Existing 2x4 wall with 1x sheathing // (2) 3/4" layers rigid insulation // taped

The toughest details are those that have to match up with someone else's work, or those done — even done well — when energy was really cheap. These details are a collection of some common — and tough — dovetails of existing work with retrofits or additions. Bear in mind that green remodeling means creating a new operating regime that is better, not worse, than the one that may well have been working just fine before. Integration of energy efficiency, moisture management, and indoor air quality is much more important and challenging in remodeling than in new construction.

Just as with new walls, Green Building Advisor employs rigid exterior insulation for retrofitting. It gives great bang for the buck thermally (insulation and air sealing), warms the existing framed wall reducing the potential for condensation in the cavities, and makes continuous air and thermal barriers much easier than working from the inside.

TAPE ALL JOINTS

Taping joints in rigid insulation

To restrict the flow of both heat and moisture, tape all joints. If the assembly contains more than one layer of rigid insulation, stagger the joints to further restrict the movement of air and water.

RIGID INSULATION AS A WRB

Tape all joints of rigid insulation to make an effective air barrier

The primary reason for using rigid insulation on the exterior of a wall assembly is to keep the wall assembly warm to reduce the chance of condensation or moisture accumulation. If desired, rigid insulation can be taped — according to some, but not all, experts proper taping allows the rigid insulation to act as an exterior air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both. — and can also serve as the wall's weather-resistive barrier (WRB).

To be either of these, the tape must bond well to the surface of the rigid insulation. That means the rigid insulation must be clean and dry. It also means that you should match tapes to insulation; use tape specifically designed for the rigid insulation you are using.

Not all types of rigid foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. can be used as a WRB. In order to meet code requirements for WRBs, the foam manufacturer must submit its product for testing by the ICC-ESThis is the International Code Council Evaluation Service. ICC-ES is a non-profit public benefit corporation that evaluates building products, issuing final reports on code compliance of building products and materials. These reports on then made available at no charge to the building community at large.. If the ICC-ES has issued an evaluation report that qualifies the foam as a WRB, it can be so used — but only if the builder uses the same tape and flashing details shown in the ICC-ES report. For more information, see Using Rigid Foam As a Water-Resistive Barrier.

If conditions — weather, mud splash, careless workers — make a good clean bond between the tape and the rigid insulation unlikely, then use housewrap, asphalt felt, or building paperTypically referring to Grade D building paper, this product is an asphalt-impregnated kraft paper that looks a lot like a lightweight asphalt felt. The Grade D designation has come to mean that the building paper passes ASTM D779 (minimum 10-minute rating with the “boat test”) and different products are called out as “30-minute” or even “60-minute” based on D779 results. At times confused with roofing felt, roofing felts and building paper differ in two ways: felts are made of recycled-content paper, building papers of virgin paper; felts are made of a heavier stock paper; building papers a lighter stock. See also roofing felt. as a WRB.

EXISTING FLOOR ASSEMBLY

Insulating/air-sealing a rim joist in an existing floor assembly

It is quite common for a band or rim joist to be insulated with just a square-cut fiberglass batt. This doesn't work, however, because it can't air-seal the rim joist, a common source of significant air leakage. Take out the existing insulation and insulate with spray foam or rough-cut blocks of rigid insulation and spray foam, as shown in details 9-00300 and 9-00301. With the latter, you'll need a lot less spray foam and can use scraps of rigid insulation.

RIGID INSULATION

Rigid foam is a versatile and effective type of insulation that can be used in walls, roofs and foundations. Even the least thermally efficient variety of foam has a higher R-value than fiberglass or cotton batts and blown-in fiberglass or cellulose.

There are three types of foam board—expanded polystyrene, extruded polystyrene and polyisocyanurate. Key differences are R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. per inch, water resistance, compressive strength, how permeable they are to water vapor, available facings and, of course, their cost.

Board insulation is manufactured with the help of a “blowing agent” that creates tiny bubbles in the foam which slow down the flow of heat. Manufacturers of expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) and polyiso board use hydrocarbon blowing agents that don’t damage the earth’s ozone layer. A switch away from the ozone-damaging hydrofluorochlorocarbons (HFCFs) used to make extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.)is not expected to take place until 2010, giving this type an environmental disadvantage for the time being.

Expanded polystyrene. The least expensive of the three, EPS can be manufactured in different densities to increase its compressive strength but it’s still not as strong as extruded board and it’s more susceptible to crumbling at the edges and to other job-site damage.

R-value: About 4 per in.

Permeance: 2 to 5, depending on the density of the board.

Extruded polystyrene. Because of it shigher strength and water resistance, XPS is often used below grade to in-sulate slabs and foundation walls.

R-value: About 5 per in.

Permeance: 1.1 per in. up to 2 in. then 0.55 per in.

Polyiso. With higher insulating values and no ozone-depleting blowing agents, polyiso board has some advantages over XPS for above-grade use (it’s not recommended for below-grade applications because it can absorb water). Polyiso often comes with a foil facing, which gives the material a very low vapor permeance.

R-value: up to 6.5 per in.

Permeance: 0.03