Q&A / 

Rigid Foam Insulation

DEAR TIM: I am currently installing rigid foam insulation around the exterior of my foundation. I’m installing 3 inches of it to a depth of 10-13" below grade to keep the heat in and the cold out. I told my neighbor about this idea and he thinks I’m wasting my time. A clerk at a local home center agrees with me, but he may be biased and trying to sell products. Who is right? Do you have any tips for installing these great insulation products? Randy McL., Gilmanton Iron Works, NH

DEAR RANDY: You’re not going to like my answer, but you’re both right to a degree. There is no doubt that the rigid foam insulation boards you’re installing work. All you have to do is take a modern picnic cooler to a New England Patriots tailgate party. Those coolers typically contain foam insulation, and they do a great job of keeping sodas or other liquid courage icy cold inside the insulated box.

This exposed concrete foundation needs rigid foam insulation to minimize heat loss. PHOTO CREDIT:  Tim Carter

This exposed concrete foundation needs rigid foam insulation to minimize heat loss. PHOTO CREDIT: Tim Carter

The reason your neighbor feels that you’re wasting time and money is because he probably knows the frost depth in New Hampshire. Where you’re located, it’s 4 feet down below the surface of the top of the soil. This means that the soil, to a depth of 4 feet all around your home can freeze into a solid block of ice.

Six or more inches below the 4-foot depth, the soil temperature could be in the upper 30-degree F range. Go down to 6 feet below the surface and the soil temperature might be in the 40-50 degree F range. That’s still pretty cool. This cold soil is touching your foundation, and since cold is the lack of heat, the cold walls are pulling the heat from your house into the surrounding earth every minute of every winter day.

Putting the rigid foam insulation basement panels as shallow as you’re doing is futile in my opinion. The exposed foundation above and below the narrow strip of your insulation is going to be very cold in the middle of winter. This will negate any benefit whatsoever as the temperature of the surface of the interior of the foundation will probably be as cold as if you had done nothing.

For you to get an effective result for the cost of the rigid foam insulation, you need to cover more of the foundation. To get the best result, you need to cover the foundation that’s exposed above grade and is directly exposed to the frigid air temperatures. However, this extra step is not easy to do after a house is built.

I don’t know the clerk you spoke with at the home center, but I have to tell you that my experience with most of them has been dismal. I’ve asked many of them pointed questions over the years about product installation techniques, and most have given me the wrong answer.

Many of these clerks have great attitudes, are hardworking and well intentioned. But many have limited, or no, hands-on experience working with the products they’re selling. I’d be very leery of getting information from young employees that are at the bottom of the pay food chain in that business. How much construction experience can a 22-year-old adult have? It’s always best to take the time and get the real answers from the manufacturer of the product.

If you want to really get some benefit from the rigid foam insulation panels, I suggest you do the project the correct way. You need to install them to a depth of 6 feet or more below grade. To get the best result, extend the panels all the way to the top of the foundation.

Consider using the special foam panels that have drainage channels cut into them that allow water to pass quickly to the foundation drain tile. Any foam insulation that’s above grade needs to be covered with a masonry stucco for fire-prevention reasons and aesthetics.

Should you decide to cover the exposed part of your foundation above grade, you’ll need to install a special flashing on the top of the foam panels. This flashing should extend up behind any wood siding or other exterior wall material. It then slopes down slightly and bends over the top of the foam insulation. This flashing prevent water from getting behind the panels.

The R value of rigid foam insulation increases with the thickness of the product. It’s possible to get a panel that’s 4x8 feet in size and 2-inches thick that has a R value of 10. That will do a very good job of keeping a concrete foundation from conducting cold back into the basement.

Concrete, because of it’s density, conducts both heat and cold quite effectively. If you put your hand on the inside of an uninsulated concrete foundation in the middle of winter here in New Hampshire, you’ll feel like you put your hand inside a refrigerator.

Be sure to read and follow all written installation instructions provided by the manufacturer of the foam you choose to use. Taking the time to do this will save you time and money in the long run.

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One Response to Rigid Foam Insulation

  1. you mean cover the concrete slab under my trailer instead of applying insulation to the belly board will do a better job?i am confused

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