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2″ or even 4″ of foam on exterior of garage/workshop in zone 7

Tim Brown | Posted in Energy Efficiency and Durability on

I am just about to put siding on my 24 x 30 garage/workshop. It is 2 x 6 construction (16 centers) with R50 blown cellulose in the ceiling and R20 batts in the wall cavities.

I’m adding 4″ foam of exterior foam vertically along the 12 ” thickened edge slab below grade (as deep as 48″ if the rock below will allow… some places it will be closer to 24″) in an effort to keep the slab from robbing all the heat.

It has three windows (tri-pane double hung vinyl)..two on the east and one on the west, the man door is on the south side and two 7 x 9 insulated garage doors are on the north side.

It occurred to me that there is little if any insulation in the headers above the overhead doors and man door. Up till now it has been unheated…. I want to heat it to about 40-45f to keep my tools rust free continuously and bump it up to about 60f when I’m working (wood work) in the dead of a Canadian winter (zone 7 Kenora Ontario).

I have a lot of extra EPS foam and could if it makes sense either…

1-add 2″-4″ (or more) of foam to the INTERIOR of the walls over the OSB (interior of garage is OSB not drywall) to deal with these substantial thermal bridges or

2- using way more material add either 2″ or even 4″ of exterior foam to the entire garage before siding it. The problem here is I would have to furr out around the windows and doors to allow for the foam,,,essentially making them innies.

The building will be occupied about 10-20% of the time.

Option #1 is cheaper and easier…will it work in a garage?

Thanks

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Tim,
    Interior foam is easier (but not as elegant from a thermal perspective) as exterior foam.

    Interior foam will work -- but you will lose interior space.

    For more information, see Walls With Interior Rigid Foam.

  2. Tim Brown | | #2

    I was planning on only adding the foam over the headers (on the inside) for the OH doors, windows and man door.

    fyiw I have built insulated shutters (R8, 2 inches of EPS) that fit over the windows in the winter and am building stable style (side hinge) doors (insulated R24, 6 inches of EPS) that fit in front of the OH doors helping insulate and air seal them. They will (I hope) improve the look of the insulated sectional garage doors and add a country look to the garage.

  3. Tim Brown | | #3

    Martin..I read the article you mentioned...thanks for the link.

    It makes me revisit a question I have seen asked in the past... about adding interior insulation to a cathedral ceiling.. the cathedral ceiling is really just a sloped exterior wall,

    in my case my house has a rather poorly insulated cathedral ceiling on the east side (R40 in two layers of batt insulation) which is also poorly vented..if at all..it looks like the builder just stuffed the rafter bays full.
    My question is about adding insulation to the interior of that roof assembly...say 4 inches EPS (R16)...

    How does that differ from adding the same interior foam to a wall?

    If the condensation plane is interior can't it dry through the EPS?

    if its in the existing batt insulation, can't it dry through the roof like it always has?

    Thanks in advance.

  4. GBA Editor
    Martin Holladay | | #4

    Tim,
    If you have a poorly ventilated cathedral ceiling insulated with fiberglass batts in Climate Zone 7, then the roof sheathing above the fiberglass batts is at risk of moisture accumulation and rot. Considering that fact, I wouldn't advise adding any interior rigid foam until the riskiness of the existing situation is addressed.

    You either need to provide true soffit-to-ridge venting for every rafter bay -- something that may be difficult -- or you need to add an adequate amount of rigid foam above the roof sheathing to keep the sheathing warm and dry during the winter.

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