Building Science
Designing and building durable structures that provide a safe, comfortable, efficient, and healthy living environment starts with an understanding of building science. All buildings have forces acting on them that create pressure differentials. Most building envelope problems arise when we place an air permeable insulation, like fiberglass, over a big gaping hole in the building envelope and then expect it to stop heat transfer, air movement and associated moisture movement. We need to understand that every building is a system made up of multiple sub systems that all interact.
Air Leakage
Stack effect —warm air rises, leaks out of holes at the top of the building and is replaced by cold air leaking in at the bottom of the building.
Wind effect —the windward side of the building is pressurized forcing air to leak in. The leeward side is under negative pressure causing air to leak out. Movement of air over the roof also creates a negative pressure in the attic and sucks air out of holes in the ceiling plane.
Flue effect —this is a variation of stack effect. Hot flue gases accelerate the rate of flow out of a chimney. The negative pressure created by wind blowing over the roof also increases the flow rate. Cold air leaks in at the bottom of the building to replace air leaking out the flue.
Exhaust fans, clothes dryers, and leaky duct systems tied to air handlers create additional "Induced air leakage" in the building.
Moisture
- Bulk liquid — Water moving as a result of gravity, air pressure and hydrostatic pressure.
- Capillarity —Water film deposited by rain is drawn into the building envelope by capillary draw.
- Air movement —When a hole exists, and a pressure differential acts as a driving force, air will move through the building envelope carrying water vapor with it. Once it reaches dew point temperature it will condense.
- Vapor diffusion —When the relative humidity is higher inside the space than outside; water vapor will diffuse through building materials that are vapor permeable.
Note: We obsess about the need for vapor retarders in spite of the fact that water moved by vapor diffusion is miniscule when compared to water moved by air leakage. In many cases, a vapor retarder installed to keep moisture out of an assembly, actually prevents water deposited by air movement from drying to the interior. Since air leakage is a far more significant vehicle for moisture moving into assemblies, many building scientists believe vapor barriers should be used on a case by case basis, only when specific conditions warrant such use. Unfortunately, codes and code officials are not always up to speed with the science.
Our goal is to tighten the building envelope as much as possible. We should insulate the entire building envelope with a monolithic system that eliminates gaps and compression while minimizing air flow through the insulation. (Spray applied products like cellulose and foam are less subject to inherent installation flaws of a batt that must be cut and fitted.) Thermal sheathing should be considered to avoid thermal shorts through framing. Foundations should be insulated properly. We should use only sealed combustion appliances (including fireplaces) that draw 100% make-up air from outside the building eliminating the possibility of back drafting associated with tight buildings. Provisions should be made for radon mitigation. Install mechanical ventilation to introduce dry dilution air from outside the building. This will reduce high relative humidity associated with tight buildings as well as improving indoor air quality. All duct work should be kept out of unconditioned attics, exterior walls or cathedral ceiling cavities. At a bare minimum, duct work placed in attics should be sealed at all joints with mastic and insulated. Avoid the use of can lights in attics. If they must be used, consider air tight cans and seal them with two component foam. Drywall is typically our interior air barrier. Care must be taken that drywall extends behind tub enclosures and behind fireplace boxes.
Designers should not expect the general contractor and subs to fill in the blanks left off on drawings. Communicate your expectations clearly and find enlightened generals and subs that are willing to cooperate. Generals and subs that get training in these concepts can act as a second and third set of eyes that help you carry out your design concept.