Posted by: lcowie | November 16, 2010

BPI Building Analyst & Envelope Professional

An important component of the BPI Building Analyst and Envelope Professional exams is understanding the thermal boundary – what it is, what it does, where it lies, etc. The thermal boundary is the separation between the conditioned and unconditioned environments of a building.

Unconditioned spaces include basements, crawlspaces, attics, and attached garages. I like to think of it as the places where it’s always humid. It’s humid because the thermal boundary blocks off these spaces from the rest of the house, so when the air conditioning is running in the home, these spaces do not get the benefit of the conditioning (hence the name unconditioned space).

The thermal boundary must be continuous. When I say this, I mean that there can be no gaps in the thermal boundary. If there is a gap in the thermal boundary, air conditioning will find a way through to the unconditioned space. Gaps create a huge list of problems.

Imagine if there was a hole in the thermal boundary connecting to the attached garage, and the homeowner keeps gasoline and pesticides in the attached garage. The fumes from these containers will seep through the hole and into the house. The same goes for crawlspaces; a gap in the thermal boundary connecting to the crawlspace will enable all kinds of dust, animal droppings, and dirt into the home.

In the BPI Envelope Professional training course, you will learn the distinctions between building shell and thermal boundary. They are very similar but very different! The exterior shell is the exterior surface of a building’s construction – the walls, windows, roof, and floor. The thermal boundary, or envelope, is the sum total of the parts of a building separating conditioned space from unconditioned space. Insulation is located in the thermal boundary, which is where the pressure/air boundary should be located as well. An attic will function as part of the building shell but not as part of the thermal envelope. The attic is on the exterior surface of the building’s structure, but it is not conditioned or considered a livable area.

Unconditioned versus conditioned space is going to be a big deal when you start talking about blower door tests. If the blower door detects a certain amount of pressure in an area that is connected to the house (building shell) but is considered outside (thermal boundary), the BPI Building Analyst will be able to figure out whether or not there is a gap in the insulation and where.  I will discuss this in length in another blog post!

Any questions about conditioned versus unconditioned space? Leave a comment!


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