As I understand it most radiant installations require about an R10 insulation between heated floors. What is a good work around when my ductwork will be in the joist cavity? If a room has 25-30% of the cavities filled with ductwork is it worth it to even add insulation to the rest? Thank you in advance brian

whatever you can do is better than nothing. I'd want to know that the downward loss wasn't going to cause a problem though, before I had the system installed. it *probably* isn't, but if you had a high water temp requirement anyway, it could push you higher than you want to go. or in the case of geo, higher than you can go, perhaps.

Making the joist cavity air-tight is just as important as the insulation, which is specified by first determining the heat load for the areas heated (or unheated), the design water temperature and the floor covering. Once again, their are few useful "rules of thumb".


DIY design, always the first mistake.

How do you plan to get the tubing in the joist if the duct work is occupying the space? I would suggest you use Low-E Reflective Foil Insulation which looks like metalic bubble wrap. It is very thin and flexible and may allow you insulate the space even with the duct work in there. I would suggest putting the insulation in separating the ductwork form the radiant tubing and alum. plates so that you are not heating the ductwork with the radiant heat. I think sacrificing the air space in the joists with ductwork is a better compromise than not insulation at all.

the reflective insulation is a supplement to, not a replacement for actual insulation.

Posted By NRT.Rob on 03 Mar 2010 09:30 PM
the reflective insulation is a supplement to, not a replacement for actual insulation.

... and is nearly useless for plated installations, since the emissivity of the aluminum plates are so low.  Only the heated wood subfloor has enough emissivity area to to matter, and the fractional surface area of the exposed wood in a plated system is low compared to staple-ups and suspended tube.