There is a product specifically made for that. It fits between the rafters and has got channels that run from the peak down to the lower edge of the roof, creating air flow under the roof deck toward your ridge vent.

I'm sure someone here knows what it's called.

DO NOT FILL THE SPACE WITH INSULATION. I made this mistake, and I have a HOT upper room that I'm going to have to do over. Not looking forward to ripping out lighting, ceiling, insulation, and starting over.

 

I'm not sure where you live, so this advice may or maynot help. Ventilation is just as important...if not more important...during the winter months. Even with a vapor barrier some moisture will escape and condense on the interior of your roof deck. Ventilation will help this condensation evaporate and not create a mold/rot situation. Also, some heat will escape thru your insulation. Without adequate ventilation to remove this heat, it will melt snow on your roof and create ice dams at the eaves.

 

I guess I don't understand the reason to have a vent (ridge vent, soffit vents, etc.) if there isn't any space to vent....so why create a space between the roof decking and the insulation??

As Cabin Fever stated. And to prolong the life of the roof/shingles, etc.

 

What cabin fever said! I made that mistake on a cabin I built and had mold and some rot, ice dams, and degraded insulation performance due to the freezing moisture buildup in the insulation. Learned with my cabin mistake before I built my house and the 2 inch gap between the insulation and the roof deck works perfectly.

The things they make for that are called roof ventilation baffles.

 

The things they make for that are called roof ventilation baffles.

The ones i used were formed styrofoam, very light, cost about fifty cents each or something ridiculously cheap like that (each piece about 3ft long, and fits between 16" o.c.). At that price, you can't afford to not put them in...

 

I used spray foam, which they spray directly onto the osb decking. Seems to work pretty well, but I am only 2 years into to it now. We'll see. They have done a lot of them around here that way.

 

I'm in cold/hot, windy Northwest Ohio.

I was wanting to spray foam also. Won't that create an impermeable vapor barrier? I'm not sure how moisture will GET to the interior of the roof decking.

If I end up putting in vents (sigh), I was thinking of using foam boards with wooden spacers glued to the back of them, and then foaming around the outside to seal them into the 16" cavities. And then filling in the rest of the 10" rafters with blown in (densepack) cellulose (which would be on the living space side).

Bad idea?? Is that putting a vapor barrier on the OUTSIDE of the insulation?

All of this is giving me a huge headache......

CC

 

I'm in cold/hot, windy Northwest Ohio.

I was wanting to spray foam also. Won't that create an impermeable vapor barrier? I'm not sure how moisture will GET to the interior of the roof decking.

If I end up putting in vents (sigh), I was thinking of using foam boards with wooden spacers glued to the back of them, and then foaming around the outside to seal them into the 16" cavities. And then filling in the rest of the 10" rafters with blown in (densepack) cellulose (which would be on the living space side).

Bad idea?? Is that putting a vapor barrier on the OUTSIDE of the insulation?

All of this is giving me a huge headache......

CC

If you use the baffles, you don't need wood spacers, the shape of the baffle holds a channel for air. They also fit snug into the 16 inch cavity, so you don't need to foam them in. (the baffles go at the top of the cavity). Then you can blow in cellulose, and install a vapour barrier on the warm side of the cellulose.

If you were going with spray on foam, I would be tempted to put the baffles in and then spray the foam in after. You probably wouldn't need a vapour barrier then. I would check with the manufacturer to make sure though.

 

Hi,
Around here, its very common to use the polyurethane spray foam to completely fill the rafter cavities and have no ventilation above. The foam does not allow water vapor to pass through it, so you don't get condensation that would with FG or cellulose insulation as water vapor works its way out to a surface that is cold enough for it to condense. I think its a good way to go -- it gives very high R values because the R value per inch is high for this kind of foam, and you don't lose the depth of the ventilation cavity. Corbond is one of the brands of polyurethane foam.

You might want to post the same question on the Fine Homebuilding "Breaktime" forum -- lots of insulation and building pros there.
Gary

 

I understand the importance of letting the "attic" breath, but if I understand the original post there is no attic. If there's no attic then there's no need to make an air space. My brother had a house with a cathedral ceiling with no attic and it had fiberglass filling the entire cavity, just like a wall cavity all the way from the outside wall to the peak. 20+ years in that house and still the original shingles and no moisture issues.

If that's the case, no attic space, I'd fill it full of spray in foam.