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Workshop/garage — Unvented attic & insulation questions

user-795783 | Posted in General Questions on

I live in a mixed-dry area (Lubbock TX). Technically I am within Zone 3B but from a practical standpoint, I am right on the border of Zone 3B & 4B. We average about 18” of rain a year but it’s been much less in recent years.

I am building a detached workshop/garage. The building will be L-shaped with a hip roof. It will be fully insulated as well as heated and cooled. I’ve done some research on the subject of unvented attics but most articles are focused on cold, hot humid, or hot dry climates. I’ve learned just enough to be dangerous but it seems like an unvented attic could work in this mixed-dry climate.

I’ve chosen mineral wool insulation for the walls and would like to use it in the attic as well. The attic will be a fairly low volume attic. The attic will not be used for storage but it will contain HVAC ducts if I can build an unvented attic.

Questions Assuming an Unvented Attic:

1) Can I insulate with mineral wool batts in the rafter bays and in direct contact with the roof deck?

2) Can I split the insulation between the rafter bays and the ceiling? The rafter bays aren’t deep enough for R30 mineral wool batts. I could fairly easily put R15 below the roof deck and another R15 between the ceiling joists. Are there any issues with this approach other than that the HVAC ducts would see more heat gain than if the entire R30 was under the roof? The HVAC ducts will be insulated.

3) How concerned do I need to be with air tightness in this attic? Should I seal the numerous ceiling boxes and the various wire penetrations through the top plate as I would have with a vented attic? Or would it be best to allow some conditioned air to end up in the attic?

4) The rafters are 24” O.C. and the ceiling joists are 16” O.C. Although I haven’t checked everywhere yet, it doesn’t look like mineral wool is locally available in 24” widths without special ordering. Assuming I could split the insulation between ceiling plane and roof, would there be anything wrong with using the mineral wool batts for the ceiling and then using R19 fiberglass batts for the roof? If this works, does it make any difference if the fiberglass is kraft faced or not?

Thanks for the help. It is much appreciated.

DC

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    D.C.,
    Q. "Can I insulate with mineral wool batts in the rafter bays and in direct contact with the roof deck?"

    A. No. That would be a code violation, and you would risk problems with moisture accumulation in your sheathing. If you want an unvented roof assembly, you need to either install rigid foam above the roof sheathing, or spray foam insulation under the roof sheathing. For more information on this issue, see Creating a Conditioned Attic and also How to Build an Insulated Cathedral Ceiling.

    Q. "Can I split the insulation between the rafter bays and the ceiling? The rafter bays aren’t deep enough for R-30 mineral wool batts. I could fairly easily put R-15 below the roof deck and another R-15 between the ceiling joists. Are there any issues with this approach other than that the HVAC ducts would see more heat gain than if the entire R30 was under the roof?"

    A. As far as I know, your local building code official can still insist that either your roof assembly or your attic floor must have enough insulation to meet minimum code requirements. There is no provision in the code to add up the R-values of these two different assemblies.

    Q. "How concerned do I need to be with air tightness in this attic? Should I seal the numerous ceiling boxes and the various wire penetrations through the top plate as I would have with a vented attic? Or would it be best to allow some conditioned air to end up in the attic?"

    A. You have to be concerned with the airtightness of your building's thermal envelope. If you move the thermal envelope to the roof assembly (the plane of the rafters), then that is the assembly that needs to be airtight. If you do that, then you don't have to seal leaks in your attic floor.

    Q. "The rafters are 24” O.C. and the ceiling joists are 16” O.C. Although I haven’t checked everywhere yet, it doesn’t look like mineral wool is locally available in 24” widths without special ordering. Assuming I could split the insulation between ceiling plane and roof ..."

    A. You can't.

  2. user-795783 | | #2

    Thanks. I'll just go with a vented attic. I thought that might be the answer but I wanted to see if the thoughts on unvented attics had evolved any since those articles you linked were first written. In this climate, we can get away with things other areas can't such as bathroom exhausts being ducted into attic space so I just wanted to make sure of my plan.

    But that brings up one more question. After price checking, it is going to be less costly to put two layers of R15 on the attic floor as compared to 1 layer of R30. I plan to use flexible ducts from the main plenum to the supply vents. R6 ducts are readily available and I can get R8 ducts for not much more cost if it would help. Given my mixed-dry climate, can I sandwich my air ducts between the two layers of mineral wool?

    DC

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