# Zonal Pressure Testing

I got into a discussion with a college the other day about testing the attic connection to the house.

When the house is depressurized to -50pa wrt outside, and the attic wrt house is -49pa, isn’t the attic more “outside” then “inside”? When the house was depressurized to -50pa I also looked at the attic wrt outside, and got just -2pa. My understanding is that if the Blower Door is putting -50pa on the house interior, and the attic only goes -2pa wrt the outside, the ceiling is pretty darn tight.

Any thoughts on this? We talked zonal pressures until my head got foggy. Any other ways to describe this testing process would be greatly appreciated. I may even get a lunch out of it if I can explain it better!

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### Replies to This Discussion

If I am understanding this correctly when the house is at 50Pa relative to outside and the attic is at 2Pa relative to outside then there is not much connection between those two zones. The 2Pa in the attic could even be due to stack. Sounds like they have a very tight ceiling.

Silvie Votrubova, Retrotec

Here are a couple of things to remember when measuring zonalpressures.  First is that you want tomeasure the change in pressure caused by the Blower Door, so you will need torecord a baseline attic to outside pressure. Secondly zonal pressures do not tell you the size of the leakage betweenthe house and the attic.  The zonalpressure tells you the leakage ratio between the house and attic compared tothe leakage between the attic and the outside. For example, if the attic to outside number is 25 pa, then the hole sizebetween the house and attic is the same as between the attic and outside.  If the attic to outside number is 2 pa, thenthe ratio is 1 to 8.  It is possible tohave a attic to outside zonal pressure of 2 pa on one house and 25 pa onanother house and have an equal hole size between the house and attic.

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