The most recent blogs at Green Building Advisor

[Editor's note: Roger and Lynn Normand are building a Passivhaus in Maine. This is the 24th article in a series that will follow their project from planning through construction.]

UPDATED June 17, 2013 with new product information

Twenty years ago, very few residential builders knew what 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. was. These days, however, it’s no longer unusual to see siding being installed on vertical furring strips or a plastic drainage mat. As rainscreens become more common, mainstream builders are beginning to ask, “What’s a rainscreen? How do I know if I need one?”

This article will pull together information to answer the most common questions about rainscreen gaps between siding and 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. .

With any house, there are so many variables that influence the decision to choose one particular mechanical system over another: climate, house size, cost, local availability and cost of fuels and materials, and the lifestyle and preferences of the occupants. There is no “one-size-fits-all” system that we can reliably prescribe for all projects. Phil and I sat down over a good winter cocktail to share our views, anecdotes, battle scars, and wisdom on this important subject.

In many parts of the country and for many applications, natural gas is considered a panacea to our energy challenges.

If you install fiberglass batt insulationInsulation, usually of fiberglass or mineral wool and often faced with paper, typically installed between studs in walls and between joists in ceiling cavities. Correct installation is crucial to performance. * with a kraft paper vapor retarder in a home, which way do you face the vapor retarder? To the inside of the home or the outside of the home? For many building science questions, the answer is, “It depends.” For this one, however, the answer is clear.

SPOILER ALERT: The answer is in the next paragraph — so if you'd rather wait and find out when you see the movie in the theater, don't read any further.

Whether they actually do it or not, I think almost everyone involved in high performance buildings recognizes that the best place to put our ducts is inside conditioned spaceInsulated, air-sealed part of a building that is actively heated and/or cooled for occupant comfort. . Most builders in my area haven’t made the change, and with the exception of the occasional house with an insulated basement, they still put most air handlers and ducts in the attic.

Michael Roland is designing a new house and trying to choose the right wall assembly. It’s down to a choice between a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. filled with fluffy insulation, or a single wall wrapped in a layer of rigid foam insulation.

I tried an experiment this week during our cold snap. We've kept the door closed to the first floor ell (bedroom and bath) and let it run cold, because the Fujitsu wasn't sized to heat that space too. I opened the door early in the cold snap, and let the heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. go, leaving it set on 70°F. What I found was that overnight the main space went to 66°F, and the upstairs and back bedroom were 3° to 4°F lower.

My calculated heat loss in these conditions is about 24,000 BTUBritish thermal unit, the amount of heat required to raise one pound of water (about a pint) one degree Fahrenheit in temperature—about the heat content of one wooden kitchen match. One Btu is equivalent to 0.293 watt-hours or 1,055 joules. /hour, and the heat pump is rated at about 17,000 BTU/hour at about 10°F. You'd think it would not be able to keep up.

Most of us want to do the right thing in improving the energy performance of our homes. We research energy-saving products like appliances and insulation. We search the internet or clip ads from the paper looking for products that will save us the most energy (and money). We look for the most R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. for the money. Well-meaning homeowners do this all the time.

One thing that invariably surprises people when I walk them through a house during their first blower-door testTest used to determine a home’s airtightness: a powerful fan is mounted in an exterior door opening and used to pressurize or depressurize the house. By measuring the force needed to maintain a certain pressure difference, a measure of the home’s airtightness can be determined. Operating the blower door also exaggerates air leakage and permits a weatherization contractor to find and seal those leakage areas. is how much air leaks in through the electrical switches and receptacles. On a recent Friday, we went out to do the last home energy rating in our latest HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. rater class, and we got to see something even better. But first, let's talk about that air leakage. We have a number of surprises waiting.