating. Do you agree?
Stephen,
I'm not sure what you mean. The attic floor is insulated, not the attic ceiling.
Is this a trick question? The attic is the open space above the collar ties. The attic has an R-60 floor. The vented, sloped portion of the roof appears to have a different R-value, and is coupled to the attic, but is not part of the attic space. Nobody's breaking any laws or stating anything that's incorrect.
Martin, I think I have all I need... Now I'm pointing out the obvious to put this beat horse down. This sloped ceiling area under the roof, in the top floor living space is not r60. They did not address thermal bridging and there is no mention of this as being a challenge or detriment to the project. I would not say this home has an r60 roof.
Stephen, nobody but you is claiming it's an R-60 roof.
Stephen,
You wrote, "I would not say this home has an R-60 roof."
I agree. And so does the Building Science Corporation. That's why they wrote that the house has an "R-60 (nominal) vented attic," not an R-60 roof.