Stairs to an unconditioned space

[Sam]

I was asked to undertake a blower door test at a property that had a set of stairs down to the garage. The stairs are in conditioned space but under the stairs is a cupboard accessed from the garage which is an unconditioned space. It is my understanding that if following TSLA 1, with a floor over an unconditioned space you would measure the volume to the top surface of the floor, if it were a stair entirely in a conditioned space you would ignore the volume the stair takes up.

Below is cut through of the stairs and my original attempt at allowing for the volume. The issue is exacerbated by a winder at the bottom of the stair. Not all insulation and framing is shown.



In this situation where it is a stair with one side conditioned and the other not where would you take the volume measurement?

Do you:
1. Measure to the tread and going of each stair, which gives the most accurate volume but increases the surface area.
2. Measure to a slope halfway down the tread of each step, this should give the same volume but reduces the surface area.
3. Measure to the underside of the stair where the airtightness membrane is, above the insulation, assuming the stair is inside the conditioned space and the voume of it can therefore be ignored.
4. Something else entirely?

 

[Paul Carling]

You basically ignore that the stairs are there unless they are solid concrete or stone with no storage or room underneath them. The volume is therefore the room at the top of the stairs connected to the room at the bottom of the stairs by the volume of the opening in the floor void.

 

[Sam]

Thanks @ATT Numpty I believe that is the case if the room underneath is inside the airtightness envelope. In this case, the stairs are essentially the floor of the conditioned space. The cupboard is accessed from the garage and is an uninsulated area and not part of the airtight volume so it wouldn't make sense to ignore the stairs and include it as part of the volume of the house.

 

[Paul Carling]

In that case you need to measure the volume based on the air barrier of the stairs.

 

[windsailor]

@Sam - common sense prevails.