Insulation for 1:12 standing seam metal roof
Newbie to GBA but we are interested in building an efficient home. We live on the edge of zones 4 and 5 in Illinois. 100% of the exterior will be EIFS. The plan is for our exterior walls to be 2×6 with cellulose in the cavity and rigid foam over the sheathing.
My question is about how to ventilate our standing seam metal roof (1:12 pitch). Our architect didn’t seem to have a good grasp on green building techniques.
My questions to architect:
Should we use deep trusses for our roof? 12 inches of insulation then have an air gap of 6 inches between the top of the insulation and the roof sheathing? Then having ventilation in the soffit or fascia at a height that would be within the 6 inch air gap (above the insulation). Do we need ventilation in the center of the roof (like a cupola)?
What type of insulation 12″ thick are you going to get that won’t make the drywall ceiling sag? Maybe batt? If batt at 12″ you can get around R38 in the cavity. With 6″ air you will need to vent. I would only vent on ends OR top and bottom to cause cross ventilation. I would not vent in center. If there is no unconditioned space (stale air), there is no need to vent. So you could spray insulation directly to the bottom of the roof deck. 1″ rigid supposedly means R5 to R7. You will need to spray around 6″. This is hard to accurately measure. In this case, the strength of the roof members shall dictate the height for the will be greater than 6″. I think either would be fine. But I feel the spray insulation is a better insulation. On my home we have the same situation. We did the spray foam without ventilation. I think some contractors thought this was a bad idea because of lack of ventilation, but why ventilate when there is no air to ventilate. And on steel buildings we insulate directly under the roof panels and do not ventilate. There should be no difference.
Ok now this is me again. My question for all of you …. what do you feel is the best approach to insulating and ventilating our roof? Sorry, all of this is new to me and I’m trying to learn and make the best decision for our home. Thanks to all in advance!
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