Hi Everyone!

I was wondering what would be a good recommendation for attic floor insulation for a non vented roof in the house and vented (decorative) roof in the garage? It is ICFed from bottom to top with regular trusses for roof.

Cost wise I was thinking cellulose for an R50 approximately 12 inches I believe to get that R rating but if it doesn't compare to polyurethane then maybe we should do that.

Penny for your thoughts? :)

THanks in advance,

Ken

If I read your question correctly you are thinking about not venting the attic but still putting insulation on the ceiling, right? If so, that's exactly what you don't want to do. The only time you would have an unvented attic is if you're insulating the roof deck thus bringing the attic space into the conditioned envelope. If you want to insulate the ceiling then you must vent the attic.

Please clarify and get back to us.

Hi DM,

Thanks for the reply. It is an unvented roof so we are intending to put it on the attic floor. The garage we will put in the ceiling.

Ken

I'm still confused. The attic floor is the ceiling for the house, isn't it? What do you mean exactly by unvented roof? Usually we talk in terms of vented and unvented attic in which case the roof deck is the upper surface of the attic space. An unvented roof would normally refer to something like a cathedral ceiling where the ceiling is so close to the roof deck there is no attic. There are two approaches to the roof deck, insulated and uninsulated. If it's insulated you need to have enough insulation so the temperature of the underside surface of the roof deck stays close to the outdoor air temp. In this case the attic is not vented but it is conditioned with house air. If the roof deck is uninsulated then you have to keep the attic air close to outdoor air temp to keep the underside surface close to outdoor air temp. This means the attic has to be vented with outdoor air. What you have to avoid is having warm air contact the underside of the roof deck. That causes moisture to condense on it and that in turn is big trouble.

I see you are a new poster and I'm just wondering if you're using terminology in a way that isn't usual.

Hi DM,

Yes I'm new here and just as confused but I was refering to a cathedral style roof. Hence it is an unvented attic.

I'm assuming then that I can spray the attic floor but I think my concern what would be best used and economical.

Hope this clears up some more confusion

Thanks,

Ken

Now I'm confused!

Vented attic w/floor insulation:







Vented cathedral ceiling insulation:







A cathedral ceiling is a finished sloped ceiling above conditioned space. Being condtioned space, you wouldn't be insulating the floor.  If it's not conditioned space, but unvented, the "safest" approach is to use spray foam on the underside of the roof deck, or rigid foam & nailer  layer above the roof deck.

Depending on your actual climate, you may/may-not be able to get away with insulating the attic floor.  Another climate-dependent alternative would be to dense-pack cellulose against the roof deck, but you may need some amount of exterior side rigid insulation in colder climates:



....or....



Where are you?

In general, blown insulation will outperform fiber batts due to minimal voids. At high-R cellulose can be pretty hefty.  At 12" settled depth you're talking 1.6-1.8lbs/ft, which can represent a significant dead-load for long spans & trusses. Be sure it's rated for it.

In the unvented attic, insulate the roof deck, but be sure it's done appropriately for your climate.

Hi Dana,

Thanks again for your reply. I am in Canada, Montreal to be precise so it can get pretty cold.

Sorry for the mix up guys but yes it will be a vented cathedral ceiling. It's a newbe thing.

It's mainly whether to go cellulose or polyurathene or mix the 2 for the best insulation. The diagrams were very helpful. Trying to be as economical as I can while making it as airtight as possible.

Ken