13) Triple Pane Windows, Insulation, ICFs & Thermal Bridging
There are 5 main principles when it comes to Passive House design as shown in this graphic:
Taken from the German Passive House site
We'll start with windows. Without going into the exact specifications, windows have to meet a strict low level of heat conductivity. And only triple pane windows can do this:
Airtightness is critical in a passive house. In most older homes heated air leaks out in a thousand places. Besides the doors and the windows, you've got range and dryer vents, chimneys, electrical outlets, attic hatches, foundation cracks, just to name a few. To avoid these problems and conserve the hard-earned heat we need to wrap the house in an impermeable envelope. A passive house actually has to pass an air flow test to qualify. But the savings are significant:
There's a downside of living in an air-tight house: stale air. In fact it's downright unhealthy. The answer is a mechanical ventilation system; we'll turn to the fresh-air strategy in the next post.
Thermal insulation is probably the most obvious factor in making a home energy efficient. And it's all about the R-factor. Passive houses have two to three times the insulation of regular houses. Here's a breakdown of the suggested insulation level of homes through out the US. Our location is a little colder than Burlington due to our altitude, so we fall somewhere between Zone 6 and 7. Well, it could be worse, like if you lived in Fairbanks, Alaska!
Our plan is to have double insulation with walls in the neighborhood of 12 inches thick for the above-ground walls.
Below ground we'll be using an innovative technology, the ICF: The Insulating Concrete Form. Described as Lego Blocks, these forms lock together to form the foundation. The sides consist of 3 inches of attached foam, both inside and outside. The interior is empty, and once in place, is filled with concrete. Thus the foundation will be 12 inches thick. And unlike a normal house where the cold ground is in direct contact with the walls of your basement, here the wall itself is separated from the earth, and from the interior you've got a solid 3 inches of insulation between you and the wall. Super tight, super insulated!
Somewhat related is Thermal Bridging. Essentially this is where a structural component is exposed to the outside environment and conducts cold directly inside the house without an intervening layer of insulation. Normal houses using 2' x 4' or 2' x 6' construction have studs that support the walls. Outside cladding or clapboard is nailed to it. On the inside drywall is screwed into the same studs. While there's insulation between the studs, it really has no effect on the wood framing itself. Direct contact, heat can conduct right out of the house.
To avoid thermal bridging you need to create a thermal break; that is, you need to put insulation between all the solid elements of the outside and inside of your house. The way we'll do it is to have two walls, an exterior one which supports the clapboard siding, an a separate interior wall that supports the inside drywall. In between there's a 'break' of insulation so that internal heat can't flow directly out of the house. It's called double-stud wall design:
