High-performance insulation in cathedral ceiling
Claire Anderson
| Posted in Green Building Techniques on
I’d love to hear feedback on the most efficient and cost-effective way to insulate a cathedral ceiling constructed with scissor trusses (residential construction). I’m considering having the trusses built with 16″ or 20″ heels and blowing in cellulose to achieve the insulation levels desired. Another option could be spray-foam (closed cell). I’d love any feedback, i.e., pros/cons to each or a better method.
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It depends on where you are located but I'd say that you are likely to be better served with the closed cell foam than the cellulose in most geographical areas.
I agree with Michael on the selection of foam, although I tend to lean towards open cell over closed cell. There is a raging debate in the industry and between the open and closed cell manufacturers about this. My opinion (and it is just that) is that open cell provides you with a better opportunity to identify a roof leak before it causes significant damage as any bulk water will usually leak directly through the insulation into the attic or living space while closed cell is more likely to hide the leak or disguise the location as water moves down the framing and exits somewhere other than the location of the leak. Some closed cell companies market their product showing a video of a house insulated with open cell that soaked up water and became totally moldy, but I believe they insulated the house before the roof was on which is a stupid practice in any case.
I just joined this forum, so this response may be too late - but no one else answered your request for pros and cons.
Spray foam, if closed cell, offers more R per inch which makes it sometimes cost-effective for limited spaces. But it can be problematic on a roof assembly which typically cannot breathe to the outside and would require breathing to the inside to prevent moisture from being trapped in the sheathing. Open cell foam offers no R-value advantage and a cost disadvantage over cellulose. While it's open structure will allow roof leaks to eventually drain, it will also contain moisture against sheathing.
Studies by Rose, et al, indicate that attic ventilation has at least modest advantages in all but the most humid environments - in dissipating water vapor, in limiting roof and attic temperatures, in minimizing summer radiant heat gain, and in controlling ice dams.
Since your high-heeled scissors trusses have more than adequate insulation space, cellulose insulation would almost certainly be more cost-effective and may increase the livability and durability of the structure, particularly if there was a ventilation channel above it which could double as a secondary drainage plane in the event of a roof leak.
Cellulose is a far more environmentally-benign material than any kind of plastic foam, and also offers significant fire-retardant, rodent-retardant and insecticidal advantages, as well as significant sound attenuation.