Basic physics. Two rules to keep in mind: warm air rises, and heat goes to where there isn’t any. That’s contrary to the common “heat rises,” which is not accurate. But all things equal, warm (or moist) air is slightly more buoyant than cooler (or drier) air and will tend to rise. But the dramatic difference in temperature you may have experienced in older or poorly built homes is largely due to air leaks, allowing air to enter at the basement or first floor level and to exit at the roof, carrying warm (and moist) air with it. When you stop those air leaks, and have a thermally efficient envelope, you will experience much less difference in temperature at different stories of your house. On the other hand, heat goes to where there isn’t any, so poorly insulated areas will conduct heat to the cold outdoors. Even more than energy and carbon concerns, mold and fungus are the things we worry about the most. Warm air can hold more water (in vapor form) than cooler air, so when warm air is in contact with a relatively cool surface, the moisture condenses out of the air and accumulates as liquid (or adsorbed) water. The psychrometric chart shows us what will happen at different temperatures and humidity levels. Keeping the indoor relative humidity below 50% is safe, and above 20% provides occupant comfort.
The 18 Woodland Grove house has two large ducted Sante-Fe dehumidifiers. One is located at the highest point in the center of the house and one is located in the basement. Together, they maintain a constant level of moisture in the air for a healthy indoor environment, increased durability and a more comfortable feeling home.
Roof venting (or not). A vented roof is usually more forgiving than an unvented roof, and in many cases it’s easier (and cheaper) to build it that way. But in some cases, an unvented (or “hot”) roof is desirable, and there are ways to do it safely. They usually involve using closed-cell foam, in either sprayed or board form, so for carbon emission reasons we generally avoid hot roofs. The primary reason for venting a roof is to allow moisture to dissipate before it becomes a problem. There may also be a slight reduction in roof temperature, leading to potentially longer-lasting roofing, but the effect is minor.
The 18 Woodland Grove house is built with an incredible double roof. The first layer is air tight, vapor open and insulated with cellulose insulation. This airtight layer allows for a vapor open assembly that drys to the inside and outside. The second layer is another layer of sheathing and roofing. This gap between the layers is ventilated, allowing both to dry. This roof has no penetrations, is water tight on both layers and extremely robust. Added expense is hidden in the roof that no one can see, but the owner can appreciate. The shingles could literally be ripped off the roof and it would not leak. Not many roofs could be more durable and of higher quality than that.
Rain screens. Perhaps the single most important thing you can do for your walls is to include a rain screen, which is like venting for your walls. Even a tiny gap of 1/16” allows water to drain, extending the life of the wall and the cladding. Bigger gaps allow faster and more complete drying. Ideally, provide at least ½” gaps, open to the air at both the top and bottom of the wall.
The 18 Woodland Grove house is designed with a ¾” gap, that allows for draining and just as importantly drying. With the walls increased ability to stay dry, the siding and paint will last longer as well. This is another added complexity that is not visible to the common home buyer or “market” forces. The system dramatically increased durability in the wall system and siding. The lower moisture content in the wall reduces risks and increases health, comfort and safety.
Air barriers. You should be able to draw a cross-section of your house and trace a continuous line around the entire perimeter, representing the airtight layer. The airtight layer is not a single material, but a collection of materials with connection details that create a relatively airtight building, typically measured in air changes per hour at 50 pascals pressure, or how often the total volume of air in the house would be replaced with a roughly 20 mph wind blowing on the house. A good target is 1.0 ACH50, but 1.5 or 2.0 ACH50 is still Pretty Good. If you can get below 1.0 ACH50 you will save a few dollars per year and improve building durability. The Passive House standard is 0.6 ACH50, and the tightest homes in the country are 0.1 to 0.2 ACH50. Another measure of airtightness is measuring the air leakage per surface area. Do not confuse the airtight layer with the vapor control layer; in some cases they may be the same material, but they serve different purposes. The term “airtight” makes some people feel claustrophobic, but there is always enough air available that you won’t suffocate, especially with the high-efficiency mechanical ventilation used in tight homes. Windows and doors are always part of the airtight layer. Concrete in good condition is a good air barrier.
The 18 Woodland Grove house would fit into the category of some of the tightest homes in the country at .23 ACH50. Having been designed, built and every detail looked over by the same person was the only way these numbers can be achieved.
Vapor retarders. Classified by how open they are to water vapor movement (borrowed from the Building Science Corp’s website):
Class 1 vapor retarder, 0.1 perm or less. (May also be called vapor barrier.) Also considered vapor impermeable.
Class 2 vapor retarder, more than 0.1 perms and up to 1.0 perms. Also considered vapor semi-impermeable.
Class 3 vapor retarder, more than 1.0 perms and up to 10 perms. Also considered vapor semi-permeable.
Vapor permeable (or vapor open) is a material greater than 10 perms.
In old homes there is often no vapor retarder, homes less than 50 years old often have plastic or foil vapor retarders, and Pretty Good Houses have a variety of them, depending on the location. Under slabs (either basement floors or slab-on-grade foundations) by code you need at least a 6-mil vapor retarder, but for better durability and radon control use 10- to 15-mil polyethylene. Vented roofs are forgiving, so painted drywall, a Class 3 vapor retarder, may be enough but a Class 1 or Class 2 vapor retarder is generally considered better. When it comes to walls and unvented roofs, things get tricky. A new type of vapor retarder called variable permeance membranes (or smart, or intelligent membranes) are effective and forgiving for these locations.
The 18 Woodland Grove house utilizes multiple strategies based on the location of the vapor control. The walls and ceilings have variable perm rating membranes. The roof is covered with a vapor open but air tight membrane and the exterior face of the walls are in the calls 2 range. The entire home is air tight but designed to have increased drying potential and constant ventilation. The walls and roof are also monitored with sensors to prove moisture accumulation is not an issue.
Sorption and solar drive. Now we’re getting nerdy. Everyone knows the term absorb, which is when a material takes in water. Adsorption is when water vapor adheres to a material’s surface. Together they are called sorption. A more general term is moisture accumulation. In general practice in the building world we often call it condensation, though that technically is a different process. Solar drive, or solar vapor drive, occurs when a moisture-loaded material such as wet siding or cladding is exposed to direct sunlight, the sun has enough radiant energy to push the moisture through the material and into whatever is behind the cladding. Rain screen gaps are an effective barrier to solar vapor drive, especially when they are fully vented, but a WRB is also necessary.
The 18 Woodland Grove house roofing and siding are separated from the structure with rainscreens. The capillary break in materials prevents the solar vapor drive from pushing moisture into the building. Michael explains this well, it’s nerdy, but we actually consider these things in the entire process of construction, from design to building to testing.
Foundations. Every house needs a foundation, but there are many types to choose from. In New England, foundations with full basements are common, and are relatively inexpensive square footage. But they are also susceptible to moisture issues, the amount of concrete required has a huge carbon footprint, as does the insulation needed (but not always used) with a full foundation. Other options include crawl spaces, slab-on grade, and pier foundations. Each has pros and cons, but all can work with proper detailing.
The 18 Woodland Grove house has a full foundation. The advantages of additional space for mechanicals, storage and living space is an advantage and is expected in this area of the country. This is not a wet, dark hole in the ground like most basements. This is the same filtered air that is in the living space. The concrete has a drainage management system for bulk water, a vapor control layer, the slab and interior are completely inside the thermal envelope and it feels like any other above grade space. The entire perimeter is framed walls and fully insulated.
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.