HELP! Suspended slab radiant heating insulation question

CWirickJanuary 23, 2012

We are doing hydronic radiant on both floors of a new rambler, 1900 sq ft per floor. In the basement we put down 2" foam board insulation under the tubing before pouring the slab (with the sides insulated as well), so we should be good to go on the basement level.

On the main floor the tubing is laid atop the plywood subfloor, which will be covered with 1.5" gypcrete (suspended slab). The question is, do I need to insulate between the floor joists below the subfloor even though the basement level is also heated? I assume every bit of insulation helps, but from a cost/performance standpoint, would filling between the floor joists with fiberglass batting make enough of a difference to pay for itself, or would the improvement be minimal?

If this were a staple-up installation this would be a no-brainer, but with a suspended slab above a heated basement I didn't know if the in-floor insulation would be cost-effective. Any advice appreciated!

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fsq4cw

The improvement may only be incremental. It is recommended, or suggested, for both maximum efficiency and for peace of mind.

SR

    Bookmark   January 24, 2012 at 11:39AM
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CWirick

So which approach is best? Reflective foil? Fiberglass batting?

    Bookmark   January 24, 2012 at 1:20PM
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ionized_gw

The question of "do you need to" would better be "will it pay off" as I see you understand. The answer has to be that it depends. If both the basement and main floor to be used a lot and heated at the same time, insulation between them will have little benefit. The added insulation will have the most benefit if you heat the basement a lot when the upstairs cooler. It will have some benefit if you are heating the upstairs when not heating the basement.

You are really going to have to look at your projected use and what temp you will keep "unused" areas set back to. It could be the case that the heat that leaks by even with no insulation will pretty much serve to keep the area at the level that you want so no heat calls are necessary in that zone. If you add insulation, you might be running the circulator in the cooler area when you might not be otherwise.

    Bookmark   January 24, 2012 at 1:48PM
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CWirick

Yes, both floors of the home will be heated equally and at the same time. Kids bedrooms are in the basement, master bedroom is on the main floor. So I have no need to shield either from colder temperatures in the other. I just wonder how much of my energy is being wasted by heating the dead air between the floor joists. An insignificant amount? I figure it will cost me $600-800 (maybe a little more) to insulate between all the floor joists, and that is with me doing the work myself. Is that money I'll never see again in energy savings?

    Bookmark   January 24, 2012 at 2:03PM
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ionized_gw

Then don't bother with insulation between floors unless you need it for the quiet. Make sure that the area between the foundation and the floor (rim joist/sill plate) is well-sealed and insulated and be done with it.

This is, however, a difficult area to handle. In cold climates, there is often a lot of condensation and decay there because it has not been given enough attention. Good gaskets or sealants durning construction are essential and spray-foam may be the best material. See http://www.buildingscience.com for help.

    Bookmark   January 24, 2012 at 7:20PM
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