Does Foam Board Insulation Under Siding Help Save Energy?
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.