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Community and Q&A

Insulating my detached garage has me baffled!!

treeguy147 | Posted in General Questions on

Hey everyone! I’ve been learning a lot on this site, but haven’t been able to piece together a plan to address the project I’m working on.

I’m looking to insulate my detached garage, which previously had a mold issue. I’d like to use green materials, however, moisture and budget take priority.

Building Details
Detached Garage (35’x25’)
4c climate (marine)
1 story (plus “attic like” space)
No plumbing or gas, has electricity
45 degree roof pitch
15’ from soffit to roof line
Straight pitch roof, no valleys, hips
Soffits are not vented, however there are visible gaps between roof deck and soffits
Roof is vented

Ceilings used to have faced R19, no drywall, insulation was removed during mold remediation.

Rafters are 16 on center, 2×6, so not a lot of room there.

My primary concern is to keep the RH down since it is very moist where I live. I plan on having a dehumidifier, and may add heat later as I use in the space year round.

From my understanding there are two potential ways to approach this project, air sealing and using baffles. I don’t know what is the best method for my situation.

If using baffles… I would plan to add vents in the soffits, run baffles up to the roof ridge, fill the rest of the rafters with as much insulation as possible and cover with drywall.

If air sealing… Seal the roof vents, soffits and any other leaks. Then add rigid insulation to all exterior walls, seal insulation and cover with drywall.

I’d really like to know what approach would give the best results in keeping the building dry. Budget isn’t much, expensive materials aren’t realistic for me. I’d much appreciate being pointed in the right direction by folks who have understanding of the challenges that marine climates present.

Many Thanks!

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  1. Expert Member
    Michael Maines | | #1

    Treeguy (I like the screen name--I'm Wood Guy on other forums),
    Unless you're also in a windy area, venting the attic is usually the easiest and least expensive option. In Marine zone 4 you need R-49 insulation in your roof or ceiling, although there are some fine points that may allow reduced R-value (see N1102 here: In any case, you're not going to meet insulation requirements with cavity insulation in 2x6 rafters. You'll either need to add insulation above the roof deck, or extending below the existing rafters.

    In general, the best approach is to add insulation above the roof deck (typically foam) to create a fully insulated, unvented assembly, but if you're trying to keep costs down, my go-to approach is to use "gusseted rafters"--basically hanging 2x4s below the existing rafters, until you have a big enough cavity for the required insulation. With cellulose or fiberglass, that's 14", plus at least 1" for a vent baffle.

    Is it important to you for the attic to be accessible? The least expensive approach is to blow loose insulation into a sealed attic, but if you want the attic for storage or something else, you can use the gusseted rafter approach all the way to the ridge.

    More information here:

  2. treeguy147 | | #2

    Hey Wood Guy, thanks for your reply. I hadn't thought about extending the rafter space with gussets, great idea! I would like some some storage there in the attic. I've attached a shot of the attic like space. Also took a shot of the gaps between the roof deck and the soffits. I'm wondering if they would provide enough airflow (1/16" to 1/4" gaps) or if adding proper vents would be better.

    Thanks again!

  3. walta100 | | #3

    First you need to find the source of the moisture that feed the mold and stop it.

    Was the roof leaking?
    Is water coming thru the floor?

    Once you get the water out then you need vents near the peak. I like gable and soffits vents.
    Please take the time to read this

    Depending on your electric rates and moisture sources you could spend $100 a month running a dehumidifier and still grow mold in the garage.

    I see no point in insulating unconditioned spaces. If you want to warm the place up occasionally put in a ceiling and a big wood stove.

    If you just put it back the way it was the mold will come back.


  4. Expert Member
    Michael Maines | | #4

    Treeguy, you need more vent space than the daylight shown. The article Walter and I linked to has a lot of relevant information for you. I prefer ridge vents because they usually allow a lot more air flow than gable vents, but either can work if sized properly.

    I'm also curious if you have fixed the source of the mold. You hadn't asked that question, but there is no sense investing in energy upgrades if they will contribute to additional problems.

  5. treeguy147 | | #5

    Great points about the moisture source. I believe that there was a roof leak the previous owner addressed. It has a new roof and there are areas that have new plywood on the roof deck. The roof has ridge vents.

    With it being unconditioned and uninsulated, I took some readings in attic space during the warmer months and have gotten RH above 65%, which was consistent with outside RH. The RH here can be 100% for days, weeks sometimes. I need to do more testing to see how high it can get in garage this summer. In the mean time spending $100 a month to condition the space sounds like a bad idea. However, it isn't much use to me if it is moist enough in there for mold to grow.

    Thank you for the ideas and the link, this has given me a lot to chew on!

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