Attic insulation upgrade
MONTANARED
| Posted in Green Building Techniques on
I’m struggling with getting the correct answer on how to proceed with upgrading my attic insulation.
I live in Pittsburgh PA climate Zone 5A.
My existing attic space has batt insulation on the floor with no air barrier and is vented at the gable ends.
I would like to spray foam on the underneath of the roof deck sheathing in-between the truss joists and on the gable end walls to meet the required R38.
I’m struggling to understand if I should use open cell foam or closed cell foam.
Please let me know your thoughts. I would also eliminate the current gable end vents in the roof. Maybe I need to add an air exchanger to the space?
Thanks!
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Replies
Forrest,
Either closed-cell spray foam or open-cell spray foam can work. Open-cell spray foam is more environmentally friendly. However, open-cell spray foam must be detailed carefully to avoid moisture problems in the roof sheathing. That may require the installation of interior gypsum wallboard covered with vapor retarder paint.
For more information on these issues, see these two articles:
Creating a Conditioned Attic
Open-Cell Spray Foam and Damp Roof Sheathing
Forrest,
I couldn't tell from your post exactly what your goal is for the attic. It sounds like you currently have a vented attic, with insulation on the floor. Do you currently use the attic for living space? (Doesn't sound like it.) Is the intent of switching to a conditioned, unvented attic to create living space? If you don't intent to live up there, and you don't have ductwork in your attic (they don't do that in PA, do they?), it's probably a lot cheaper to air seal the attic floor and then add a bunch of blown insulation (probably cellulose) on top.
Of course, if you specifically want a conditioned attic, follow Martin's advice.
Martin and Andrew thanks for your responses.
Martin to help clarify to install the gypsum board with a vapor retarder would be almost impossible in this space, at least to do it successfully. That is why I really think it should be closed -cell foam because you get the vapor barrier.
Andrew I currently have a vented attic with insulation on the floor you are correct. We use the attic space for storage, it has removable plywood floor. I want to create conditioned attic space because I believe it will be better insulated and should help reduce heat gain during the summer. There is not duct work or plumbing in the attic. Since we use the attic space for storage we cannot air seal from below and fill up with cellulose.
Forrest,
Before you have closed-cell spray foam installed between your rafters, you should check with your local building department to ascertain whether local building codes require the foam to be protected with a layer of gypsum drywall. Some local codes require such a layer; others don't.
Martin thanks for the comment. County would require the foam to be fire protected and I would plan on using a spray applied fire stop product that is approved. This is really only a requirement when you use the attic space or have mechanical equipment in the space when using spray foam.
R38 of closed cell foam between rafters is an environmental disaster, and the assembly would under-perform R38 implemented as open cell foam due to the shorter thermal bridging.
For fire-code control many locations would allow intumescent paint sprayed directly on the foam. For moisture control, a layer of smart vapor retarder (Intello Plus, Certainteed MemBrain) outperforms vapor-barrier latex on wallboard. MemBrain is a sheet of 2-mil nylon, and if exposed still meets fire code without a thermal barrier. But the foam would still need the intumescent paint, since MemBrain is not a thermal barrier. While it's better to cover it to limit damage if the space is being used, it's not a code requirement. A smart vapor retarder is downright cheap compared to the higher cost/lower performance of doing it with 6-7" of closed cell foam.
Closed cell foam would have to be installed in at least 3 lifts, maybe 4 (2" max per lift), with a cooling period between. With open cell foam you can get to R38 with two lifts of 5"- 5.5" each with a cooling period between lifts. Taking short cuts like deeper lifts or skipping the cooling period has both shrinkage & adhesion issues, and a short-term fire hazard issue, since these materials generate heat while curing.
Open cell foam uses about half the polymer per R as closed cell foam, and it's blown with water instead of HFC245fa, which has a global warming potential ~1000x CO2. Until the manufacturers switch to HFO1234_ _ variants for blowing agents (less than 5x CO2), the blowing agent issue alone is reason enough to design it out of the stackup whenever you can.
Almost all closed cell foam in the US is currently blown with HFC245fa, under a variety of trade names. Icynene makes a couple of ~R5/inch ~2lb foam products that are water blown that would be appropriate (MD-R-200 and MD-R-210), and the small regional player Aloha Energy in NY has a range of 1.8lb water blown foams at about R6/inch. At R38 either of the Icynene water blown ~2lb products are sufficiently low permeance to be fully protective of the roof deck.
Dana, thanks for all of your information, I had to read it twice, it was so detailed!
I think what I gathered from your response was, that an open cell foam application with a smart vapor retarder such as CertainTeed is a really good option to go away from a closed cell foam application. I think this could be very successful, assuming the install of the smart vapor retarder is done with no flaws. The risk is that the tight spaces don't get sealed correctly with the smart vapor retarder and then moisture is introduced.
Your other option you suggest using a closed cell foam, Icynene water blown product, so I will need to see if anyone in the area is using that product.
I'm fearful of using the open cell foam because of many of the roof deck issues I've read about. I don't want to create a roof deck rotting issue and if the space did not have to be accessed I would to a flash and bib system using the closed cell foam.
You can fill the attic floor with R50 blown -in after air-sealing. Build a storage platform area before insulating and insulate the best you can under and around the storage platform area.