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Community and Q&A

Insulating Room With ICF Walls and Open-Web Truss Floors

tippsand | Posted in Energy Efficiency and Durability on

Hi –

This is in Nashville TN, in a 30×30 garage with a full size bonus over top unconditioned garage.  The walls are ICF to the eaves, the bonus subfloor and ICF wall intersection was sprayed 3″ closed cell foam.  The trusses are 24″ deep, and the entire garage is drywalled.

The insulation contractor believed that spray foam solves every problem, and decided we didn’t need other insulation before the drywall was installed in the garage (required by code over trusses and ICF foam).

The bonus room is noticeably cooler in the winter and warmer in the summer.  The rest of the house (also ICF) is pretty thermally amazing, but the 900sf bonus room is a giant temperature differential that sucks.

How do I beef up the insulation for the bonus room now that we’re in the house?  (Thanks for not commenting on how steel beams, LVL-and-joists, or other after-the-fact design change would make my life better. That ship has sailed.)

Blown in fiberglass (drill holes, let it settle on the drywall ceiling)?  Blown cellulose?  Rigid board on the exposed side of the garage ceiling?  More foam?  Cut an access scuttle and try to roll batts?


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  1. charlie_sullivan | | #1

    I think blown in insulation, blown through holes, will work well. I think the choice of material to blow in should be driven by the preferred material of the best installer you can find.

    1. tippsand | | #2

      Thanks - that was my hope. Your opinion on the "insulation must touch the warm side" issue? (ie, the conventional wisdom holds that insulation needs to be in contact with the bonus floor in this situation, but that's gonna be impossible due to the depth of the cavity.)

      I've read that if the bonus floor is air-sealed (it is, with closed cell), then the added insulation can sorta be anywhere that's convenient. Any thoughts?

  2. Expert Member
    Michael Maines | | #3

    Sandy, 3" of closed cell foam will perform at about R-15 to R-18, not quite up to the R-19 IRC requirement for floors in your climate zone but not terrible either. What are your wall and roof/ceiling assemblies? How is the bonus room heated and cooled?

    1. tippsand | | #4

      We have a full ICF home (2 stories plus full basement, 6" poured concrete, 2.5" foam either side, from basement to eaves). The roof deck is sprayed nominal 6" open cell. The bonus over the garage isn't really separate from the main house, so it shares the same geothermal forced air, not zoned. The bonus floor / garage ceiling is the one area where I trusted the insulation contractor against my better judgment - he said we'd only need 3" of closed cell and nothing else.

      3" of closed cell foam may well be close to code, but there is a noticeable thermal difference in the bonus room once you leave the cozy thermal envelope of the "main house". Maybe it wouldn't be as significant if the rest of the house wasn't so comfortable, but I am 100% convinced that it's because the bonus has approx R18 in the floor over an unconditioned (currently cold) space versus a (functional) R50 in the walls and a conditioned attic. R18 in the bonus floor cannot defend against the ambient temperature in the void below - the garage is 30x30 with 12' ceilings. That's a lot of cold air (or hot air, in the summer).

      I'm not fighting air intrusion or humidity. Just an under-performing space relative to the rest of the house. The only difference I see in the bonus versus the rest of the house is the bonus is sited over 1000sf of unconditioned space versus the rest of the house being conditioned.

      1. Expert Member
        Michael Maines | | #5

        Thanks for the additional information. I don't doubt your experience, just trying to get the full picture. R-18 floors over open air will definitely feel colder or warmer in extreme temperatures than floors over a conditioned basement, but in your zone the difference in heat loss (or gain) shouldn't be significant. Colder or warmer surfaces will change how you feel in the space, though, through radiant heat loss or gain directly from your body to the floor.

        If you only have 6" of open cell in your roof, that's a red flag. The IRC requires R-49 for your zone but 6" of open cell is R-21 to R-24. Because roofs are open to the sky they experience greater heat loss than bonus room floors with enclosed garages below, which moderate temperature and air flow. I think you would find it more effective to add insulation to the ceiling than to the floor.

        HVAC distribution is often not even, regardless of the heat source. Is the bonus room at the end of a long run serving other rooms?

        It sounds like you are not going to be dissuaded from your opinion about the floor; if not, your options are to fill the cavity with dense-packed blown-in insulation, or to remove the drywall and install batts or more spray foam. Adding a few R's won't make a big difference; here in zone 6 (Maine) code minimum is R-30 but I usually aim for at least R-40. Spray foam has a larger carbon footprint than most other insulation; the blowing agents in conventional closed cell spray foam are potent, persistent greenhouse gasses. Cellulose is much greener, and fiberglass or mineral wool have lower impact than foam.

        One more note about insulation levels: ICF manufacturers often use deceptive marketing regarding R-value; you probably have R-20 to R-30 walls, unless extra rigid insulation was added. Thermal mass does not have as much impact as many people think, especially in a relatively mild climate like yours.

        1. tippsand | | #6

          Appreciate the detailed response.

          6" open cell on the roof deck was all that anyone insulation contractor would spray when we built in 2009. And you're right that it is insufficient, even though code when we built was R38. But I don't know of an easy way to upgrade that, given that the attic is open-web trusses, 24" oc. The webs and members are fully foamed, and there's no easy way to add additional insulation, since I don't know how I'd secure it (batts - no staple surface and not enough member depth anyways; loose fill - nothing to hold it in place against the foam). I've looked at the Owens Corning netted system, but that needs a rafter to staple to, and the rafters are buried.

          So I agree - there IS a thermal issue in the roof deck, but the easier problem to address might be the floor, especially since the floors feel cold and the ceiling of the bonus doesn't. In fact, the attic is very close to the same temperature as the house, whereas the garage isn't - ever.

          Regarding HVAC, the upstairs handler is basically centered in the house, so the bonus actually gets great flow, and has it's own separate return. (There are a few other returns in the other "cut up" parts of the upstairs.) It's a WaterFurnace 5 series matched system, variable speed, and puts out a lot of air when it's running on high fan (which is never really does).

          Every custom home has stuff that could be improved a second time.... we tried to put our money into structure and envelope, but missed in a few places, despite having what we thought was the best available advice. In 2009, building this way was a real challenge in the south, since performance-oriented homes weren't even on most contractors' radars. So now, I'm trying to address the stuff that might be done with minor annoyance versus major reconstruction.

          (If you've got some clever suggestions on how to fix the roof deck / attic insulation stuff, I'm all ears. There is nothing on the attic floor - I *have* considered blowing loose insulation up there, and then adding a return in the attic to address moisture. There's a 4" supply up there already.)

          Thanks for the help! Maine is cold - I grew up in Toronto. I get it.

          1. Expert Member
            Michael Maines | | #7

            The building science field has grown rapidly over the last 20 years. In 2009 this was all new to many of us! You did great, considering the information available at the time.

            Have you had a blower door test done? You may have more air leakage at the roof than you think. Just an idea.

            Return air vents can often make a space feel cool, due to air flow.

            For the roof, the most practical approach is probably to just deal with what you have until the roof needs to be replaced. That's a good time to go over the roof with additional insulation. There are various ways to do that, and there will be more by the time your roof needs to be replaced.

  3. logicalIthink | | #8

    If you want a quick and economical to implement solution, a ductless single exchanger unit should take care of the needed warmth and cool. Those units are very efficient and because it can be operated as and when needed its economical to operate as well! A tiled floor in that room would contribute to the feeling the heat and cold as well. So if possible, an insulating layer under high quality padding and a nice thick carpet will also assist in thermal comfort is my guess.

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