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Exterior rigid insulation for attached garage?

Lyle_N | Posted in General Questions on

I am building a house in 6A/7A.  I am going with 3 inches of EPS exterior ridged foam and dense pack in the walls.  My question is…  I will have a heated 3ish stall garage attached to the house. It covers part of the front and one whole side.  I originally decided to put exterior insulation around the garage but now rethinking it.  Is the cost and complexity of the exterior insulation worth it for a garage (garage doors, windows, exterior doors)?  It will be heated on average around 40 degrees Fahrenheit (4 Celsius) most of the winter. With garage doors and vehicles going in and out… thermal bridging is not a major source of heat loss.  Or am I off pace and put exterior insulation on?  Let me know your thoughts. Thanks

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    Your decision depends on your performance goals and your budget. If you decide to install 3 inches of exterior rigid foam on your heated 3-car garage, then (a) your garage will have much better wall insulation than most garages, and (b) your heating costs will be lower than most homeowners with a heated 3-car garage.

    Is it worth it? Hard to say. Probably not, from a financial perspective (if you are expecting the fuel savings to justify the installation cost in a reasonable amount of time) -- but the answer to that question requires math, and depends on (a) the installed cost of the rigid foam, (b) your fuel cost, and (c) your payback time frame.

    1. Lyle_N | | #2

      Thanks Martin!! My name is Lyle.

      Just to give you a quick background. I am my own general contractor for my house. The main reason is your site and finehomebuilding. I meet with many local builders and they are not big into building science (build a house the same way for 20 years). I wanted better quality and more robust house, since I hope to live in the house for 20 years.
      I have had to learn with many of my subs along the way, but I think it has been worth it so far. Your info and site have been invaluable in this process for showing the subs to getting the inspectors on board. I hear a lot of “I have never seen anything like this before. And you are well educated for a guy in the career of information technology”. All the credit I give to you, greenbuildingadvisor and finehomebuilding.

      After that nice long intro…..

      As far as my garage goes. It is a 2x4 Zips wall, 11ft ceiling and radiant floor heating (water and fuel type natural gas, which is very affordable around here). Since wrapping your house in foam is not common here…. The only way I could get most subs to install is by the hour, which can be a rabbit hole. I plan on doing what I can after work, but day are getting shorter here in the North and will probably hiring most of it out. If I insulate the cavity I can do it after the electrical is installed (on my own time).

      Is there issues if I wrap one side of the garage in insulation but not the other? The reason I ask is the wall with the garage doors. After you put the doors in there is not much area between them 2ft and about 3ft on top of them. That is pretty much the whole wall.
      And along those lines… If I don’t’ wrap the garage in insulation. On the back side it is one big wall garage and house. If I wrap the house, but not the garage there will be a spot where the sheeting will be at a condensing point (I would guess and the garage front corner as well). I am fairly confident that I can keep the WRB tied together making the transitions, between wrapped and none wrapped insulation. But am I asking for failure at the transition points? Any info is greatly appreciated.

      Thanks Lyle

  2. Lyle_N | | #3

    Any additional thoughts on this?

    Thanks Lyle

  3. user-2642926 | | #4

    I can't answer your technical questions, but I can say that even in my Southern California location, having the rigid foam on the exterior of my garage makes for a very stable environment in the garage, both during winter and summer.

    If you have any plans to do things in the garage - car guy, woodworking, etc - it's quite nice to have a well-insulated garage.

    Given that you likely have very few door or window penetrations to worry about, it seems to me the labor for rigid foam for the garage shouldn't be too crazy, and it does end those concerns about condensation issues.

    1. Lyle_N | | #5

      Thanks Mike.... I am a DIY guy and enjoy working in my garage. I will probably do it.

      For a building science perspective..... I am curios about the condensation part... Starting and stopping exterior ridged foam on a house/garage. If it can be successfully done or not, and best practice if it can.

      Martin/Others do you have any thoughts on this?

  4. jaccen | | #6

    IMO, pay attention to air sealing the garage door. Note that if you do a decent job of air sealing, air pollutants are a concern (ie. carbon monoxide).

    1. Lyle_N | | #8

      Hello Jaccen,
      Thanks for the reply... I do have two vents that are going to be timed with the garage door opener and run for a bit after the garage door has been closed to help with pollutants. A perfect air tight garage is great as an energy stand point, but not always the best for occupants (considering the stuff people store in them).

  5. GBA Editor
    Martin Holladay | | #7

    If one wall of your garage is insulated, and another wall is uninsulated, your garage will not stay as warm in winter, or as cool in summer, as it would if all the walls were insulated. Other than that, there is no particular problem with leaving a wall uninsulated.

  6. Lyle_N | | #9

    Thanks Martin,
    I guess I should have explained my wall a little better (sorry about that). I do still plan on insulating my wall on the inside (dense pack).

    It comes to the all interior walls are dense pack insulated and only part of the same wall is insulated on the exterior with 3 inches. The question I have is, where I stop on that continuous wall will that be an issue? Or is there enough heat transfer through the sheathing that my condensation point will be on the non insulated part of the wall.
    thanks Lyle

  7. lance_p | | #10

    Lyle, I'm not sure what climate zone you're in, but I've researched the "minimally heated" garage issue here before and come to these conclusions:

    1. I plan to put INTERIOR continuous foam in my garage. I will separate the slab from the foundation with 2" foam and continue that foam layer all the way up to the ceiling. The foam will be foil faced and serve as the vapor retarder once taped. The 2x8 framed walls will sit flush with the interior side of the foundation and will be stuffed with cellulose or fiberglass batts, sheathed on the outside with taped/sealed OSB. Horizontal 1x3 strapping will hold the foam in place and provide mounting for drywall.

    2. With a well insulated shell and doors that don't leak too much, a garage here in CZ6A (Ottawa Ontario) should stay above freezing on its own if there's no insulation below the slab. This is a trade-off as I will have a small workshop area actively heated when in use, and with a relatively cool slab will take more energy to heat. The rest of the garage should stay warm enough to keep snow melted off cars etc. It should also help keep the garage cool in summer.

    3. Drawback #1 is the potential for summer time condensation on the cool slab. This shouldn't be too much of a concern except for metal objects sitting on the floor. Keeping metal things elevated may be necessary.

    4. Drawback #2 is the potential for slab cracking should a door get left open in the winter and the ground below the slab freezes. Diligence with doors will be a must!

  8. GBA Editor
    Martin Holladay | | #11

    There are no potential moisture issues associated with a change in insulation strategy for half of your garage walls.

    Half of your garage can have walls with dense-packed cellulose plus exterior rigid foam, and the other half of your garage can have walls with dense-packed cellulose and no exterior rigid foam -- and there will be no moisture issues or building science problems associated with the transition between these two approaches.

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