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Thread: Insulating the roof of an old detached workshop/garage

  1. #1

    Insulating the roof of an old detached workshop/garage

    I've got a detached garage workshop. I want insulate the roof a bit to help with occasionally heating it up in the winter to make it more comfortable to work inside. I'm in Denver, CO where it's usually nice enough during the day in the winter to work out there (inside of the garage in the high 50s), but it gets colder in the nights evenings. Right now, uninsulated, I have 2, 110v heaters in there. One radiator, one regular. With that, if it's in the 30s outside, it's in the high 50s inside after running the heaters for a bit. I'm upgrading that to a since 220v 6000w heater, which gets installed tomorrow. That alone should make a difference.

    Here is a picture of the roof: https://bit.ly/3icL4My The roofing is directly on those planks and there are some janky, off square 2x6's screwed in to the stick built joists. I want to reinforce those a bit, and and make them even, but that's a little beside the point.

    How do I better insulate this thing? I want it to be better than nothing, but in no way will this be a conditioned space. The only goal is to make it retain the 6000w generated heat a bit more, to make running that heater more efficient.


    I got some advice about just installing some R13 batts up there, but since it's a unvented space, I was told there could be condensation issues. Is that something to even worry about? I'm in a very dry climate, and I won't be heating the inside more than a 30 degree differential, and at most, it'll be 10% of the time in the winter only. My current plan is to install the fully encapsulated R13 batts and leave a bit of space above them, since those are 2x6 studs. In case condensation forms, it has room to dry out, and the fully encapsulated batts won't absorb it.


    Are there any other/better options here? For around similar price points as R13 batt ($350 for this size roof)? Is this something to even worry about because of the very occasional heating of the space? Spray foam is not an option because of coast. It would be 4x the price of the R13 batts.

    Thanks!

  2. #2
    Join Date
    Nov 2013
    Location
    Waterford, PA
    Posts
    1,225
    Without knowing the square footage, I don't know if paper-faced ISO board (Commercial roofing insulation) is a comparable price point. That said, I'd consider 2" thick ISO board applied to the bottom on the roof rafters on the lower portion and the bottom of the collar ties in the center. That will give you an R 12.4 package with greater air volume between the insulation and roof decking which should alleviate/reduce any condensation issues. You can seal gaps between sheets or around penetrations with the spray foam in a can. Our old workshop was insulated this way and it worked very well.
    Last edited by Lisa Starr; 01-22-2021 at 12:56 PM. Reason: Corrected typo

  3. #3
    Join Date
    May 2018
    Location
    Lancaster, Ohio
    Posts
    1,346
    my 20x20 garage has 6" of fiberglass between trusses and then 1" foil faced foam board on inside. definitely makes a difference, 2kw wall heater
    first place I had the two story barn was built in 1913 and visqueen plastic stretched on the underside of the rafters helped alot, had a wood stove
    Ron

  4. #4
    Join Date
    Mar 2003
    Location
    SE PA - Central Bucks County
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    65,688
    Personally...I'd go right to closed cell spray foam. You don't have venting and the bottom of the deck is right there. Closed cell spray foam will provide R 7+ per inch, provide a superior vapor and air infiltration barrier and provide significant additional structural rigidity. Yes, the cost is a little higher, but it will make things much more pleasant in the winter when you are heating and also do the same in the summer, even if the space isn't conditioned.
    --

    The most expensive tool is the one you buy "cheaply" and often...

  5. #5
    Thanks for the replies. For the record, it's a 450sq ft space. Right at 20x22.

    I do really like the idea of the 2" thick boards applied on the bottom of the roof rafters. The 5" of space between the roof insulation would certainly get rid of any condensation issues, especially in this climate. I'll price these out and check them out compared to the equivalent encapsulated fiberglass. Thanks!

    Jim, I appreciate the closed cell spray foam advice. I looked at doing DIY and it was going to be around $1500 to get 2" of it DIY and $2500+ to get it professionally done. I forgot to mention in my original post, that this is not our forever home. We're going to be here for probably 2-3 more years before we make a move into something much more longer term, so I can't justify $2000 vs $350 for this.

  6. #6
    Join Date
    Mar 2003
    Location
    SE PA - Central Bucks County
    Posts
    65,688
    That makes economic sense, Tom.
    --

    The most expensive tool is the one you buy "cheaply" and often...

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