Replies

1. 
   user-2310254 | Feb 23, 2019 01:07pm | #1

   Joe:

   Martin addresses your question here: https://www.greenbuildingadvisor.com/article/how-to-build-an-insulated-cathedral-ceiling

   Here is the key text:

   "In a word, no — the code explicitly forbids this method. Cellulose can only be used in an unvented roof assembly if there is an adequate layer of rigid foam above the roof sheathing or an adequate layer of closed-cell spray foam under the roof sheathing. Cellulose alone won’t work.

   However, in some areas of the country, especially in the Northeast, insulation contractors have been dense-packing unvented rafter bays with cellulose for years. Because the method has deep roots in New England, many building inspectors accept such installations.

   If you’re building a new house, however, here’s my advice: if you want to insulate with cellulose, make it a ventilated roof by including ventilation channels under your roof sheathing. Leaving out the ventilation channels is risky."

2. 
   Peter Yost | Feb 23, 2019 01:10pm | #2

   Hi Joe -

   My first inclination when I saw your question was to suggest that you only hire a cellulose installation contractor that is 3rd-party certified and trained. But when I visited the Cellulose Insulation Manufacturers Association (CIMA) website, I was pretty surprised to learn:

   1. that this trade association (unlike the spray foam insulation industry) does not support or have a 3rd-party certification/training program for installers

   2. CIMA refers visitors to its website to hire members of the Insulation Contractors Association of America (ICAA), which is yet another insulation trade association that does not have 3rd-party certification and training.

   So, I would do three things:

   1. Use the pretty basic but still important CIMA guide to hiring an insulation installation contractor: https://www.cellulose.org/HomeOwners/QuestionsAskContractors.php.

   2. Don't take the lowest bidder: any insulation installation contractor is in a cut-throat business and the lowest bidder will most likely NOT result in a best-practice installation and effective insulation is all about high-quality installation.

   3. Hire the insulation contractor with the most regard for air sealing. MANY folks in the cellulose insulation industry don't have enough deference to air sealing because cellulose insulation is more airtight than other air-permeable insulations. But NOTE: while cellulose is more airtight than fiberglass, cellulose insulation is 50 times too air-permeable to be classified as an air barrier material.

   Hire the cellulose insulation contractor that understands what a blower door and blower door test results mean AND the one that itemizes all the air sealing they will accomplish BEFORE they install the cellulose.

   Peter

1. 
   joenorm | Feb 23, 2019 01:51pm | #3

   Sorry this was not clear but I am asking assuming a ventilated roof with an air gap

3. 
   Jon_R | Feb 23, 2019 03:15pm | #4

   You might also consider Johns Manville Spider, which can also be sprayed on before sheet rock is installed (and trimmed flush).

4. GBA Editor
   Martin Holladay | Feb 23, 2019 03:29pm | #5

   Joe,
   For some reason, everyone is misunderstanding your question. I think what you want to know is found in this article: "Will Thick Cellulose Cause Your Ceiling to Sag?"