Q&A Spotlight

Peter Fusaro is building a high-efficiency house on spec, and his plans include insulating the foundation walls. The question is how.

One option is to apply rigid foam to the outside of the foundation. That would leave roughly 1 ft. of insulation above grade, and Fusaro is concerned about how durable the foam would be.

Another possibility is to use a sandwich of 2 in. of foam between two outer faces of concrete, each 4 in. thick, making an assembly with both structural and thermal properties. He’s been told a wall built that way would have an effective R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of 19.27.

Thaddeus Cox’s 1944 Cape Cod in Portland, Oregon, has a roof that needs some attention. Not only is the roof under-insulated, but it’s currently covered in two or three layers of asphalt shingles installed over the original layer of cedar. Roof 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. consists of 1-in. thick boards.

The International Energy Conservation Code recommends R-38 for the roof in this Climate Zone 4 house, far more than the R-11 batts Cox thinks are currently in place.

Andrew Homoly is building a house in Kansas City, Missouri, and plans to use a “perfect wall” system consisting of 2x6 studs, IcyneneOpen-cell, low-density spray foam insulation that can be used in wall, floor, and roof assemblies. It has an R-value of about 3.6 per inch and a vapor permeability of about 10 perms at 5 inches thick. open-cell foam insulation and an additional 1 1/2-in. layer of rigid foam on the building’s exterior.

His question in this Q&A post at GreenBuildingAdvisor concerns the best way to install window and door flashing, and where to install housewrap.

It is now a given that high-performance houses have high levels of insulation. It is not uncommon for a new cold-climate home to have R-40 walls and an R-60 roof, as builders do their best to lower a home's energy requirements.

But is this premise in favor of thick insulation weighted toward houses in cold climates, where heating is a higher priority than cooling? Does it make just as much sense to insulate houses as heavily in hot, humid climates? Or does a lot of insulation actually make it more difficult to keep the house cool?

Finding effective ways of beefing up under-insulated roofs is a perennial problem, affecting countless houses built to minimum energy standards.

Even roofs that meet minimum code requirements are susceptible to thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. when only the rafter bays are insulated.

UPDATED on July 7, 2011 with comments from Tedd Benson

Jay Hersh is building a house in northern Vermont that will have a roof of structural insulated panels. Although his plans are fairly advanced, Hersh is still stumped about detailing the SIP(SIP) Building panel usually made of oriented strand board (OSB) skins surrounding a core of expanded polystyrene (EPS) foam insulation. SIPs can be erected very quickly with a crane to create an energy-efficient, sturdy home. seams to prevent the migration of moisture-laden air from inside to outside. He's also looking for a foolproof way of heading off any leaks from the outside.

Eric Dymond lives in a 1920s Baltimore row house that needs a new roof. He plans on replacing the low-slope, built-up roof with an EPDM membrane, and the question is how to insulate it correctly.

Currently, the built-up asphalt roof is installed over Homasote (or something similar) and a roof deck made of wood planks. Although there’s some “sparsely distributed” insulation in the space between the ceilings and the roof deck, it won’t meet current recommendations for Dymond’s Climate Zone 4 house.

Brandon M. is building a three-story house in Seattle whose design includes cantilevers on the second and third floors. The designer has specified steel I-beams to provide the structural support in this modernist design, and this is what’s giving Brandon pause for thought.

Readers who post a question in GreenBuildingAdvisor’s Q&A forum typically look for advice on very specific building problems. Whether the challenge is detailing a rainscreenConstruction detail appropriate for all but the driest climates to prevent moisture entry and to extend the life of siding and sheathing materials; most commonly produced by installing thin strapping to hold the siding away from the sheathing by a quarter-inch to three-quarters of an inch. , selecting windows with the right solar heat-gain coefficient, or using thermal massHeavy, high-heat-capacity material that can absorb and store a significant amount of heat; used in passive solar heating to keep the house warm at night. to store solar energy, the focus is usually narrow and technical.

Morgan Martin’s dilemma is what to do about the crawl space beneath the house, identified in a recent energy auditEnergy audit that also includes inspections and tests to assess moisture flow, combustion safety, thermal comfort, indoor air quality, and durability. as a source of air leaks and energy loss.

The joist bays above the crawl space have been insulated with fiberglass batts, but the subflooring has not been air-sealed. A local green building company recommends removing the batts and taping and caulking all the gaps in the floor (that is, the crawl space ceiling).