Dew point in deep cavity walls & Spray Cellulose
Home located in central mountains of Idaho @ 5,000 ft elevation.
To curb possible condensation problems & air-seal In a double-stud wall cavity, I was planning on spraying 2lb closed cell foam 3.5″ thick then spraying remaining 6.5″ of cavity with cellulose for a total r-value of around 44.
…then a reputable local insulation company that does all kinds of spray foams as well as cellulose had this to say: “with spray cellulose that has an adhesive, and with our climate & wind loads, the cellulose air-seals the home & there is no condensation in the wall cavity, and no other sealing of seams is necessary.” He is trying to save me money but said he’ll spray as much foam on as i would like …so I’m wondering if I do a 12″ deep cavity instead of the planed 10″ cavity for a comparable r-value that is completely filled with cellulose.
My concern is that in the 12″ deep cavity, that there would be condensation on the cold interior side surface of the exterior sheathing if use all cellulose for the air-sealing as well as insulation. This company’s pamphlet states that it is 14x tighter then National Energy Star, 5x tighter than Northwest Energy Star, and 3x-14x tighter than LEED requirements.
advise regarding my trust issues?
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Replies
Eric,
Many people have been debating your question; the jury is still out.
My personal opinion: as long as you don't use OSB sheathing (instead, use plywood, diagonal board sheathing, or fiberboard), and as long as you include a vented rainscreen gap between your siding and your sheathing, you don't have to worry about this problem in a double-stud wall insulated entirely with cellulose.
Here's an article that discusses the question: How Risky Is Cold OSB Wall Sheathing?
Thanks Martin. What are your thoughts about the claim of the spray cellulose being used to make the home air-tight & not need any taping of seams, etc.?
Eric, there is lots you can do to tighten up a home. I agree with Martin and your cellulose use. Next you have to pay attention to all the other joints for air leaks and install some Panasonic bath fans of your choice to vent with if you did all my way.
Eric,
Dense-packed cellulose slows down air flow, but it is not an air barrier. You still need to pay attention to air tightness. That means that your entire thermal envelope needs an air barrier material (for example, gypsum drywall or plywood), and that all seams and penetrations need to be sealed.
again, thank you. If I went with spraying the foam layer first, then I would not have to worry about sealing all other penetrations, correct? e.g. electrical boxes, etc... One advantage that I can think of if I spray foam first, then I could work through the winter months doing all the electrical & plumbing & have an insulated building to R-21 before finishing the insulation with spray cellulose & drywall....but just not sure if the cost justifies it all...also thinking that the 3.5" of foam will add some to structural integrity of the wall?
The spray foam will add greatly to your plans. But you still need to seal under sills etc... the more you seal the better. The spray foam costs extra money but will perform. Sealing all the rest costs extra time but is well worth the effort in new or existing construction.
And yes the spray foam will make that part of your home bullet proof. But, to really add strength next you would have to anchor the super strong walls completely to the foundation and to the roof using the same materials used in hurricane and high wind construction. You could do this, not that hard to do. Where you are building should have an exposure to wind noted, whether through an engineer or checking with your building permit office.
thanks AJ