There are many construction and insulation approaches which allow a builder to create walls and ceilings with high R-values and low levels of air leakage, creating a much better envelope than is achieved with standard framing methods. Structural insulated panels (SIPs), insulated concrete forms (ICFs), double-stud walls, and advanced framing can all produce more energy-efficient buildings than the ol’ stick-built number.
The one thing they can’t do is to 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 only suitable for 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 require the installation of between 2 and 6 inches of rigid foam insulation (polyisocyanurate, EPS, or XPS) on the exterior side of the wall framing. The window openings, door openings, and roof edges are detailed to prevent water infiltration, and finish siding is added. These days, the type of siding is either vinyl siding (typically installed on furring strips) or stucco that has been back-vented for drainage.
This approach is can be applied to almost any existing house, although more complex house shapes require greater attention to waterproofing details. It’s an approach that can be used to make older homes more energy-efficient without harming their essential character.
Disadvantages of exterior foam
Let’s tackle 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 you do on the outside of your house will be pounded by nature — and that’s where we’re insulating.
Moisture: Exterior insulation will get wet; it’s a given. The real challenge is not the moisture so much as finding ways to minimize 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.
Pests: Insulation on the exterior may increase the chance of 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, which was insulated with 6 inches of exterior EPS foam. Two simple errors in flashing fed moisture behind the foam board layer, providing life-sustaining moisture to burrowing bugs.
There was no structural damage (except maybe to Lstiburek’s pride) because of the water controls in place, but a lesson was learned.
How to prevent this: just as wood framing needs to be out of contact with the ground, so does exterior foam board insulation. Sufficient space from the ground, good flashing details, and pest shields will help prevent bugs. Keeping the insulation dry will discourage any pests 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 not a 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 down the water-control details. You need to avoid errors like reverse-lap flashing. You need to include sill pan flashing (to prevent water infiltration around doors and windows) and other details 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, which leads to smaller (and less expensive) mechanical systems.
Greenness: 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, fiberboard insulation, and high-density rock wool board don’t have the same issue.
Advantages of exterior foam
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 while retaining the house’s character. In New England, this is a consideration, especially given the age of existing 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, the foam stops thermal bridging. That thermal bridging is compromising the existing wall cavity insulation, meaning that the whole-wall R-value will rise by more than you might think (especially if you base your assumptions on the nominal R-value of the batts inside the wall).
As building and energy codes improve, alternatives to standard framed houses are being explored. An exterior foam retrofit may not be the greenest option, but it’s one of the best performing options for improving insulation in older homes.
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