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The external walls of the timber-framed house from outside in:

  • EPS cladding
  • studwork with batts that are the same thickness as the width of the studwork
  • battens for an airgap
  • drywall

I requested the air gap for ventilation in case moisture gets into the batts and as a cavity for wiring. I didn't like the idea of the wiring being sandwiched between the batts and the cladding. The climate is temperate but it gets very hot in summer. Also, it is often very humid and mold in the batts is a concern to me.

I have had to find a new builder after my previous builder had an accident and is recovering. The previous builder only pointed out that the air gap would open into the roof cavity at the top and suggested fitting some mesh to keep insects out.

Initially, the new builder suggested using wider studs with the standard-width noggins because it would be cheaper and sturdier than the battens. I thought that was ok but I realized the top plate would now cover the air gaps so I asked the new builder if it were possible to use a standard-sized top plate with the wider studs.

He now tells me that encapsulating the batts with the drywall would not potentially compromise the R-value of the wall like my open-at-the-top air gap idea. Is that correct?

Edit: Note the roof cavity will be well ventilated and the plan is to seal the baseplate to the slab.

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  • Cellulose rather than batts (fiberglass or rockwool, presumably) offers better moisture management.
    – Ecnerwal
    Aug 9, 2022 at 17:21
  • Normally, external wall cavities are completely filled with insulation with no air gap at all. That strongly points to your desire to having an air gap as very non-standard. Of course, "non-standard" doesn't inherently mean "wrong", but I think that in this case, it's a pretty good sign.
    – FreeMan
    Aug 10, 2022 at 15:40
  • @Ecnerwal, you may need to substantiate that. Cellulose is shredded newspaper. It's not what I'd consider a good steward of moisture. Maybe there's more to your statement than I realize.
    – isherwood
    Aug 10, 2022 at 15:51
  • @FreeMan, plenty of walls have things like sound channel which create air gaps. It's far from odd and there's no reason to change course.
    – isherwood
    Aug 10, 2022 at 15:52
  • cchrc.org/media/CelluloseSnapshotFinal.pdfgreenfiber.com/uploads/documents/…cellulose.org/GreenestInsulation/2020/01/24/… • or DAGS yourself on cellulose insulation moisture management
    – Ecnerwal
    Aug 10, 2022 at 16:31

1 Answer 1

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No, but (or yes, if)...

You lose insulation value any time there's a path for air around the insulation. In fact, such a path can almost completely negate the value of the insulation if there's enough airflow. By introducing a gap behind the drywall you create opportunity for this. If the insulation layer is sealed well against outside air intrusion or inside air escape, it's not a concern. But if air can get around the insulation you lose efficiency.

Another way of saying all that is that the drywall can (and often does) act as an air barrier, even if only a supplemental one. If you don't need that because your insulation layer is well sealed, all good.

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