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Community and Q&A

Insulation help

John Matousek | Posted in Energy Efficiency and Durability on

I am planning to build a 32×40 garage, gambrel trusses with a bonus room in the second story. I also plan to have radiant floor heat in the slab and in the joists to the bonus room. I live in Mid-Michigan.

My question is what is the best way to insulate. I would like to spray foam a portion of the building (roof/ceiling) and conserve costs by going with batting on the walls. I am having 2×6 construction on the walls which will give me some extra room there.

Is spray foaming the roof deck and not venting the building the way to go? Or should I vent and insulate around the bonus room and the ceiling of the first floor? I have read that this can cause some moisture problems and that applying the spray foam to the roof deck is the way to go as it provides a moisture barrier and insulated directly from the outside temps, but will create a larger space to heat?

As always, your suggestions/experience is appreciated.


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  1. GBA Editor
    Martin Holladay | | #1

    You are located in Climate Zone 5 or Climate Zone 6. According to the 2009 IECC, the minimum R-value for wall insulation in these climate zones if R-20 (or R-13 between the studs IF you also include at least R-5 of exterior foam sheathing). So 2x6 walls don't give you any "extra room" -- they represent the minimum depth of wall framing unless you plan to install exterior rigid foam.

    There are many ways to insulate a ceiling, walls, and floor, and all of them can work -- as long as you include at least as much R-value as stipulated as the minimum value in the code, as long as the insulation is installed meticulously, and as long as you pay attention to air sealing. (In the case of fiberglass batts, that means that you need an air barrier on both sides of the insulation. For more information on fiberglass batts, see Installing Fiberglass Right.)

    Here is some information on creating insulated cathedral ceilings: How to Build an Insulated Cathedral Ceiling. The article includes information on unvented as well as vented cathedral ceilings.

    Most bonus rooms have cold floors, so you need to pay meticulous attention to floor insulation and the floor air barrier. Of course, if you plan to keep the garage heated to 70 degrees all winter, floor insulation is less important.

    I don't recommend using fiberglass batts to insulate a bonus room floor, unless the batt fully fills the joist bays, and unless you also install a continuous layer of rigid foam insulation across the underside of the joists.

  2. John Matousek | | #2

    I am thoroughly in zone 5.
    I plan to run another circuit of radiant to the floor of the bonus room. I am hoping that I don't have to insulate the floor of that room at all as I want to take advantage of the heat rising from that floor heat as well as the slab to keep that room comfortable.
    I am leaning toward insulating the roof deck with rigid spray foam, but am still unsure about the non venting. The articles you provided give some really good information, but I guess the fear of the unfamiliar is consuming me. I don't plan to heat the whole garage to 70 all winter, but will need to keep it above freezing (probably closer to 50deg). I'm not sure if this plays into my decision to vent or not to vent.
    As always, you advice is appreciated.

  3. GBA Editor
    Martin Holladay | | #3

    If you know that your garage will be cooler than your bonus room all winter long, then you definitely need to insulate the floor of your bonus room. And if you plan to include hydronic tubing for in-floor radiant heating under your bonus room subfloor, then your insulation installation quality needs to be very high.

  4. John Matousek | | #4

    My plan for the bonus room will be for limited use and will at most times be the same temp as the rest of the garage.

  5. Torsten Hansen | | #5

    Venting a gambrel roof is tricky. We have insulated a goodly number of buildings like yours with spray foam in direct contact with the roof deck. It works well and our customers are pleased with the results. I completely agree with Martin's comment about fiberglass in the floor. If you go that route, air sealing the cavities is extremely important. Be sure to have your spray foam contractor seal the ends of the joist bays.

  6. John Matousek | | #6

    Thank you. I am getting to the point of deciding to spray foam the roof and not vent. I think this is the best way to go with the style of roof I am planning to install (gambrel),

    My next question is how thick do I need to spray the foam on for best performance? 2inches looks like the minimal, but 4 inches (getting me over the R20 mark) will provide the most condensation prevention. Thoughts?

  7. Aj Builder, Upstate NY Zone 6a | | #7

    John, 2" of closed or 4" of open

    How are you getting the rest of the R in your assembly?

    DIY or contractor?

  8. John Matousek | | #8

    I'll contract out the spray foam. I have a guy that is relatively reasonable and did a real nice job in my house remodel.
    I'll DIY for the rest of the R in the assembly. Probably supplement with batting with foam sheeting over it. Might see how the spray foam does on its own for a season.

  9. Aj Builder, Upstate NY Zone 6a | | #9

    Foam, fiberglass, foam is not the best. Good luck DIYing.

  10. John Matousek | | #10


  11. Darryl In Winnipeg | | #11

    Is foam, fiberglass, foam worse than foam - airspace - foam?

  12. Expert Member
    Michael Maines | | #12

    Darryl, the rule of thumb is that in any airspace greater than 3/4" or so you will get convective currents that negate any benefit of a dead air space--so yes, foam-airspace-foam is worse. At least with fiberglass in the middle you get a little resistance to air currents.

    John, I would suggest a thicker layer of spray foam, as much as 5 or 6 inches, and fill the remainder of the cavity with fiberglass (or even better, cellulose).

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