By Brad Oberg
A well-insulated bonus room means that every component of it has been insulated carefully. In order to reduce energy loss (heat loss in winter and heat gain in summer), insulation must fit tightly and be in contact with all surfaces of the cavity (drywall, exterior sheathing, and framing members at the top, bottom, and sides).
Cavity insulation should completely fill the cavity, leaving neither voids around the perimeter nor voids adjacent to any wiring, piping, or mechanical component passing through the cavity. And insulation should be in contact with a sheathing element on the exterior, where installed, even when facing attic conditions.
Why is a sheathing element such as housewrap, structural board, or insulated sheathing needed? Framing irregularities and workmanship will almost always result in several cavities not retaining the insulation fully, so some durable closure of the sixth side reduces unwanted air intrusion, which is important to long-term performance.
In addition, it is important to seal the insulated areas to prevent air infiltration through the walls. This sealing will help to ensure durability and safety. Without airtightness, the moisture in warm, humid air could pass into the assembly and condense on any surface with a colder temperature.
Condensation is problematic for several reasons:
- The wood in the assembly can rot, causing deterioration.
- Mold and mildew growth impact indoor air quality.
- Sealing against air infiltration is particularly critical above the garage to prevent the entry of carbon monoxide and other gasses and vapors that present health hazards to the occupants of the house.
Paying close attention of insulating the bonus room will make it more comfortable, more energy efficient and safer, both for the people who live in the house and for the house itself.
Brad Oberg is co-founder and chief technology officer of IBACOS, a building-science company based in Pittsburgh. He has directed extensive research into the integration of ventilation strategies, residential ductwork design and performance, improved residential airtightness approaches, and durable construction approaches.
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