DIY Hydronic Floor Heating

Exeric · 08-21-12, 04:29 PM

I think the biggest advantage to under subfloor pex (if you don't have extremely cold winters and you have a crawl space) is that it allows much better flexibility for installing different floor coverings. For instance, in my house I have 3/4" diagonal planks for the subfloor over which I'm installing 5/8" plywood as an underlayment.

A back of the envelope R value for 3/4" hardwood is about 0.9 or so. Assuming the plywood and planking are about the same that would come out to around R = 1.8 before any floor covering is laid. That is actually a good thing provided you back insulate it for an under subfloor pex installation job. It means that the difference between a linoleum or a thick carpet floor covering will have much less effect on the required heat output from the pex.

Taking the opposite case with the pex installed directly under the floor covering you will have a change in the R value between a linoleum and carpet installation where it goes from an R value of about .25 (1/4 plywood) and about R = 3 for heavy carpet. Like I say, these are back of the envelope estimates just to show the trend. There would be no easy way to accommodate those different temperature requirements between those extremely different R values that the heat must penetrate. That will definitely effect the value of it on resale of the house.

The other problem with above floor installations, except for pex in concrete, is that the pex radiates directly into the floor covering. Even with aluminum spreaders there will be significant cold spots where the spreaders don't exist.
On an underfloor pex installation on the other hand the heat will migrate in all directions through first the subfloor and then the underlayment. This will provide a lot more even heat to the floor covering.

I think maybe you have overstressed the importance of direct heat conductivity through short distances to the floor covering. If you have to do it that way because of limited access underneath, well fine. But if not then below subfloor installation has a lot going for it in milder climates.

08-21-12, 04:29 PM	#11
Exeric Apprentice EcoRenovator Join Date: Aug 2012 Location: California Posts: 274 Thanks: 19 Thanked 37 Times in 28 Posts	I think the biggest advantage to under subfloor pex (if you don't have extremely cold winters and you have a crawl space) is that it allows much better flexibility for installing different floor coverings. For instance, in my house I have 3/4" diagonal planks for the subfloor over which I'm installing 5/8" plywood as an underlayment. A back of the envelope R value for 3/4" hardwood is about 0.9 or so. Assuming the plywood and planking are about the same that would come out to around R = 1.8 before any floor covering is laid. That is actually a good thing provided you back insulate it for an under subfloor pex installation job. It means that the difference between a linoleum or a thick carpet floor covering will have much less effect on the required heat output from the pex. Taking the opposite case with the pex installed directly under the floor covering you will have a change in the R value between a linoleum and carpet installation where it goes from an R value of about .25 (1/4 plywood) and about R = 3 for heavy carpet. Like I say, these are back of the envelope estimates just to show the trend. There would be no easy way to accommodate those different temperature requirements between those extremely different R values that the heat must penetrate. That will definitely effect the value of it on resale of the house. The other problem with above floor installations, except for pex in concrete, is that the pex radiates directly into the floor covering. Even with aluminum spreaders there will be significant cold spots where the spreaders don't exist. On an underfloor pex installation on the other hand the heat will migrate in all directions through first the subfloor and then the underlayment. This will provide a lot more even heat to the floor covering. I think maybe you have overstressed the importance of direct heat conductivity through short distances to the floor covering. If you have to do it that way because of limited access underneath, well fine. But if not then below subfloor installation has a lot going for it in milder climates. Last edited by Exeric; 08-21-12 at 05:10 PM..