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Community and Q&A

Wall prep for radiant heating

newenglandwest | Posted in General Questions on

I have a leaky old Cape that I’m converting from electric heat to hydronic, and I have 2 areas that I’m curious about how to go about prepping the walls for radiant heat. I’m located in Zone 5, Connecticut.

1. Uninsulated garage wall
The walls are 2×4 with 1/2″ ply and cement fiber siding on the exterior side.  Tar paper in between sheathing and siding.  I plan to utilize radiant heating in the wall for heat… no sense adding it to an uninsulated slab.  I’ve sealed the framing/sheathing well, and added R15 rockwool to the stud bays.  I plan to add a layer of 2″ rigid foam… probably polyiso… to the studs and then start building up the pex tubing assembly.  If I use 2″ thick foam for this last substrate layer, will the wall assembly give me the necessary oomph to push the heat back out into the garage rather than to the cold side?  Note that the wall in question is to the Northwest and very shaded.  The garage will become a frequently used woodworking shop with lower thermostat settings… typically just enough to keep things above freezing, with low 60’s being the norm when in use.

2. Attic knee wall
I’ve insulated and sealed the attic crawlspace side… R15 rockwool, 1″ rigid foam.  I’ve blocked and sealed the joist bays under the knee wall and the rafter bays above.  My question here is whether I should also add additional rigid foam over the drywall to the interior before I begin building up the radiant heating assembly?  Would I be better off adding additional foam to the exterior side instead?  This difference would basically be one of convenience… as you might imagine, I’d prefer to work on the ‘civilized’ side of things, rather than crawling around in the attic!  That said, if it’s worth doing, it’s worth doing well.  Interested in your thoughts.  I’m also curious as to whether I’ll be creating any moisture/vapor issues by having multiple layers of drywall… from the outside in, the layers will be 1″ foam, R-15 rockwool, 1/2″ drywall, 2″ foam(?), grooved/gapped 1/2″ OSB with pex in aluminum transfer plates, 1/2″ drywall.

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  1. gusfhb | | #1

    I would go with a panel radiator in the garage, unless by garage you mean room with couch, TV and overhead door. Waste of time and money I think

    This comes from someone who has built and owned a radiant floor heated garage. But it was constructed as such, and I work on cars as a hobby.

    I actually had to set the setback thermostat to turn the heat on once a day or so, because the boiler was in the garage and it never got cold enough to turn the regular heat on.

  2. Expert Member
    Akos | | #2

    Your garage wall is around R27.

    Assuming you are looking at around 10000BTU of heat for the garage, that would be around 500sqft of wall that will run about 90F. In my area (zone 5) of 2F design temperature, you loosing (90-2)*500/27 (detaT*sqft/Rvalue) so around 1700BTU so almost 20% of your heat input to the great outdoors. Part of that would be the heat loss from the wall even if not heated but still not the most efficient way to heat a place.

    For you knee wall you can do a similar calculation to figure out the best R value.

    As for the location of the rigid, provided the exterior 1" rigid is vapor permeable (unfaced EPS/GPS or felt faced polyiso), you should be fine with adding rigid to warm side. If it is faced, it becomes a bit more problematic as you can create a vapor barrier sandwich. These can be made to work but not ideal, you would be better adding extra rigid to the exterior.

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