Attic Ventilation questions/clarification

[Awitte58]

Just another insulation question. Garage and house.

So I read this: http://www.insulationman.com/vent-or-not-vent-question

I planned on having 2" of spray foam throughout the attic floor of my house and garage and then do bats with OSB for the walls. I like what I read about how it stops air infiltration etc. Then I would blow cellulose on top of that to increase the R value.
My roof is less than 3 months old and is vented up high and the soffits are vented as well.
This part concerns me if I go the sprayed route:
"Installing roof ventilating in a foam roof can lead to limited moisture problems because there will be days in spring and fall when snow is on the roof, but it is relatively high humidity outside with outside temperatures in the high 30’s to low 50’s. that air gets pulled into the roof vent space and condenses because the roof vent space is much colder due to the snow cover. The water runs out roof the vents and forms icicles in the soffit and/or it runs between the exterior finish and the wall sheathing. If it is trapped in the soffit or between the sheathing and finish, it can cause rot in those areas. Since the ventilation is not necessary either by code or science, and it can cause this rot phenomenon, we recommend you do not vent a foam roof."

Any recommendations or insight of what route to go?

 

[DC73]

Your question relates to spraying foam on the attic "floor". Their answer relates to spraying foam under the roof deck, i.e. the attic "ceiling". Two different animals to some degree.

I think you'll be fine with your plan but best to run it by the experts. Go to GreenBuildingAdvisor.com and post your question on the Q&A forum. The rest of the site had good info and articles related to your question as well.

Another good source of info is BuildingScience.com

Good luck,

DC

 

[djb25]

I'm not sure why a "foam roof" would be more likely to suffer that problem than any other vented roof.

The only difference I can think of would be the water would condense on the underside of the roof and then drip into your insulation, rather than running out the eaves. That's probably worse.

Although, it would seem that in the situation described, the vented roof wouldn't draw the warm air in. If the roof were snow-covered and the outside air was humid and warm, I would think the roof wouldn't vent much of anything.

 

[Awitte58]

Thanks for the responses and advice. I found a building insulation forum and asked it there. I got plenty of good feedback. Also someone pointed out spraying the entire attic floor isnt worth the cost because the areas between centers that isn't broken drywall doesn't leak air. Its more cost effective to seal the airs like top plates etc and then just do batts or blow cellulose.