Does Foam Board Insulation Under Siding Help Save Energy?

Updated on December 20, 2012

Many people install vinyl siding or concrete board sding to improve the look and value of their home. With budgets tighter than ever now, contractors and do-it-youselfers are tearing off the original siding, and installing a thin foam board underneath claiming it will improve the energy efficiency of the property. Is this true? Depends.

1/2" Polystyrine Foam Board Insulation

This is typically used to go over the existing siding in order to create an even surface to keep the siding plum during installation. This is most common with vinyl siding. The foam board is nailed in place with washer head nails should be taped at the seams to prevent air movement. The typical R-value of this product is R-2.5 to R-3.5 depending on the density of the foam board. This amount of R-value will make no difference in efficiency. What may help though, is the taping of the seams and the caulking of all penetrations through the siding once completely installed. This type of installation typically sees a 0-5% savings in energy, 5% being a rare scenario.

Foil Faced Foam Board Insulation

Foil faced foam board insulation is a great idea but is not a good choice for hot climates or cold climates as an exterior insulation product. We will start with hot climates.

Hot climates already have a lot of heat and vinyl siding tends to expand and contract with temperature changes up to 1" if the siding is made of recycled vinyl material. When the foil faced material "reflects" that heat, the area between the siding and the reflective covering turn into an oven-like environment and can permanently damage the siding.

In cold weather climates, you actually want your building to absorb as much heat that the sun gives off, not reflect it. So it doesn't make a lot of sense to install a product like this for an exterior application unless you live in a perfect world of 70 and sunny in which case, you wouldn't need insulation.

Standard Polyicynine Foam Board Insulation

Basically, in order to get a good insulation value out of foam board, it has to installed directly on the sheathing of the house. If the house is brick or stone, the foam board may be applied directly to the brick or stone. The best installation of foam board insulation is at least an R-10 nailed to the sheathing surface with all seams taped. The reason the foam board should be at least an R-10 in because the thermal transference will be much less between the outside and the inside making condensation issues from thermal conductivity much less likely. In fact, a perfect insulation system would be installed on the outside of a building only. We'll get to that a different time.

Basically, if you want extra R-value for your walls but do not want to tear off your siding, don't bother with the foam board. You are economically better off spending your money on dense pack cellulose insulation for the stud wall cavities and the company performing the work will charge you less because they can just drill through the existing siding without taking it off.

If you are tearing the siding off anyway, it would be beneficial if you are adding at least an R-10 to the exterior. You can still do the dense pack cellulose.

If you are not sure about the kind of energy efficiency project to perform, you should hire an energy consultant. They will go through your home with you and give you a report with the energy efficiency upgrades that will help you save money.

Questions & Answers

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      • profile image

        Touchet 

        7 years ago

        Our house is built like this. They didn't even use the pink board, they used the white stuff. I feel like its a cheap ripoff of a house. I have read many different sites and the ONLY cons are that it supposedly has a better R value in certain climates (not all of them). Even then, you can make up that R value with better insulation. But the main con is that it is cheap cheap cheap. And i bet you, that your house is just as expensive as the guys down the street who has plywood. The only person who saved that money was the contractor who charged you as if he had used plywood. GOOD contractors use the plywood, then the Rboard--flashing it between the joints (cheap contractors don't do this for extra moisture protection), use spray foam between the gap left at the bottom of the Rboard, then the exterior wall. This setup gives you the best of both worlds.

        Rboard is bad when used for anything other than exterior insulation (where it can breathe). They need to make it illegal for houses to be shealthed in it just for the security reasons alone. You might as well have your possesions in a tent.

      • profile image

        Bill 

        7 years ago

        Yes, indeed! It really helps us save energy but it still depends on the people living on the house with foam insulations in it. Well, I hope everyone does save energy these days.

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