This is the third post in our series on Passive House Design. If you missed either of the previous, click on the links below to get up to speed! LEAP’s intelligent design process consists of four main steps, each building on the previous. Site analysis is the first step, because it informs all the other steps. You can have all of the best windows, doors, and insulation, but their effectiveness is diminished if the orientation of the structure isn’t correct.
Passive House Design Process
Today we explore the importance of insulation, and avoidance of thermal bridging.
Insulation to Minimize Heat Loss
A typical modern house loses and gains approximately 150 kWh/m²a of heat, where the units refer to energy per floor area. A “leaky house” will have double those losses — think older windows, no wall insulation, and degraded door seals. On the other hand, a passive house will be 20x more efficient compared to the leaky house, and 10x more than a typical modern house. A big part of how Passive House Design minimizes thermal gains and losses is through super insulation.
Passive House Standards
Per the definition of Passive House—it can use no more than 1.4 kWh per 1 ft² of living space annually. For example, a 2,000 ft² house would only use 2,800 kWh per year, which comes out to $280/yr (@ 10 cents/kWh). To achieve this efficiency, we’ve discussed how the structure’s envelop must be air-tight, but we also need to insulate the heck out of it.
The insulation itself is generally comprised of multiple layers, all with high R-values. This insulation covers the entire envelope of the structure, including under the footing,with the only exception being the windows and doors. To be continuous, the insulation goes on the outside of the framing, opposed to between studs in a conventional building. The outermost layer of the insulation-sandwich is a water/wind membrane, which tends to be UV sensitive. This necessitates installing siding to cover and protect the membrane.
This configuration significantly reduces the heat transfer through the walls, roof and floor compared to conventional buildings. A wide range of thermal insulation materials can be combined to provide the required high R-values. Special attention is also given to eliminating thermal bridges.
A thermal bridge is a break in the insulation surrounding the house. In a traditional home, this would include all framing members of the structure, and things like porches and overhangs. Thermal bridges lead to massive heat loss, negating the benefit of “over insulating” the structure.
Much Ado About Porches, Decks, and Overhangs
We don’t pay special attention to these “add-ons” for nothing. There are lurking thermal bridges…ready to let all the heat in (or out) of our carefully crafted structure. So instead of penalizing Passive House structures and sending them to the corner with no porches or decks, we work around it. Normally a ledger board would be affixed to the structure as a supporting member for the deck or porch. Instead, we design it to stand-off, and put all of our support posts in the ground. In this manner, we avoid creating a break in the continuous insulation wrap.
So to wrap up, (pun intended), think of super insulation for a passive house like Ralphie’s brother in a Christmas Story. He is bundled to the max, layer upon layer of winter clothes, along with socks, boots, gloves, and his hood pulled so tight that he can barely see—and all on the outside of his “frame”. I guess Randy and Ralphies’ mother understood the dangers of heat loss and thermal bridging back in the 50’s. That lady was well ahead of her time.
[box type=”bio”] Interested in Passive House or Green Building Design? Whether you want to start from scratch or renovate—we can help! Every $1 spent on design saves $10 in construction costs. Don’t shoot your eye out. Let us provide you the best design possible. Contact us to get started.[/box]