Insulating the foundation for radiant heat

Insulating the foundation for radiant heat

Here in the South (climate zone 4), we usually build houses on a slab or crawlspace. Sometimes you see houses with basements, usually walk-out basements on hillside lots but for the most part, it’s slab or crawlspace. For our home, we’re doing slab on grade.

Why all this business about foundations? I’m glad you asked. For one, the footings for the house were dug today and two, the builder and I just decided how the footings and slab should be done considering insulation for radiant heat.

In my research into radiant floor heating, I learned that it’s important to insulate concrete slabs used for radiant heating, particularly at the edges because that’s where where a slab will lose most of its heat. The Department of Energy estimates that insulating the exterior edge of a slab can reduce heating bills by 10% to 20%. I suspect that figure is likely higher for in-slab radiant heating where there’s a greater temperature difference between the slab and the ground. Certainteed estimates that 60% of heat loss from the foundation is lost at the edge. In houses like the rental we’re living in now, you can definitely observe this heat loss effect on cold days. Around the perimeter of the house in areas where there’s tile flooring the floor is cold, several degrees colder than 2 or 3 feet further away from the exterior wall. Now imagine the load it would put on the heating system trying to keep that floor heated to 80 degrees or so and the money you’d be losing in the process.

It turns out there’s more than one way to insulate a slab. There are two basic options:

  1. Insulate between the slab and foundation wall.
  2. Insulate between the foundation wall and the ground.

To show you what I’m talking about, here are some foundation insulation detail drawings I’ve found:

[![Insulation between foundation wall and slab.](](
Insulation between foundation wall and slab.
[![Insulation between the ground and the foundation wall.](](
Insulation between the ground and the foundation wall.
The builder was planing on building a conventional block foundation wall which is economical, but makes it tough to get the “L” shaped detail required at the top of the wall so there’s room for insulation at the edge of the slab while still maintaining a solid surface to support the slab. To properly insulate this way, between the slab and foundation wall, it’s best to go with a cast stem-wall foundation. Of course, that’s more expensive and time consuming than a block foundation.

Insulating between the foundation wall and the ground is not without its compromises either. Both are probably equally effective at insulating, but insulation outside the wall is prone to damage, especially in our case where we have siding on the exterior and not brick. The siding has to end 8 inches or so above grade to protect it from water and bugs. To protect the exposed insulation I found Ground Breaker from Nudo. It’s essentially a tough fiberglass-plastic material that is used to cover the exposed portion of the insulation. It only comes in one color, gray, which is actually perfect for us and it’s cheap, only about $75 for a 50 foot roll.

Another problem with putting the insulation between the ground and the foundation wall is the potential for termite infestation. The rigid foam can serve as a conduit for termites to make their way to the wall and walls are made of wood and termites eat wood and nobody, including me, wants that. According to this handy Termite Probability Map from the International Code Council, you can see we’re at moderate to heavy risk for termites.

![Termite Probability Map](
Termite Probability Map
The solution is two-fold. First, treat the ground for termites. Second, use a termite shield on the sill plate. This is a bit of flashing that keeps termites out. Both of these things are typically done in our area and were already in the plans.

So what’s the best and most economical approach to insulating a slab for use in radiant heating? After looking at the options, it appears that insulating the outside edge of the foundation is the best overall approach. In addition to the edge insulation, we’ll do 2 feet of inch-thick rigid foam under the perimeter of the slab as dictated by code. We’re going a little outside the norm for our area with applying insulation to the edge of the slab/foundation so hopefully that doesn’t generate any headaches for us with inspectors down the road.

Adding this insulation means we exceed the International Energy Conservation Code (IECC) 2003 requirements which is the version of the IECC that Arkansas has adopted. An R-value of zero which means no insulation is required by IECC 2003. One inch of XPS foam insulation gives us an R-value of five. If we look to the requirements in IECC 2009, it dictates R10 slab-edge insulation, and an additional R-5 for heated slabs for a total of R15. This would mean three inches of insulation which I don’t think will fit.

Some resources:

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