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Where to create thermal envelope in attic while using open-cell foam in rafters?

hifiaudio2 | Posted in General Questions on

I am having foam put into my attic and turning it into an unvented attic space this week. My home has a slightly odd envelope situation where I have an adjoining 1 car garage that is having its attic space foamed, but the roofline for that attic also slightly extends over to cover an entrance hall and laundry room of my house. The foaming company wants to seal off that attic so that it does not communicate with the rest of the house, but that would put that hall and laundry room into that area while the rest of the house is below the “other” sealed attic. That laundry and hall does have insulation above it, but I was under the impression that I *want* the house to communicate with the attic once the attic is sealed.

Is there a correct either/or answer as to whether to allow these two attics to communicate?

I suppose I could have them rig up some studs where the hall and laundry room stop and create a thermal wall in that spot so that only the attic above the single car garage is on its own.

I *do* have plans at some point to install a mini split for that garage and condition it and the above attic, but as I do not know the costs yet, I have no idea how far down the line that would be done.


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  1. hifiaudio2 | | #1

    Here is a pic with an outline of the attic in red over the house plan

  2. GBA Editor
    Martin Holladay | | #2

    I don't recommend that you include your garage within your home's thermal envelope. (After all, that means that the car exhaust and the fumes from stored gasoline are now within your home's thermal envelope. It also means that you need to insulate your garage slab and your garage walls -- have you done that?)

    So, you definitely need a stud wall in the attic that separates your home from your garage.

  3. hifiaudio2 | | #3

    Ok thanks. The slab wouldnt be insulated in any way that I know, but the walls are. No cars or gas in this one car garage... but your point is taken.. I dont know what may get stored in there in the future.

  4. hifiaudio2 | | #4

    The vented attic that currently connects with the rest of the attic does not have any roof vents in it ... only soffit vents. If I cut off the rest of the house from that attic but leave that attic vented, do I need to add roof vents to that single car garage attic?

  5. GBA Editor
    Martin Holladay | | #5

    I'm confused. How many attics do you have? I thought that you were converting your attic from a vented unconditioned attic to an unvented conditioned attic.

  6. hifiaudio2 | | #6

    Sorry for the bad explanation.. The "main" house attic is getting foamed... but if I choose to create a new false foam wall where the laundry meets the garage in the picture above, then what is left is a vented garage attic that is all by itself, with no roof vents, only soffit vents. I assume that will let air in with no place for the hot air to escape. So that is why I am wondering if I should put a roof vent in that attic above the garage once this is all done.

  7. GBA Editor
    Martin Holladay | | #7

    If you choose to install spray foam on the underside of the roof sheathing above your garage, there is no reason to vent the attic.

    On the other hand, if you want a vented attic above your garage, don't install any spray foam in the attic above your garage. It's your choice.

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