12 Comments

1. 
   user-723121 | Feb 04, 2021 02:20pm | #1

   Randy,

   Fabulous article, thank you for making the connection between ACH50 and CFM50. Both measurements are used and the calculation needs to be understood. Everything I thought about when I read the title, you covered, great work !!

1. 
   Randy_Williams | Feb 04, 2021 05:37pm | #2

   Thank you Doug!

2. 
   charlie_sullivan | Feb 04, 2021 06:19pm | #3

   Great explanations and discussion.

   I agree that CFM50/(sq. ft.) is a better number to use if we can get people to the point of thinking that way. But I have to admit that with GBA's font, I was thrown for a loop by seeing it written as FT2, since it looks like 2 is the subscript. I think sq. ft. is a better way to write it when the technological capability to write it as ft² isn't available. (There's also a mistake later where it's written SF2, which looks like sulfur difluoride, and translates to square feet squared.)

   A question: I agree that it's good not to test when there's vermiculite insulation. On old houses, how do you know? Do you take a peek in the attic before setting up the door, or interview the homeowners?

1. 
   Randy_Williams | Feb 04, 2021 07:42pm | #4

   Hi Charlie, that's my fault with the sq ft2, didn't look for the alternate character like °. As far as the vermiculite, I do ask and check when testing older homes. I've also seen it loose filled at a rim joist, very strange seeing it there. If it's in the walls, chances are I won't catch it unless the homeowners know. Good question.

1. 
   pmarsh4 | Apr 05, 2021 09:15pm | #11

   Randy,

   When you have come across it loose filled at rim joists is it part of a balloon framed house? Or just kind of filled in at the joist.

   Thanks for the article!

1. 
   Randy_Williams | Apr 06, 2021 06:58am | #12

   The time I ran into vermiculite at the rim, the home was platform framed, not balloon framed. Someone had installed a 2x inside of the rim joist with a small space between the floor framing and the top of the 2x to allow a space for pouring the vermiculite into the cavity. Someone went through a lot of work getting that space insulated.

   Thanks for the question.

2. GBA Editor
   Kiley Jacques | Feb 05, 2021 08:45am | #5

   Hi Charlie,

   Thank you for pointing out that confusion and inconsistency. I've updated the post to reflect your comments. Keep 'em coming...

3. 
   tundracycle | Feb 09, 2021 11:03am | #6

   Randy, good article!

   I agree with moving to CFM50/ft² for two reasons. First is that as Martin and others have pointed out it is simply a better and more informative measure.

   The other is that it will help us compare ourselves to other countries better. ResNet uses the total envelope interior (including dead space between ceilings & floors) for the volume measurement (denominator) which results in a lower and perceived better ACH50 than other countries who use the actual conditioned space where air is actually changed (floor to ceiling).

   A 1.5 ACH50 house in Europe is much tighter than a 1.5 ACH50 house in the U.S.

   Also, what are you seeing for houses built within the past 2 or 3 years as far as ACH50? Are they improving each year or have we kind of plateaued?

1. 
   Randy_Williams | Feb 09, 2021 06:39pm | #7

   I'm seeing improvement. The tightest house I had tested three years ago was 1.75ACH50, that's now close to normal for me. I have a few that are less than 1 ACH50. The insulators, who do most of the air sealing in my area are very good. Thanks for the comment!

1. 
   tundracycle | Feb 10, 2021 10:04am | #8

   Perhaps you can send your insulators down to the cities :-)

   Are builders in your area beginning to understand how important sealing is as the project progresses rather than as a one-time 'the insulators will do it' thing?

   Are you seeing much in the way of exterior insulation? We had planned to do exterior insulation on our house but got hard pushback from architects and builders. They'd be OK at first talking about it but then get cold feet and run back to what they knew.

4. 
   john_prospect | Feb 11, 2021 02:45pm | #9

   Thanks for the great article. I like how you provided an example house.

   One thing I would be curious about is how quickly surface area increases (relative to volume) as a building design gets more complex? I don't have enough knowledge of building fashions regarding bump-outs, turrets, dormers, courtyards, etc. to guess what is seen in the real world.

   We see that ACH50 punishes small home design. Will surface leakage punish simple home design, or is envelope "complexity" not much of a factor?

   Edit: I can see that envelope complexity is probably self-limiting as there are more places to make mistakes on the air sealing.

5. 
   Randy_Williams | Feb 15, 2021 08:00am | #10

   Hi John,
   You're right, the more complex the design, the harder it is to air seal. You're also right with very small homes being punished by ACH50, very large volume homes get a break. There is something interesting going on in Washington State's code, they are limiting the ceiling height to 8.5 feet with regards to the volume calculation. An attempt to level the playing field between large volume and small volume homes. Hoping to have an upcoming article that talks more about that code.
   The question about the relationship between surface area and volume, I haven't tried to find a correlation between the two. No one I'm doing testing for is paying attention to the CFM/Sq Ft number, I do believe it's a better metric, but the majority of testing I'm conduction only concentrates on the ACH number. I've recently started including the CFM/Sq FT on all my tests, I should start a spreadsheet to track the relationship.
   Thanks for the question!