The Four Essential Components of a Better-Performing Home
By Mark LaLiberte

Much of what goes into a high performance home lies beneath the surface. As architects, builders and manufacturers, it is our job to understand and attend to the inner workings of a house, and to help our customers appreciate the benefits of a better-built home.

1. The Building Enclosure

The building enclosure is simply the boundary between the outdoor weather and the indoor environment. It consists of a water boundary, the thermal boundary, the pressure or air boundary, and the vapor boundary. All architects, builders and sub-contractors should be able to locate these boundaries in a building. Though this may seem simple, it is surprising how many building professionals confuse these principles, overlook critical areas or misapply materials. Let’s review what each boundary is and how it works.

First, is the water boundary, which protects against rain, melting snow and soil moisture. Draw an imaginary line of isolation or control around the entire home, including the foundation, slab, walls and roof system. Start at the roof, move down along the eaves to the walls, flash windows, doors and penetrations and drain that water away from the house. Next you would look at how the site and soil type could contribute to the moisture flow. We need to isolate materials from sources of moisture, using capillary breaks or hydrophobic products. The building must be designed with four principle systems: draining the site (perimeter drains, draining backfill and swales); draining the building (roofs and gutters); draining the assembly (house wraps and membranes); draining the components (flashings and weep holes).

Insulation is used to control energy flow and create a thermal enclosure. Most insulation systems are fiberglass, cellulose or foam. R-values vary among different insulation materials, but one must also consider how well the material will seal cracks, voids and other breaks in the building envelope. Complexity of the installation, the compatibility of the adjacent materials, and any possible air quality contaminants should be evaluated as well.

Air carries more heat with it than any conductive losses through insulation, so an effective air barrier system is critical. Besides sealing or reducing voids in the insulation, we must also seal gaps at the foundation with a sill sealer, and around the rim joist and headers. Sealing the many holes from electrical to plumbing is essential for establishing the continuity and effectiveness of the air barrier. If properly installed, a house wrap can often contribute to both the water management of the home and provide air sealing benefits.

The thermal barrier needs to have another important feature. These materials should be “vapor open”, which means they do not block the movement of moisture. This is an advantage because building assemblies must be able to dry if they get wet. Even in a high-performance home, some water – hopefully teaspoons rather than gallons – will inevitably get past the water boundary. This will not be a serious problem as long as the moisture isn’t trapped within the building enclosure.

2. Crawlspaces

To seal or not to seal the crawlspace is a question that many builders struggle with. I believe that whenever a house is over a crawlspace, the best approach is to seal it. Treat it as a short basement, with adequate insulation on the walls, and the ground covered by poly or a concrete slab to prevent ground moisture from rising into the crawl space. Like a basement, this space will then be considered part of the total building enclosure. The air inside will be conditioned and, in most areas, dehumidified. This can be done in various ways that can be very cost effective and can improve the comfort and durability of the home. Some studies have also shown a reduction in energy use when the crawl space is properly conditioned.

One of the primary reasons for sealing the crawlspace and keeping it inside the building enclosure is that we can then put the HVAC system in the crawlspace. The attic or an unconditioned crawlspace is the wrong place to put HVAC equipment for reasons of energy efficiency, safety and health. By conditioning the crawlspace, we can now run ductwork through this space without worrying about the energy loss that typically occurs when an R-6 insulated duct system is used in a hot or cold climate.

Though keeping ductwork inside a conditioned air space will increase performance, it’s important to underscore that any HVAC ductwork should be sealed just like plumbing. Air is a fluid, and we want to push that air through a sealed vessel to maintain system efficiency and occupant health.

3. High-Performance Windows

Good windows are an important component of the building enclosure. Unfortunately, in many parts of the country we see window frames made of poor quality hollow-extrusion vinyl or of aluminum channel. These windows are will prove to be shorter lived and will have a greater possibility of leakage and seal failure that higher quality windows. It is important to remember that windows play a critical role in the thermal and moisture boundary. They are also our link to the outside, and can add beauty and quality to the finished home.

Window selection should take into account the local climate and other criteria much broader than the cost of the unit. Considerations in that decision process should include the window’s design pressure rating, which is affected by the weather and the location of the home. When a window will be exposed to wind or lots of rain, a higher DP rating becomes more important. The higher the DP rating the more resistant it is to wind-driven rain getting through the assembly.

Every window should have a label from the National Fenestration Rating Council, or NFRC. There are two key numbers on this sticker to look for:  The U-value and the Solar Heat Gain Coefficient. The U-value is a measure of the amount of heat conducting through the unit, and the Solar Heat Gain Coefficient is a measure of the radiant heat flowing through the glass. In both cases, make sure your windows are below .4. The lower those numbers get, the better the window will perform.

Finally, make sure the window has a good warranty on the insulated glass unit. There are plenty of windows out there with poor warranties, good for only for a few years. These units can lose their seal and fog too soon, leaving the homeowner disappointed. Look for a window that has at least a 10-year warranty against seal failure.

4.  Controlling Indoor Air

To ensure a healthy and comfortable indoor environment, we need to keep the air at a comfortable temperature, maintain a reasonable relative humidity, filter particles out of the air, and provide fresh air.

The focus of the HVAC system is typically on heating and air conditioning to keep the air temperature comfortable. Ventilation – maintaining a supply of fresh air – is often the missing letter in HVAC.

It is vital to the health of the building’s occupants, that there is a supply fresh air at the standard level of 62.2 set by ASHRAE. This standard requires 7½ CFM per person of fresh air flowing into a house on a continuous basis. It also requires an additional contribution of .01 CFM/ft2 of conditioned floor area to be part of the total designed capacity.

HVAC contractors and distributors often focus on filtration rather than ventilation. Filters are important, but it’s only one piece of the puzzle. Filtration will take out the particulates, such as dust, but it won’t take out gases. Carbon monoxide, for example, is NOT something you filter. Formaldehyde or other toxic emissions from building materials, furniture and household cleaners cannot be filtered easily. These must be ventilated. 

Once we have fresh air coming in, then we can filter it. Filters should have a MERV (Minimum Efficiency Reporting Value) rating of at least 8 – the minimum level recommended by the American Lung Association. The higher the MERV rating, the higher the filter efficiency.

Interior relative humidity is also very important. Here, too, ASHRAE has set a standard between 30% and 55% relative humidity. Essentially, you don’t want the air to be either too dry or too moist. The ventilation system can have various design strategies from simple fans to energy recovery systems. A house located in an area with consistently hot and humid conditions, has different ventilation issues than a house in a cold, dry climate. The design should provide filtered ventilation – exhausting stale air as it brings in fresh air. It is very important to distribute this air to all inhabited areas on a constant and reliable basis.

High relative humidity is a challenge for most building, as it increases the chances that moisture-laden air will condense on a building surface and lead to mold or rot. The integrity of the building enclosure is your best defense. While indoor relative humidity can be controlled, we’ll never control outdoor humidity. Only an intact air barrier will keep large volumes of moisture-laden moist air out of walls, and only a forgiving wall assembly that is vapor open will protect the structure when water does get in. There are wall systems that are different from conventional assemblies and can utilize various materials, strategies and techniques to control these forces of nature in very effective ways. Before selecting an alternate approach, remember to consider all the necessary principles of physics and the need for proper installation and maintenance.

It is also worth underscoring the conditions that support mold growth. Mold needs moisture, and it needs a source of food – usually wood fiber or paper. Paperless gypsum eliminates one easily accessible source of food for mold, but it won’t eliminate mold. Mold spores are always present in the air. If there’s enough moisture to support the growth of mold, the spores will find somewhere to grow.

In summary, a home that’s built with an emphasis on improving the building enclosure – including better insulation, improved air sealing, high-performance windows, and ventilation – is a building that will demand significantly less energy to make it comfortable. A better building enclosure is good for the environment, good for the customer, and also good for the builder.

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