I live in Southern Arizona and every time I mention the idea of putting insulation under a slab I get a polite but confused look. Here, where the climate is defined by extreme heat in the summer, it is not common practice to insulate a slab. In fact, no one would think to put insulation under a slab unless they are doing a radiant floor.
Why? Because it has always been done this way. Cheap energy has made it reasonable to do this. Times are changing though, and insulating a slab in the desert (or anywhere) is not such a crazy idea.
As I sit here (on our un-insulated slab) in late December I am bundled up in long pants, long sleeve shirt, sweater, scarf, shoes and socks and I have a heater on under my desk! I can’t help but think that insulation (even just perimeter insulation) would make things a lot more comfortable.
Types of Insulation
There are certain qualities underslab insulation should have. It needs to be able to support the load of concrete without deforming and it should maintain it’s R-value over the long haul. Suffice it to say the options are limited. Foam is the default option, though it’s not my favorite, because it has issues. Namely, with chemical additives, blowing agents that aren’t friendly to the environment, and moisture absorption. Let’s just say that you can put foam under a slab, but you do it with your fingers crossed.
On the hunt for a better option, I’ve run across a product called foamglas. It’s a mineral based material (glass) made by Pittsburgh Corning and has a long history of use mainly for industrial applications. Foamglas is recommended specifically for underslab purposes and is made for the construction industry. It comes in 18″x24″ blocks in thicknesses from 1 1/2″ to 7″, available in 1/2″ increments.
Unfortunately, it’s not something you are going to find at the local hardware store or even from your insulation contractor, but you can purchase it directly from Pittsburgh Corning. They have manufacturing facilities around the U.S., and will ship from their factory to your jobsite. Lead times vary, but it’s wise to check with the manufacturer to get an estimate of how long it will take from the time you order until it arrives on your site.
The benefits of foamglas are numerous though it does have one main drawback which is cost. No surprise, it’s on the expensive side. Right now in December 2013, the quoted cost is $1.25 per board foot. That is more than you will pay for foam, by quite a bit.
Here’s where it makes sense to weigh the benefits versus cost: Confidence. I have a lot more confidence in foamglas than I do in foam, and for that reason, while I don’t like spending extra money, it may be worth it for this application. It’s like deciding to buy the car with the good safety rating even though it is more expensive.
The foamglas warranty is 20 years, but since foamglas has been around since the 40′s and has a proven track record as a durable insulating material I wondered why the warranty isn’t longer. I asked foamglas about it and they said they don’t have plans to offer a longer warranty because any issues with R-value retention will occur within the first 5 years and certainly within 20 years. They added that it is worth comparing warranties to see what is included, noting that in addition to R-value retention they include compressive strength, dimensional stability and moisture resistance which not all manufacturers will warranty.
Benefits of Foamglas
Foamglas’ benefits are listed on their website, but here is the quick rundown:
- It is waterproof. It won’t absorb water and will maintain it’s R-value.
- It is naturally pest resistant. Rodents or insects (think termites and carpenter ants) won’t eat it or nest in it. There are no added chemicals to make it pest resistant.
- It has high compressive strength. It’s very high at about 90 psi, good if you need to drive a truck over it, but way more than you need to pour concrete over it. Your structural engineer will tell you what psi you need, but usually it is in the neighborhood of 15 psi.
- It is noncombustible. Non combustibility is a big thing for insulation because it means no added fire retardents, which are on the red list.
- It is air and vapor tight. Note: foamglas recommends installing with a vapor barrier, so that the insulation doesn’t bond with the concrete.
- It is dimensionally stable. It is what it is for the long haul and it’s not changing into some other shape.
- Decent R-value: 3.4 per inch
- No blowing agents to cause harm to the environment.
- It’s easy to cut.
- It can be recycled.
- It’s made from sand.
- It has low embodied energy.
How much do you need?
This is the big question and the answer is that it depends. The amount is determined by two things. #1 what your energy and comfort goals are and #2 your climate. If you live in a cold climate, you will need more insulation. If you want to be comfortable you will need more insulation. If you want to save energy you will need more insulation.
Yeah, but how much?
Energy modeling is the way to determine exactly how much insulation you need. There are several programs out there, I use the PHPP (Passive House Planning Package) which is designed specifically for low energy buildings. Not every high performance house has to be a passive house though and there are other options like the “pretty good house” (more on that in a future post) which has some general guidelines for insulation levels that are actually pretty good! In a “pretty good house” underslab insulation levels vary by climate with warm climates being R-5, and cooler climates at R-10.
Generally speaking, insulation under the slab is always a good idea; for energy savings and for comfort. The benefits of foamglas are many as listed above. It’s always a tough choice when faced with higher costs, but for this particular application I think the benefits are worth considering spending the extra money for. That’s just me though, in future posts I’ll talk about the foam options, to help you make the decision that is right for you.