First, you might ask . . .”What is a thermal envelope?” A thermal envelope consists of all the building components that protect the interior of a home from the outdoors, including non-livable space. Homes built in California must adhere to Title 24, a section of the California Code of Regulations, which govern our building codes – including energy and mechanical code standards. You can find out more about Title 24 here.
Homes that feel drafty or encounter leaks during weather events are those that don’t have a tight building envelope. The vapor barrier assemblies used in buildings that employ the best building practices focus on creating an airtight barrier at the roof, exterior walls, windows, doors, and at the sill plate, footings, stem walls, slab or raised foundation. When designing a home these issues must be thoroughly planned and incorporated to maximize performance and sustainability throughout the life of the structure.
Creating a solid thermal envelope
Preventing air and vapor starts at the foundation. In many cases, a thick polyethylene or rigid foam insulation material is placed under the slab on top of a gravel base prior to pouring the slab. This same material is applied to the footings and stem walls to create a capillary break. A sill gasket is also used between the framing and the concrete connections to further prevent air and water intrusion.
Metal flashings are applied around the perimeter face of the building where the slab meets the framing to provide another layer of defense. The windows and doors should receive a combination of metal, peel, and stick flashings to seal openings and ensure proper performance. The entire structure and roofline is wrapped in an air and water barrier product like a Tyvek system that uses special tape to seal all the seams and joints. Professional installation is the key; no matter how good the system is, it can’t perform as expected if it isn’t correctly installed.
High performing insulation methods include the framing members which should incorporate radiant barrier sheathing at the roof to help deflect the heat from entering the building. This material is installed with the foil side down, inside the structure. Insulation, no matter the type, must be carefully installed to avoid gaps and voids, which would allow air and vapor to infiltrate.
The correct insulation R-value, calculated by a Title 24 expert, should be installed on all exterior walls and ceilings. The R-value represents the insulating materials resistance to heat moving through it. An approved foam product must be used in conjunction with the insulation to fill all spaces between framing members. A cementitious product is applied at all mechanical trade penetrations to support an airtight seal.
Testing the effectiveness
To test the successful implementation of a thermal envelope, an energy auditor can be hired to perform a calibrated blower door test. This procedure measures the air tightness of a home, using a powerful fan temporarily installed at the front entry to pull air out of the structure. This lowers the pressure inside the building – the higher pressure on the exterior then flows in through all the unsealed cracks and openings to determine the amount of air leakage. The results of the test will expose any final weaknesses still present in the structure.
Through careful consideration and proper planning, a highly effective thermal envelope can easily be achieved – but the process needs to start before the first shovel hits the ground. For a home to have the maximum level of energy efficiency and comfort for its occupants, the components that make up its thermal envelope should be of the utmost importance.
In responsible building, maximizing lifetime efficiency and quality through following the latest best practices is always a number one priority, and understanding the proper implementation of a thermal envelope is a prime example of upholding that responsibility.