Radiant Heat Insulation
Thea in Maine recently emailed us a question about radiant heat insulation. He has radiant heat tubing stapled to the bottom of the plywood sub-floor in his house and the insulation that was installed by the builder is falling down and doesn’t appear to be installed properly. Thea asked me what method we recommend for radiant tubing that’s stapled to the bottom side of the sub-floor.
First let me start by saying we’re not experts in radiant heat. We can only share our experiences with it in homes that we’ve built and my own personal home.
Thea explained that the insulation in his home was stapled up tight against the sub-floor. He was also concerned that there wasn’t any type of reflective surface (foil faced or an independent foil product) installed below the radiant tubing.
My home was built with radiant tubing stapled to the bottom side of the 3/4″ plywood sub-floor. I have 1/2″ thick engineered Brazilian Cherry flooring installed above that. The floor joists are 11 7/8″ deep TJI engineered I-beam joists. I have R19 fiberglass kraft faced insulation stapled to the bottom of the joists. This creates an air space of approximately 6″ between the tubing and the fiberglass insulation.
I can tell you that my radiant heating system works extremely well. I have no “hot” spots that we’ve identified over the course of four winters. Our heating bill has been very reasonable. And I’d rate the overall performance as excellent. In fact, I wouldn’t change anything with regards to the radiant heat.
Here are our thoughts on insulating a sub-floor with radiant tubing:
- We really don’t think a reflective surface will do much of anything. We can see however why some folks might want to use an aluminum heat transfer plate. The reason would be to evenly distribute the heat to prevent hot spots. I haven’t had any problems so again this may be a case by case issue.
- For me the issue seems fairly straight forward. Heat (or cold) will travel the path of least resistance. Thermal resistance is measured with R values. I installed R19 insulation below the radiant tubing. Wood has an R value of between 0.7 and 1.2 per inch. If we have 2 inches of wood, that would be a maximum of 2.4, call it 3. So it’s quite clear to me that it’s easier for the heat to travel up through the wood than it is to travel down (which heat doesn’t do naturally) and through R19 of insulation.
- Probably just as important is to discuss this issue with your builder, HVAC contractor and friends and family with a similar system. Finding the right combination is the key, each HVAC contractor should have a combination of factors that works well for them. Ask lots of questions and you should be ok.