Every house needs to keep rain and cold (or hot) air out, and conditioned air inside. In a Pretty Good House it’s easiest and best to identify dedicated control layers, following Dr. Joe Lstiburek’s list, in order of importance:
A rain control layer
An air control layer
A vapor control layer
A thermal control layer
Rain control layer. If you can’t keep the rain (or other precipitation) outside your building envelope, none of the other layers really matter. Make sure your house will shed water before worrying about other control layers. Many traditional practices are effective, but others have been updated. Notably, despite popular opinion, siding is almost never fully watertight, and many windows and doors will eventually leak. Good flashing details, a rain screen gap and a good WRB (water resistive barrier) will make sure rain stays outside.
The 18 Woodland Grove house is built with a ventilated rainscreen and a vented over-roof. The siding and roofing could be removed the the occupants wouldn’t see a drop of water.
Air control layer. You may have heard that a house needs to breathe. That’s not accurate; it needs to control vapor movement, but air leaks are a bad way to do that. It’s far better to create a nearly airtight structure, using one or more air control layers in the building envelope.
The 18 Woodland Grove house is very airtight at .23 ACH50. Under that standard test pressure, the house leaks only 155 CFM. To put that into perspective, a clothes dryer blows around 200 CFM and the range hood is 400 CFM. But at the same time, the wall has the ability to dry to the outside or inside with the ventilated rainscreen, and a variable perm interior vapor retarder.
Vapor control. Water vapor—that is, H2O dissolved into air—is often the cause of moisture-related problems in buildings. There are various ways to address vapor control.
Thermal control. Mainly referring to insulation, it also involves windows, doors and other penetrations in the building envelope. Insulation is measured in R-value, which considers all three forms of heat movement: conduction, convection and radiation. Window and door insulating ability is measured using U-factor, which is simply the inverse of the R-value. (U-0.2 windows are equivalent to R-5 walls.)
The 18 Woodland Grove house is very well insulated. 10/20/40/60/5 are the numbers that Building Science Corp advises to use for values, based on the idea diminishing return. It stands for 10 subslab, 20 below grade walls, 40 above grade walls, 60 roof and 5 windows (.2 U-factor). This home is 10 Subslab, 23 below grade walls, 36 above grade walls, 63 in the roof and 6.66 (.15 U-factor) windows.
Portions of this page attributed to Mike Maines, "Guidelines for designing and building a Pretty Good House". Visit the Pretty Good House website to read more by clicking here.