Everyone knows that green vegetables are good for you, but for many people veggies will never make their lists of favorite foods. So is a green home good for you in the same way, where you have to make compromises and put up with a home that’s not really what you wanted? The answer is a resounding no! More than anything, a green home is one that offers tangible qualities that owners enjoy immensely. Prominent among these is unparalleled comfort and outstanding energy efficiency.
I’m sure that some of you out there are skeptical, so let me explain. Typical homes in this country lose heat rapidly in the winter – and gain it just as rapidly in the summer – as the result of conventional construction and insulation strategies that have been around for decades. These homes overcome rapid heat loss by consuming massive amounts of energy (oil, propane, electricity) that is converted to heat (by furnaces, boilers, and heat pumps) and then blasted throughout the house in the form of warm air through ducts, or hot water through baseboard radiators or radiant pipes in the floor. This generally works to keep the occupants reasonably comfortable, despite the cold exterior walls and windows, and the cold air pouring through electrical outlets and other air leakage points, but all of that energy is costly, as everyone knows.
How exactly does this happen? Well, since the 1920’s, the vast majority of our homes have been framed using a technique called stick framing where each floor of a house is built as a platform, with the 1st floor platform carried by the foundation walls, the 2nd floor platform carried by the 1st floor walls, and the roof carried by the 2nd floor walls. Each floor’s walls are framed with 2×4 vertical studs, installed every 16”. However, the studs are frequently doubled, tripled or worse under load-bearing points from above (like the roof). Also, big pieces of solid wood called headers are installed above window and door openings to transfer loads from above to the studs below. By the time you’re finished, as much as 25% of a wall is solid wood. Why is this problem? It’s a problem because wood is a pretty poor insulating material. Heat loss through 2×4 framing is so rapid that we call it “thermal bridging,” meaning that the wood acts just like a bridge to conduct the heat out of the house.
The next problem is that 2×4 studs are only 3.5” deep, and that’s not much space for insulation, so the “R” value (“R” stands for resistance to heat flow) of the insulation can’t exceed 15, given the space limitations, and that’s only in the areas that don’t have wood framing! If insulation can only be installed in the cavities that represent 75% of the wall and the rest is wood, then the average R-value for that wall is generally only 10-12, and that’s simply not enough in our climate.
The final problem is air leakage through the wall. A framed wall is completed with plywood sheathing nailed to the outside face of the studs and drywall to the interior face. However, until recently, there has been little effort made to seal any of the thousands of points at which air from outside can leak through the gaps in the sheathing and between the sheathing and the studs. And does it ever leak! You see, during the winter months, there’s a significant difference in pressure between air inside and outside the house (the same air pressure that you hear about on weather reports). This air pressure difference pushes lots of warm inside air out of the house through the ceiling or roof and pulls an equal amount of cold outside air into the house through the basement and 1st floor walls, leading to lots of additional heat loss.
How is a “green” house different? Well, the exterior walls are generally thicker than in a conventional house so that more insulation can be installed, resulting in “R” values of 30-40 in a cold climate like ours. This also allows for a large percentage of the insulation to be continuous across the framing, either to the outside or the inside of the structural studs, which eliminates the “thermal bridging” through the framing that I mentioned earlier. The final critical element is an air control barrier to completely eliminate air leakage through the walls. This could be the drywall (if installed the right away), or spray foam insulation within the framed wall, or plywood with taped joints, or one of several other methods. The result is a wall that transfers heat at a much slower rate than we are used to seeing in our homes, resulting in astounding comfort and energy efficiency.
Next month’s article will explore the many different ideas for building high performance walls. Please tune in.
Michael Trolle, a Ridgefielder, is principal of BPC Green Builders, 523 Danbury Road, Wilton. He may be reached at firstname.lastname@example.org.