Should an ancillary space be included in the air tightness test if it is thermally separated from the outdoors but not from adjoining indoor portions?

Including an ancillary space in the airtightness test is appropriate when it is thermally separated from the outdoors, regardless of its relationship to adjoining indoor portions. This ensures that the testing process comprehensively assesses the entire building's air leakage characteristics. By including such spaces, the overall performance and energy efficiency of the building can be accurately evaluated, as air leakage in these areas may still contribute to the heating and cooling loads of the building.

Additionally, measuring the airtightness of all spaces that interact with conditioned areas helps identify potential sources of energy loss and informs better design and construction practices. Not considering these ancillary spaces could lead to an incomplete understanding of the building's overall airtightness performance, which is critical in achieving energy efficiency goals.

The other options suggest excluding the space or conditional inclusion based on factors like ventilation or building size, which do not adhere to the standard practice of evaluating air leakage in comprehensively assessing the building envelope's performance.