Foam Board Insulation Under Vinyl Siding
Written by Todd Fratzel.
Installing Foam Board Insulation Under Vinyl Siding
Today more than ever before homes are being built with foam board insulation installed on the outside of the sheathing and under the siding. This detail is used with both new construction and also with renovation work. Below are some of the design details along with the benefits of installing exterior rigid foam board insulation.
Improved Moisture Control
One of the biggest advantages to installing exterior rigid foam board insulation is improved moisture control. By installing insulation on the outside of the framing it causes the location of the dew point to move to the outside of the wall assembly. This reduces the likelihood of condensation within the wall cavity.
The exterior insulation also helps keep the siding (all vinyl siding leaks!) further away from the exterior sheathing. Basically it creates another drainage plane that helps protect the sheathing and framing from damaging water infiltration.
Improved Thermal Performance
Exterior rigid foam board insulation can make a significant difference in overall thermal performance for a building. First of all it adds to the overall R value, which can be quite significant depending on the thickness and type of insulation used. If a 1 inch layer of XPS foam is installed it can result in an immediate increase in R value of 5 (20% increase for a wall with 6″ of R19 fiberglass).
In addition to the increase R value, the bigger advantage in my opinion is the improved air infiltration that’s realized with a properly installed layer of rigid foam insulation. The reduced air infiltration results in a better overall performance of the insulation within the wall cavity. This is especially true if the wall is filled with fiberglass batt insulation.
Rigid Foam OVER Building Wrap
One of the most asked questions on our site is related to this article. Readers are constantly asking if the rigid insulation is supposed to be installed over the building wrap or vice versa. Dupont says you can install it either way, however we typically install the foam over the house wrap. It’s much easier in my opinion to do it in this sequence.
To attach the foam board we use a cool tool from Paslode, their Cap Stapler does a great job holding the foam in place and also attaching the house wrap. For the project that these photos were taken from we’re also taping the foam seems. This is fairly typical however Dupont suggests leaving them “open” to help the wall assembly breath. Seeing that we’re using vinyl siding I think it’s better to tape the seam and keep out any water.
Pay Attention To Details
By installing rigid foam board insulation on the exterior walls you’ll need to pay special attention to the details. You’ll need longer nails for nailing trim and siding. You’ll need to properly flash window and door openings. You’ll also need to pay attention to window and door extension jamb depths as they will increase if the window is mounted flush with the foam board.
Energy costs are never going down at least in the near future. So investing in good insulation is always a great investment. Using exterior rigid foam board insulation is a great way to make a building more energy efficient, quieter and more likely to prevent moisture infiltration problems. The method works for both new construction and remodeling and doesn’t affect siding and trim details very much. Bottom line is exterior foam insulation is a good investment.
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