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Air movements will influence the thermal performance of the building envelope; to what degree will depend on the pressure situation in and around the structure, the permeability of the different materials, and the airtightness of joints between materials and building elements. In a multilayer structure with a high thermal resistance, different parts have different functions in order to protect against unwanted air flow and ensure airtightness.
Wind protection will give a degree of safety against air flow in the insulation, especially in situations involving large wind loads. Certain areas of the structure may be more sensitive and have a higher need for wind protection than others. In a similar way, the building envelope will be sensitive to air infiltration. The inside—that is, the vapor barrier, inside board, and joints—will greatly influence the air flow and the heat losses from the envelope.
Both types of increased heat losses from the building envelope (that due to air flow along the insulation and that due to air infiltration) will most influence envelopes with high thermal resistance. This type of envelope is typical of today's design in Scandinavia, and the results and discussions are related to this.
thermal insulation, air movements, permeability, wind protection, airtightness
Professor, Swedish National Testing Institute, Borås,