How to insulate my new garage...

[Dberglind]

Closing on a house with a 24x30 detached garage in September.

I have been doing a little research on how to insulate the 24x30.

The walls are insulated, but no roof insulation, so is not very usable in the winter here in Minnesota. It has soffit and roof vents.

My primary question: Is there a way to insulate the roof deck itself to make the rafter space usable space? (Kind of like cathedral ceiling houses?) If it isn't to cost prohibitive, I would like to make that space usable for storage.

I know I need to leave an air gap from the soffit to vent up to the roof vents. Could I attach foam-board to the bottom of the roof deck boards? Is that 3.5" enough of an air gap?

Thanks for any info.

 

[James-W]

If all you want is cold storage up on top, then I would put in a ceiling and insulate it to keep the heat down in the garage. Do you have a need for a heated storage area?

 

[DC73]

My primary question: Is there a way to insulate the roof deck itself to make the rafter space usable space? (Kind of like cathedral ceiling houses?) If it isn't to cost prohibitive, I would like to make that space usable for storage.

I know I need to leave an air gap from the soffit to vent up to the roof vents. Could I attach foam-board to the bottom of the roof deck boards? Is that 3.5" enough of an air gap?

Thanks for any info.

If you want a conditioned attic, the best thing in your climate is to create an unvented attic by closing up the soffit and roof vents and using closed cell spray foam under the roof deck.

Any other insulation cannot be in contact with the roof deck (they let mositure get through and condense on the bottom of the roof deck) so you would need an air gap under the roof deck. You'll need at least 1" and some engineers recommend 2" but if you go this route, you really need to get the details correct to avoid moisture and mold issues. Different climates have different issues. I can get away with construction details in my arid climate that would cause problems for you in your climate.

For more information, go to BuildingScience.com and GreenBuildingAdvisor.com. Both have articles on vented and unvented attics and the proper ways to insulate a cathedral ceiling.

The least costly option is to leave the attic vented and blow in insulation on the floor of the attic. You can easily do this yourself. Home Depot and Lowe's will both loan you a blower machine with the purchase of enough blow-in insulation.

DC

 

[DC73]

Here's a good article from BuildingScience.com written by Joe Lstiburek that was just published and emailed to me today. It does not apply (yet) to the OP due to his climate but it gives those of us in the south an option to put various insulation products in contact with the roof deck while dealing with the moisture issue.

http://buildingscience.com/documents/insights/bsi-088-venting-vapor?utm_source=Building+Science+Corporation+List&utm_campaign=35739b2ea6-BSC+Newsletter+Issue+%2380&utm_medium=email&utm_term=0_194890bc8c-35739b2ea6-63891105#foot05

To briefly summarize the article, you would construct a continuous ridge vent (to include the top 4' of any hip roof). Over the cut opening, you would apply a waterproof membrane that is vapor permeable (lets water vapor escape but keeps water itself from penetrating). Then put the ridge vent and shingles on. This gives the inevitable moisture a place to go. Joe also recommends a 50 cfm per 1000 sq. ft. of ceiling area supply vent to condition the attic and help remove moisture. I can see that need in the humid south but it wouldn't surprise me if those of us in arid regions can forgo the supply vent.

DC