External air barrier – how real is the benefit of non-adhered wraps?
We’re working on a remodel of a 1940/1990 house in climate zone 4 Marine (Seattle). We’ll be redoing the siding and we are hoping to put in place an external air barrier.
Our first thought was VaproShield SA, a self-adhered product. However, it appears that it won’t adhere well to our existing (old) plywood sheathing. The manufacturer is recommending VaproShield IT, which is similar to SA, but it is manually fastened (stapled) to the sheathing.
It seems to me that any loose air barrier is likely to perform poorly over time as holes in one location of the barrier allow air to transit to all locations. And more holes would likely be formed over time, so it would just degrade more quickly than a product which is stuck to the sheathing…
Am I thinking about this the right way? What is our best option for an externally applied air barrier on existing plywood sheathing? The liquid applied air barriers? Some other roll on product?
Thanks in advance!
Dave
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
> However, it appears that it won’t adhere well to our existing (old) plywood sheathing.
Is that speculation from the manufacturer, or based on an adhesion test?
You are right in thinking a self-adhered product is a more foolproof way of getting an air barrier on an existing structure. I think you can find a product that will adhere to old sheathing (perhaps hit with a leaf blower or other light dusting method). Might be worth testing samples of VaproShield and other products like Blueskin, Benjamin Obdyke, Pro Clima, and Siga.
Vaproshield is sending out a sample for an adhesion test - but they did give us a warning that it might not work. Great idea on the alternative membranes and thanks for the product recommendations! We'll try adhesion tests on those as well!
Dave,
Why not make the old sheathing the primary air-barrier instead? That would leave you free to use an non -adhered WRB, and not worry about how effectively it air-sealed.
On interior inspection, it appears that there are lots of nail holes in the sheathing so my working assumption is that it will be a pretty substantial amount of work to fill the holes, and is likely to result in a less than ideal outcome... We won't be doing a studs out interior remodel - I can only see the nails in the currently unfinished basement. Am I thinking about this the right way?
Have you considered fluid applied WRBs? 2 more products not listed in this article are Tremco ExaAir 230 and ProClima Visconn - both of these products also have high perm ratings.
https://www.greenbuildingadvisor.com/article/liquid-applied-water-resistive-barriers
What kind of siding do you have?
What Malcolm says makes a lot of sense if it's possible.
Some people say that using rigid exterior insulation to hold a mechanically attached membrane tight against the sheathing helps reduce air transit between fastener holes. I don't know if anyone has tested this, though--I heard it from builders saying their blower door scores went down, but they didn't change anything except adding exterior insulation.
Siding is currently vinyl and likely to be replaced with fiber cement boards.
I like the idea of rigid exterior insulation over top of the sheathing and we originally considered adding a layer of zip sheathing over the existing plywood, but ultimately the extra thickness meant we would have had to redo all of the interior trim on the windows which drop up cost considerably.
I would think a product like Delta SA vent applied with its primer would stick fine. Also delta has an additional strip on the front that aids in adhering overlapping sections. Blueskin also has a primer. Lastly what would be the harm in using an SA with staples if you are worried about it in spots, other than cost I suppose. Proclima products are also very tenacious. We have installed them on a number of remodels without issue, they also have a primer.