#1 - REDUCE LOADS / DEMAND FIRST - Envelope
Heat gains and heat losses occur through the building envelope. Reducing losses is a critical part of a Carbon Neutral Design strategy, particularly where heating is concerned. There is a finite amount of passive solar heating that is possible given limitations on glazing to floor ratio, and the reduced hours of sunlight in the heating months. The greater the differential between the interior and exterior temperature, the greater the drive for heat to be transferred across the building envelope.
Degree days, which are a measure of yearly heating requirements, are given by the sum of the departures of the daily mean temperature from 18 degrees C for each day on which the temperature falls below that value. The sum total of degree days for a year in a certain location acts as the basis for measuring the amount of heating (or cooling) required for a building. The higher the number of degree days, the more severe the climate.
AIR LEAKAGE:
The leakage of air out of the building contributes to the heat loss in a building. Air has moisture in it the amount which is measured by the relative humidity of the air. This contributes to the deterioration of the building envelope as this moisture when it escapes the building due to differential pressure will 'get caught' in the building materials it is trying to pass through and condense, freeze, and thaw, and freeze ...
The first principle that must be understood is that air is pressurized, via a relation to the atmospheric pressure and the amount of moisture in the air. The warmer the air is, the higher the pressure it has as a gas. If high pressure air is adjacent to low pressure air, it tries to dissipate, moving to the low pressure area to equalize the pressures. The air inside a building in the winter is hotter than outside, therefore the tendency is for the air to try to pass through the envelope of the building to equalize the air pressure. The reverse is true for an air conditioned interior in the summer.
There are two means that air 'leaks' out of a building and many means of preventing this air leakage. The first type of air leakage is through the cracks in the building, around windows and doors, through electrical outlets, etc. These can be solved by careful caulking, sealing and detailing of these areas. Weatherstripping can be installed around doors. Changes in the National Building Code around 1985 required the installation of an Air Barrier in the building envelope as a means to seal the building from air leakage.
The second means of air leakage is through vapour diffusion. This is where the air vapour travels through the wall materials. Materials each have a unique degree of permeability. This is related to the porousity of the material and its physical structure. Vapour diffusion is normally prevented by the installation of a Vapour Barrier in the wall, floor or ceiling. Polyethylene films are installed on the warm side of the construction (in our cold climate on the inside of the building just beneath the drywall) in a continuous mode to retard the flow of vapour through the wall. This will not completely stop vapour diffusion, but will greatly reduce damage to to moisture escaping into the exterior envelope.
HELPFUL LINKS IN THE AIA 50to50 WIKI:
Cavity Walls for Insulating Air Space
Materials and Assemblages
Thermal Bridging
Windows and Openings
Perhaps the Best Resource on the Internet for Building Science and Envelope Concerns:
http://www.buildingscience.com/index_html
HIGHLY RECOMMENDED TEXTS FOR PASSIVE DESIGN (written by SBSE members!):
Heating, Cooling, Lighting: Sustainable Design Methods for Architects. Third Edition. Norbert Lechner.
Mechanical and Electrical Equipment for Buildings. Benjamin Stein, John Reynolds, Walter Grondzik, Alison Kwok.
Sun, Wind and Light: Architectural Design Strategies. Second Edition. G. Z. Brown and Mark DeKay.
The Green Studio Handbook: Environmental Strategies for Schematic Design. Walter Grondzik and Alison Kwok.
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