Exterior wall cavity insulation Zone 7 and 8:

The myriad of products available for exterior wall assemblies is exhausting, and understanding which one is correct for your home can be overwhelming. There are new insulated sheeting panels, house wraps, weather and air barriers, insulations– the list goes on. At that, many architects don’t always take into consideration how the performance of each material can impact the performance of others. To help alleviate this problem, we try and specify materials that only perform the task for which they were intended. This way, one material does not interfere with the function and performance of other materials.

Preferred exterior wall insulation choices include fiberglass batts, fiberglass blown-in-blanket, 2 pound closed cell polyurethane foam, and a fiberglass batt or 2 pound closed cell polyurethane foam hybrid. In addition to wall cavity insulation, you have the option of adding continuous insulation sheathing over the exterior face of the studs. The framing contractor will install the continuous insulation sheathing if this system is desired.

The following chart shows cost, R value, and requirements for a vapor barrier for different exterior wall insulations:

2×4 framingMaterials and labor installed cost*Additional vapor barrier required?
R11 Unfaced fiberglass batt insulation$.37 per square footYes
R13 Unfaced fiberglass batt insulation$.44 per square footYes
R15 Unfaced batt insulation$.54 per square footYes
R15 Blown-in-Blanket insulation$.89 per square footYes
3.5″ Closed cell rigid polyurethane spray foam$2.80 per square footNo
2×6 framing  
R19 Unfaced batt insulation$.53 per square footYes
R21 Unfaced batt insulation$.63 per square footYes
R23 Blown-in-Blanket fiberglass insulation$1.29 per square footYes
2.5″ Closed cell rigid polyurethane spray foam with R11 Unfaced batt insulation installed over the foam.$2.49 per square footNo
4″ Closed cell rigid polyurethane spray foam$3.20 per square footNo
5″ Closed cell rigid polyurethane spray foam$4.00 per square footNo

Air Barriers & Air Sealing Packages

Regardless of the type of exterior wall insulation, you install you should install an air barrier or an advanced air sealing package. There is a common misconception between air barriers and advanced air sealing packages. Air barriers are a single source product that eliminates air travel before it enters the wall system from the exterior side as well as from the interior side. Conversely, advanced air sealing only eliminates air travel from the interior side of an exterior wall. The best cavity insulation still does not combat air leakage at stud packs, headers, and plates. It’s important not to confuse sheet vapor barriers and air barriers, as air barriers are not advanced air sealing packages and should not be considered the same.

If properly installed, the Knauf Ecoseal Air barrier product can expect to achieve a .15 or better Natural Air Change per Hour (NACH) and is the responsibility of one subcontractor. Ecoseal is a cost effective air barrier and can be installed to the stud pack, headers, and other construction seams. The costs of an Ecoseal Air Barrier system will generally run around $.70 per square foot of surface to be treated. To learn more about this product click here.

Advanced air sealing packages require sealant at the top and bottom plates, on all 4 sides of each box sill and rim joist, at exterior sheeting penetrations, at plate-to-foundation wall junctures, at the juncture of exterior wall bottom plates and substrate, and gaskets at electric receptacles that are installed by electricians. There is no requirement for air sealing stud packs, stud cavities, headers, and other framing irregularities. Advanced air sealing packages are designed to achieve a .35 NACH and can be fairly expensive for less than optimal results. Advanced air sealing also relies on several parties for installation and application, with no one party guaranteeing lasting results. We do not install an advanced air sealing package, and if this avenue is selected, we recommend that the builder coordinate the work with the subcontractors responsible for the penetrations.

Irregardless of the method you choose, your home will now exceed above-standard ratios of air changes per hour. For that reason, it is very important to incorporate an air exchange system into the home, as a home without an air exchange system will rely on leakage, opening and closing of doors and windows, and furnace/boiler/exhaust draw to exchange the air in the home. When the leakage rate drops significantly because of the air seal, the remaining means of air exchange are not adequate, leading to poor indoor air quality. Basic air exchange units for a 3500 square foot home will cost approximately $3000-$4500 installed. Generally, air barriers require air exchange while advanced air sealing does not require air exchange.

The definition of a system:

  1. A group of interacting, interrelated, or interdependent elements forming a complex whole.

The elements that make up a common exterior wall are siding (or other exterior finish), house wrap, sheathing, wood studs, insulation, wiring, plumbing, duct work, fire suppression piping, vapor barrier, drywall, and paint. When these elements are assembled in construction they create a complex whole or as we call it the exterior wall system.

Its important to understand that the exterior wall system is not something that can be learned in an afternoon. For a comprehensive examination of your construction documents please call or email.