(originally published in The Home Monthly, January 2013)
Everyone knows that green vegetables are good for you, but for most people veggies don’t top 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 second rate home? The answer is a resounding no! More than anything else, a green home is one that offers tangible qualities that owners enjoy immensely. Prominent among these is unparalleled comfort and outstanding energy efficiency.
Typical homes lose heat rapidly in the winter – and gain it just as rapidly in the summer – as the result of outdated construction and insulation strategies that have been around for decades.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 through the house as warm air through ducts, or hot water through radiators or radiant tubes in the floor. This keeps the occupants reasonably comfortable, despite cold exterior surfaces, and cold air rushing through electrical outlets.
Since the 1920’s, most 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. 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 wood “headers” are installed above window and door openings to transfer loads directly above the openings to the studs on either side. In fact, often 25% of a wall is solid wood! This is a problem because wood is a pretty poor insulating material. Heat loss through 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, which is only enough space for a thin layer of insulation. The fact is that 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 cavities between the framing! If the cavities 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 cold climate.
The final problem is air leakage. A framed wall has plywood sheathing outside and drywall inside. However, until recently, there has been no requirement and little effort made to seal any of the zillion little gaps through which outside air can leak. 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 pressure difference pushes lots of warm inside air out through the ceiling or roof and pulls an equal amount of cold outside air in 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 thicker than in a conventional house to accommodate more insulation, resulting in “R” values of 30-40 in a cold climate like ours. Most (or at least some) of the insulation is continuous, installed either to the outside or the inside of the framing, which eliminates the “thermal bridging” through the wood that I mentioned earlier. The final critical element is an air barrier to virtually eliminate air leakage through the walls. This could be a special membrane, or spray foam insulation, or plywood with taped joints, or one of several other methods. The result is a wall that loses heat at a vastly slower rate than we are used to occurring in our homes, resulting in both astounding comfort and energy efficiency.
Next month’s article will explore the many different options for building high performance walls.