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When insulating a portion of a structure such as an addition or re-roof, how can code compliance via the performance analysis method (ref MEC 402.2) be demonstrated without doing a whole-house analysis?

user-912539 | Posted in Building Code Questions on

Architect and code official are obsessed with the prescriptive method. I want to use a solid foam nailbase system with better overall performance and a thinner profile. How can I satisfy 402.2 for a small area of the roof without doing a full structure modeling analysis?

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  1. bByFhBnMZ7 | | #1

    if you are using a product that has an R-value less than the perscriptive values given for your area then the alternative is to use a program such as ResCheck to prove that the overall R-value of the building is sufficient. If the R-value of all other components is up to minimum requirements and the area you are insulating with solid foam exceeds minimum requirements and meets flame spread and smoke production standards then there should be no problem.

  2. GBA Editor
    Martin Holladay | | #2

    I'm wondering whether your claim that the nailbase product has "better overall performance" is based on the fact that it has a higher R-value than the prescriptive code requirements, as Bruce assumes, or whether you claim of better performance is based on some other (possible unscientific) claim -- especially since you note that the product has "a thinner profile."

    In other words, your architect and code official may both be right -- especially if the product you want to use has an R-value that is less than the minimum prescriptive code requirement.

  3. user-912539 | | #3

    I've been building with SIPs and foam panels for 20+ years. I routinely see full envelopes with a "rated" R-value below the prescriptive minimum yet they perform significantly above code based on both modelling and actual energy consumption. Case in point; a recent build in WV used r-24 wall panels and r-32 roof panels. It pulled a [email protected] ach and scored a 49 HERs. The modelling proves it's well beyond the energy performance requirements. However,if I take that structure (Or portion of that structure) into areas where a Prescriptive minimum of r-38 is mandated, I have to revert back to performance modelling.(such as Res check)
    I'm very confident that a thinner solid foam roof will work for the aesthetics of the roof line just as I'm sure it will perform far better than the conventionally framed and batt insulated alternative.
    My question is how can we provide performance modelling on JUST a roof? I don't want to spend the time and money to complete a full envelope model when I'm only offering a single component as an option.
    Now before you say "every home should be fully modeled", I agree. However, the client and the code official want to see proof of compliance before the commitment to a "better performing" option is selected. The solid foam option can arguably provide a significant reduction in the ach number and a roof that is void of any thermal bridging. Two items that have real impact on energy performance but no impact on the prescriptive method of code acceptance.
    Thanks for picking this one up after a month with no reply. I was beginning to wonder if I should re-post.

  4. GBA Editor
    Martin Holladay | | #4

    You wrote, "I routinely see full envelopes with a rated R-value below the prescriptive minimum." You've come here for advice for how to continue this practice, but you may not get the advice you expect. I'm not in favor of your approach. I think every homeowner deserves a thermal envelope with at least the minimum R-value required by the prescriptive code.

    If you submit an energy modeling program that shows that the energy performance of the entire envelope is at least equal to the energy performance of a house of the same size built to the prescriptive code, you can qualify under the performance path of most building codes. But since you've already admitted that you are skimping on the R-value of the roof, you'll only succeed if you provide above-code components elsewhere in the house -- better windows than the code minimum, for example.

    The code requirements for the performance path are there for a reason. If you want to follow the performance path, bite the bullet and hire an energy consultant to do the energy modeling that is required for that path. Otherwise, you are short-changing your customers.

  5. user-912539 | | #5

    Martin, I appreciate your response, but I assure you Skimping and short-changing are not part of the equation. The basis of my question pertains to the fact that the prescriptive method does not allow for air-leakage or thermal bridging. Full scale performance modelling does account for items such as bridging and air-tightness. You and many others here at GBA preach the supreme importance of sealing air leaks. I couldn't agree more with that first step approach to energy efficiency. In the case of a solid foam system that minimizes air leakage and eliminates thermal bridging, the prescriptive method penalizes the system. A system that can and has shown its ability to outperform a code-minimum system and still do so with a lower "rated" r-value.
    Allow me to restate my question: Can the performance method of code compliance be used on a portion of a structure (such as the roof) without doing a complete Res-check or similar model?

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Can the performance method of code compliance be used on a portion of a structure (such as the roof) without doing a complete Res-check or similar model?"

    A. No.

    Al, I don't buy your justification that you can build a house that performs better than a code minimum house that is leaky. So what? The leaky house is in violation of the code -- it just so happens that inspectors aren't enforcing existing air sealing requirements. Just because you can find a badly built house that is worse than yours, doesn't make it right to build houses with low R-value roof assemblies.

    For more information on this topic, see It’s OK to Skimp On Insulation, Icynene Says.

  7. homedesign | | #7

    Do you consider the Nailbase to be part of the air control layer?
    If so... how are you "connecting" the panels together?

    Are you including a fully adhered membrane or some other air control layer below the nailbase?

  8. user-912539 | | #8

    The system is absolutely part of the air control layer. How I seal and specify joint conditions is a function of location and conditions under the panel. The panel alone is typically sealed with a two component expanding foam and splined together at all joints with an OSB or plywood strip 3-1/2" wide. If I wanted to expand on air-control and install a vapor barrier to the inside, a peel and stick can be applied below the panels. If, and only if, I'm in the deep south would I install a peel and stick over the top of the nailbase panels. Personally, I'm a big fan of letting the continuous foam acting as the vapor barrier (assuming it's thick enough) and allowing the assembly to dry in either direction. If additional air-control is warranted, I will use a vapor permeable tape.

  9. homedesign | | #9

    I admit that I have never used nailbase forgive my ignorance
    I am still trying to visualize the Air Control Layer in your "Nailbase System"
    Do you have a link to the product you are using? or any photos?
    I noticed a comment in Lstiburek's Insight-036 that makes me think it is impossible to make the bottom side of the nailbase airtight(without including a fully adhered membrane below)

  10. GBA Editor
    Martin Holladay | | #10

    1. Joe Lstiburek is talking about nailbase installed over a perforated fluted metal deck, not plywood or OSB. Big difference.

    2. If one is installing nailbase over existing roof sheathing, the sheathing seams could be taped. That would be less expensive and a fully-adhered peel-and-stick membrane. Or one could use synthetic roofing underlayment; if the underlayment seams were taped, I imagine that it would work as an adequate air barrier.

  11. homedesign | | #11

    I considered the fluted deck...
    But Joe also made a similar comment about his Office/Barn Roof in the same Insight 0-36 on Page 1

    And again in Insight 056 ...
    when it was time to replace his 15 year old roof ...
    He stresses the importance of the fully adhered membrane located below the rigid insulation.

  12. GBA Editor
    Martin Holladay | | #12

    Peel-and-stick membrane is the classic recommended air barrier for PERSIST roofs and walls, and the system works very well. It's foolproof.

    Obviously, though, peel-and-stick membrane isn't such a good idea when there is interior fluffy insulation in the rafter bays or stud bays.

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