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Will have a new project underway soon and would like to ask about my proposal for a modified insulation package

user-7028257 | Posted in Energy Efficiency and Durability on

Will have a new project underway soon and would like to ask about my proposal for a modified insulation package. Just read Martin’s Musing on ‘Walls with Interior Rigid Foam” posted 3/27/15 and I have joined as a member of GBA today. Here is my description.

I would like to add and cover the interior side of wall stud framing with reflective foil, rigid insulation of likely 1/2″ thickness and the interior side of roof rafters with the same foil faced rigid insulation except at 2 inches thick for the roof. Thermax is one brand I am aware of at present. I would then strap (1 x 3) wall studs and roof rafters to receive sheetrock and or boarding of some type. This method seems to be ok for wall assembly but in Martin’s response to Steve of 4/2/15 he cautions against installing “interior rigid foam on an unvented cathedral ceiling” etc.

My cathedral ceiling proposed would be filled with the two types of foam in bays as described, which is basically an unvented cathedral roof. My understanding is with the foam insulation the venting is not possible nor is it required. My further understanding is that if the bays were to be insulated and filled with conventional fiberglass batts that venting ( proper vents, soffit & ridge vents ) must be used to keep the roof ‘cold’ and allow air to pass under the roof deck, thus venting the escaping ‘warm air or vapor’ from the roof assembly to the outside. I assume I did not misunderstand Martin’s response to say that if one adds 2″ of rigid foam to a cathedral ceiling at the interior side (with foam insulation in my case in all bays ) that one must ventilate that particular roof as is common with fiberglass cathedral roofs.
I note I have further homework as there is a further link…see How To Build an Insulated Cathedral Ceiling.

Thanks for considering my questions.

Kevin Begley, V.H. MA.


  1. Expert Member
    Dana Dorsett | | #1

    Seems you tried to stuff too much into the title. What's missing?

  2. GBA Editor
    Martin Holladay | | #2

    I'm going to shorten your question. Let me know if I got it right.

    Q. "I'm building an unvented roof assembly with spray foam between the rafters. Is it OK to install continuous rigid foam on the interior side of the rafters?"

    A. Yes.

    That said, you're choosing an expensive approach. You didn't mention whether you want to install open-cell spray foam or closed-cell spray foam between your rafters, but these two choices tend to be less "green" and more expensive than other insulation choices.

  3. user-7028257 | | #3

    Thanks Martin. Yes you have it right. Yes Dana, that was my initial mistake, so when it appeared on the site it seams to have been edited immediately and with it whatever clarity there may have been to my overall description & question.

    So edited out was the plan to 'flash & fill' as I have heard it referred to...specifically first spray foam the recommended inches (3-4) of closed cell foam in stud & rafter bays followed by netting and topping off the bay with open cell ( cellulose) bringing the second fill out to the netting. Then placing either 1/2" or 1" foil faced rigid over the 2 x 6 wall framing and placing 2" foil faced rigid over the interior face of rafters, presently planned as 2 x 10" or maybe 2 x 12" for more insulation, of the cathedral ceiling. Strap over rigid insulation with 1x3" in order to install sheetrock and or boarding for rooms interior finish.

    Homeowner is pushing for maximum insulation. My thinking was to add the rigid foam ( regardless of cost ) to boost the overall R-factor and maybe create a bit of 'thermal break' between backside sheetrock surface and interior stud or rafter face by virtue of the seperation of framing to finish wall surface and the approximate 3/4" air space behind the sheetrock via the strapping.

    Perhaps this project can pass the town' 'stretch code' insulation requirements with all fiberglass batts alone and or with the addition of the rigid. The HERS rater involved may not like it regardless of 'passibility' factor.

    Lastly pleading ignorance here Martin, but my impression was the less 'green' use of foam would be balanced out over time with energy savings via the well insulated structure. This thought given to buzz out there that fiberglass insulation is becoming a thing of the past. I very much appreciate your time and interest and look forward to your reply.

    Thanks again,

  4. GBA Editor
    Martin Holladay | | #4

    With flash-and-batt (or flash-and-fill) you have to pay attention to the ratio between the closed-cell spray foam layer (in your case, about R-19 to R-20) and the total R-value of the entire assembly, which in your case depends on the thickness of the cellulose and the thickness of the interior rigid foam.

    Here are links to two articles that discuss safe ratios:

    Flash-and-Batt Insulation

    Combining Exterior Rigid Foam With Fluffy Insulation

    If you install R-19 of closed-cell spray foam, your total roof R-value must be no more than R-27 if you live in Zone 8, or no more than R-31 if you live in Zone 7, or no more than R-37 if you live in Zone 6, or no more than R-46 if you live in Zone 5, or no more than R-61 if you live in Zone 4A or 4B.

  5. brendanalbano | | #5

    If you go for flash-and-batt (or flash-and-fill), see if you can specify a closed-cell spray-foam that uses the new Honeywell Solstice blowing agent. Two options are Lapolla Foam-lok 2000 4G and Demilec Heatlok HFO High Lift. While they are still petroleum products with a much higher embodied energy than cellulose or fiberglass, the blowing agents in these foams has a vastly lower global warming potential than conventional closed-cell blowing agents.

    Regarding if "the less 'green' use of foam would be balanced out over time with energy savings via the well insulated structure" it's a matter of some debate I believe, but on a pure R-value/energy saving basis, the blowing agents in standard closed-cell spray foam are so bad that they will basically never balance out. The big caveat is that sometimes closed-cell spray foam solves a problem that no other insulation can, and using it sparingly (like in a flash-and-batt/fill approach) is definitely the best use for it if it can't be avoided. And if you can use one of the new honeywell solstice blown foams, that's even better.

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