Building a timberframe home from scratch. Radiant tubing under a subfloor

Building a timberframe home from scratch. Radiant tubing under a subfloor

Building a timberframe home from scratch.

Monday, January 29, 2007

Radiant tubing under a subfloor

In my previous post, I described installing hardwood tongue and groove flooring directly to 3×3 floor joists on 12″ centers. thereby forgoing the typical plywood subfloor. Based on our first experiment (and 200+ years of historical precedence), it looks like that will work just fine. The brand new floors squeak a little bit, but I think I can solve that in the other areas with a shot of Liquid Nails between the flooring and the floor joists.

Where we’ll be installing tile, slate, or stone floors, we’ll need a typical subfloor — I just don’t see any way around it. This first picture is the subfloor for the foyer and master bath. I chose 3/4″ t&g plywood instead of 3/4″ t&g OSB. Although Advantech ™ OSB has a 50 year warranty, I feel that plywood is the lesser of the two evils. On top of this plywood, we’ll install Hardibacker brand cement backer board before installing the actual flooring material. Never having done this before, I’ll be using guidelines found in books and the internet. If anyone has any tile installation advice, please post now. )

The last step before installing the subfloor is installing the 1/2″ radiant heat tubing. Technically, we could install the radiant heat tubing from below, after the subfloors go down, but it seems to be a lot easier to pre-install it from above. My brother-in-law is using his router to cut channels in the floor joists wherever the radiant tubing must cross the 3×3’s. Per the instructions on radiant heat web sites (google radiant heat if you’re interested), I’m installing about 1 linear foot of tubing per square foot of floor space. The tubing costs about 40 cents per running foot. I’m trying to keep all of the loops about 200 feet in length. If you make the lengths too long, the water will be harder to pump, and will give up all of its useful heat before reaching the end of the loops. We’ve been careful to mark the «NO NAIL» regions of the subfloor because we don’t want to poke a nail through the radiant tubing after it’s all been installed!

The last picture shows the tubing beneath the kitchen, just before we installed the subfloor. Here, I used store bought fir 2×4’s on edge instead of home-cut oak 3×3’s. Using store bought lumber was a shortcut that cost me more money, but also helped to solve the problem of leveling the transition between adjoining tile floors and hardwood floors. BTW, if 2×4’s and 3×3’s sound like wimpy floor joists, keep in mind they are spanning less than 4 feet and are on 1 foot centers. After installing the plywood subfloor, this floor system has zero noticeable bounce or deflection. (Hear that Uncle Jeff? I know this has been keeping you up at night!)

Later, we’ll go back in the basement and tack the radiant tubing up to the subfloor and hardwood floors with highly conductive aluminum brackets. Keeping all of the tubing in place so we can nail the subfloor down is has been somewhat of a chore. The tubing has a spring-like shape memory and wants to coil back up like it was shipped from the factory. Beware — it will reach out and smack you or trip you whenever it gets the opportunity. We have learned the hard way!


The flooring looks good Thomas. The Poplar is harder than eastern Pine which I have in my house. Some of it is almost 200 years old, so you should be fine. I think worn floors have the coolest character. We are putting pine floors in our new bedroom addition as well. Those boards are 13.5″ wide.

A tip for your coiled tubing: Rather than pulling it out of the coil as it lays horizontally, unroll it across the floor or set up a 2×4 across a couple of horses and let it unroll as you pull. You will have less stored energy in the tubing as you lay it out across the floor. Same thing works for Romex too.

Building a timberframe home from scratch. Radiant tubing under a subfloor

I have layed hundreds of feet of tile. What are you concerned about? Are you using mastic or thinset? The key to good grout life and uncracked tile is zero flex. Areas that have heavy and/or wide pieces of tile may need more reinforcing. Use screws, not nails in the subfloor and hardibacker. My sublfoor in our last house was 3/4 plywoof with another layer of 1/2″ plywood overlapping the seams. The guy we sold the house too took out the tile a year after he bought the place, and he told me it was a bear. I just smiled.

Huh. I’ll have to check out poplar as an option for a project of mine. I’d always assumed it was softer than pine. Thanks for the correction, Jim.


You need to begin a DIY seminar for those who want to self-build a timberframe. Your blog and experiences are priceless.

This comment has been removed by a blog administrator.

Leave a Reply