Which is the Priority: Air-Sealing or Insulation
alexdorf
| Posted in General Questions on
While I recognize that both an airtight envelope and more than ample insulation work together in concert to create a high performance build, if one were to put more weight on either component , how would you rate it.
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I think conventional wisdom is four control layers of your building are prioritized thus: water first, then air, vapor, and insulation, in that order. Air sealing definitely takes priority over extra insulation.
For sample reference, see BSI-090: Joseph Haydn Does The Perfect Wall
Thank you for the reply! Makes sense.
As Zephyr7 says, it's not that hard to do a decent job of both on new construction (with the big assumption that it's a "reasonable" design, ie, adequate overhangs, fairly simple roof, etc).
Note that the reason for the listed priority isn't primarily energy, it's to prevent damage. Energy calcs don't prevent rot, so the advice would be to get your priorities straight (sorry) and then do a good job with each detail and connecting the control layers.
$0.02
load calcs can help with this as they can assign a value to the "infiltration rate" load. In my case they calculated 17K of my heating load came from infiltration losses at 3.5 Ach 50. That could be compared to how much insulation it would take to reduce heating load by the same amount as reducing infiltration would.
That's an interesting question. My first reaction is the two things need to work together as a system, so you can't skip one or the other. Fiberglass insulation especially is compromised by the lack of an air barrier, for example.
That said, I would prioritize insulation if I had to pick one or the other. A good air barrier alone isn't insulation, so at least with insulation you do get some insulating -- but that assumes there is some structure too -- not just a bunch of gaping holes and obvious air leaks.
Ideally you want an air barrier AND insulation, and it's not that difficult to do a decent job of air sealing on a new build. On a renovation project it can be trickier, but it's worth the effort to at least try to do a decent job of air sealing whenever you have things opened up.
Bill
Weather sealing is always about the weakest link.
I have a modest 2 level, 2200 square foot house under construction in Zone 6. I calculated 14.1 kBTU/hr at -5° F for the envelope with R20 under slab, R23 walls, R40 ceiling and triple glazed windows.
Each 1.0 ACH/50 requires an additional 25.5 kBTU/hr. So managing air has a lot more potential to cut operating expense than installing more insulation.
What do you (and others above) use for ACHnat when doing these calcs? How are you correlating ACH/50 to ACH/nat?
-Nate
One has to be very clear what they are trying to calculate. Peak load or average load. With very high wind, ACH/nat might equal ACH/50. This has some relevance to peak load, but little to seasonal average load.
Starting with nothing, I would prioritize getting a reasonable level of air control, then a modest amount of insulation. Going beyond basic levels, I would again prioritize air control, then super-insulation.
After years spent hearing my colleagues answer listener questions on the FHB podcast, I am confident saying that the main priority on almost all fronts is air-sealing. In fact, some people call it the Air-Sealing podcast because it seems to be the answer to so many problems.
You can argue back and forth on priorities, but sequence is usually air sealing first--you don't want to have to remove insulation to complete the air sealing you should have done first.