Vented insulated roof assembly question
We built an apartment on the rear of our garage. There is a carport in front of the garage. We tore off the roof and built a higher pitch roof with a shed dormer on one side to maximize usable space upstairs. The upstairs space is approximately 24′ x 60′ above the carport/garage/apartment. At the time we only insulated and conditioned the apartment. Now I want to finish out the upstairs to use as a woodworking shop and would like to condition the space as well as the garage also. I’ll insulate the garage walls and upstairs walls and blow in insulation above the carport ceiling. I plan to insulate the roof to raise the thermal envelope and keep all systems inside conditioned space. For this reason, we built the new 12:12 pitch roof with 2×12 rafters to provide room to vent and insulate the space. I’m planning to install foam insulation board between the rafters to create a 2″ deep vented chute adjacent to the underside of the roof decking (roof is decked with radiant barrier OSB). After the foam insulation board I’m planning to fill the rest of the space with mineral wool insulation. Areas inside the shop space will be finished with drywall. Rafters outside the shop space (behind knee walls and in the attic) will have wood slats nailed perpendicularly across the rafters to secure the mineral wool insulation. I have an attached sketch of my intended insulation showing a cross section between rafters. Hoping that this provides decent venting for the roof deck from the eaves to the ridge vent. My questions concern any issues this approach may cause with regard to air and/or vapor control. Our house is in the Houston, Texas area with high humidity year round. I’ve never insulated a roof before and am not sure if this will cause any unintended problems primarily with water vapor accumulation. Thanks for any advice or information you can provide.
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User ...430,
Have you considered using vapor-diffusion ports instead? They are approved in climate zones 1, 2 and 3, and there are no moisture control issues with them in hot-humid areas. That would allow you to eliminate the baffles and completely fill the rafters with permeable insulation.
https://www.greenbuildingadvisor.com/article/vapor-diffusion-ports
https://buildingscience.com/documents/guides-and-manuals/gm-2101-guide-building-conditioned-unvented-attics-and-unconditioned
Thank you for the suggestion and the link. Looks like I've got some reading to do. The cases I've read so far seem to suggest insulating both the top and the underside of the roof decking. This roof is already constructed with roofing felt and composition shingles. Also the roof decking is OSB with a foil radiant barrier attached on one side. So I'm not sure if using installing the insulation all the way to the bottom of the roof deck will work in this case. It would negate the radiant barrier. Thanks again for the idea. I will look into it further.
In your detail you show XPS being used for the insulation shields. Do the shields not need to be vapor permeable in this situation?
Usually it doesn't make much difference if the baffles are vapor permeable or not, since small amounts of moisture can diffuse through the wood rafters. With the OP's drawing though, the vent channels are defined by 1" XPS, so there is essentially no exposed rafter on the interior -- everything is "boxed in" by the XPS. I would fur out the vent channels with either ripped plywood strips (BTW, ripping plywood is much less messy than ripping XPS), or 1x2s spaced off the sheathing a bit. With wood furring strips, there is still a way for small amounts of vapor to diffuse up into the vent channel when needed.
If moisture is a concern, then I'd switch over to my trusty 1/4" waferboard instead of the 2" XPS, and use thicker batts. If additional R value is still needed to get the assembly up to a given R value target, I'd use foil faced polyiso under the rafters behind the drywall. Tape the seams of that polyiso and now you have a vapor barrier that will limit how much moisture can get up into the roof assembly, with is an added plus for the interior-side polyiso in this case.
Bill