There are many construction and insulation approaches which exceed the R-value and air tightness of standard framing. SIPs, insulated concrete forms, double stud walls and advanced framing all produce much more energy efficient buildings than the ole stick-built number. The one thing they can’t do: improve the efficiency of an existing house.
One of my first audit instructors stressed that we have to deal with the existing housing stock. People sometimes grouse that increasing fuel efficiency standards for cars is pointless since cars last 5 to 10 years. Imagine how hard it is to improve the housing stock. Houses can last hundreds of years; moving that number is much harder.
As beautiful as all those building options are, they are new construction. And 40 years from now we’ll be dealing with 80% of the same buildings. What are the options for radically improving their wall insulation? Exterior foam board insulation is one of the few.
What is Exterior Foam Insulation?
Exterior foam insulation systems are where 2-6 inches of foam board insulation (polyisocyanurate or XPS) are added to the exterior wall framing. The window, doors and roof edges are detailed to prevent water infiltration and finish siding added, usually vinyl siding hung over furring strips or stucco that has been backvented for drainage.
This approach is broadly usable on any house style, though more complex house framing requires greater attention to water proofing details. It’s an approach that can be used for older homes to make them more energy efficient and still retain the essential character.
We’re changing it up a bit and tackling the disadvantages first. What’s the main challenge of an exterior foam board system? Well, it’s outside. There’s a reason we trouble ourselves so much over the building envelope. It keeps out water, unconditioned air, pests and a host of things you don’t want in your nice cozy house. Anything on the outside will be pounded by nature…and that’s where we’re insulating. This leads to:
Moisture – Exterior insulation will get wet, it’s a given. The real challenges is not the moisture so much as minimizing any problems and allowing as much drying as possible. The disaster scenario is one where poor or improperly installed flashing creates an enduring leak into the building frame. This leads to:
Pests – Insulation on the exterior exposes it to pest infestation. Bugs don’t eat foam, but they will burrow through it, searching for tasty wood pulp. Exterior basement and siding foam board need protection against pest infestations.
And what attracts the bugs? Moisture. Joe Lstiburek recently posted an article detailing the findings when they pulled apart his 15-year-old beer cooler barn (6 inches of external EPS foam). Two simple errors in flashing fed moisture into the foam board layer, providing life sustaining moisture to the burrowing bugs.
There was no structural damage (except maybe to Lstiburek’s pride) because of the water controls in place, but a lesson learned. How to prevent this: just like wood framing needs to be out of contact with the ground, so does exterior foam board insulation. Sufficient space from the ground, flashing details and pest shields will help prevent bugs while the dry insulation will discourage any from staying.
Ease of Installation – This is a deceptive one. You’re installing standard built walls, then adding either weather resistant wrap or a self-adhering vapor barrier membrane, foam board, furring strips, and then siding. It is a big project but a not super difficult one.
Periodically I’ll mentally gauge how long it would take my Dad and I to strip the siding and install foam board on my house, the difficult/not easy part is that you’d better nail the water control details. Reverse flashing, pan flashing (these prevents water infiltration around doors and windows) and other details need to be down flat to control the water issues.
Cost – Another knock on this approach is cost. The foam board is all added cost over that of a standard wall, plus the additional labor cost. Like any high R-value wall insulation, it is partly offset by decreasing the heating and cooling loads thus smaller (and less expensive) mechanical systems.
Green – Anytime you muck around with polystyrene foams, you’re at a green crosshairs (that’d be XPS and EPS). You’re saving energy and carbon, but doing so with a petroleum product. That said, polyisocyanurate, fiber board and high density rock wool board don’t have the same issue.
Ease – Maybe not ease of use, but utility. External foam board systems are one of the only ways to radically improve the R-value of existing homes and you can retain the house’s character. In New England, this is a consideration given the age of houses.
Air Leakage – Adding exterior foam board can significantly tighten the building shell. Once the foam boards are attached and taped at the seams, they form a continuous air barrier.
R-Value and Thermal Bridging – This is a huge plus. External foam board systems add significant R-value to a wall system. Adding 4 inches of XPS foam would add 20 to the R-value. Additionally, it stops thermal bridging. That thermal bridging is compromising the existing wall cavity insulation meaning that the whole wall assembly R-value will rise by more than R-20.
As building and energy codes improve, alternates to standard framed houses are being explored. Exterior foam retrofits aren’t the greenest options but they’re one of the best performing options for improving insulation in older homes.