Typar/Tyvek Permeability
josephny
| Posted in General Questions on
I was going to put Tyvek on the sheathing around my house but the local place has Typar and says it is much better.
A quick looks shows the Tyvek has far higher vapor permeability.
My walls have 3.5” of closed cell foam (I know, I know) and therefore are a good vapor barrier.
my thinking is that the membrane on the outside of the sheathing should have a high perm so that the sheathing can breathe or dry to the exterior.
My supplier says both products breath outward the same.
Is that accurate?
Is my analysis on target?
Thank you.
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Replies
Typar cites this Building Science Corp. paper as to why their perm rating of 11.7 is good: https://www.buildingscience.com/documents/building-science-insights/bsi-061-inward-drive-outward-drying
BSC is a well respected source of information on the topic.
Every product needs to be considered in context of the design and other products being used. I’d suspect in most cases, Typar is a fine product to use.
(For what it’s worth, I’ve been using Benjamin Obdyke FlatWrap recently and really like the quality of it. They’ll also ship direct to your job site. My carpenter, who has hung miles of Tyvek, said it is way more pleasing to install than typical house wrap.)
Both are good products and will work, I wouldn't worry about using either one. If you can get access to tyvek commercial roll (costs more than regular stuff), but it is way stronger and can be left exposed for much longer before siding.
The only WRB you want to avoid is the woven stuff that looks like a tarp, those are best left for covering lumber at the lumber yard.
https://www.greenbuildingadvisor.com/article/the-complicated-role-of-a-water-resistive-barrier
So I don’t have to worry about sandwiching the sheathing between two vapor impermeable materials?
Thank you!
What are the "two" vapor impermeable materials? Typar, at 11.7 perms is not anywhere near impermeable.
Ah! You found my mistake.
The two materials I am referring to are (1) the cc spray foam and (2) the typar
But you saying that the typar over the sheathing won’t prevent drying of the sheathing to the outside?
Joseph,
All the WRBs that have been mentioned in this discussion - Typar, Tyvek, Flat-Wrap, and Commercial Tyvek allow adequate drying to the outside.
Typar is not a vapor BARRIER, it's a vapor RETARDER, which means it slows, but does not prevent, moisture migration -- drying, in this case. Latex paint, as an example, is often mentioned as "allowing drying", but is only about 5-9 perms -- less than the Typar's 11.7 perms. 11.7 perms is more open than even a class III vapor retarder (up to 10 perms).
A true vapor BARRIER like polyethylene sheet is typically under 0.1 perms.
The short answer is Typar allows drying, it's not a vapor barrier.
Bill
That is great to hear and to learn.
Thank you all again very much.
Typar is not a vapor retarder either; it's vapor open and will hardly slow down water vapor on its way through the wall assembly. It's a WRB--weather resistive barrier (or water resistive barrier, if you prefer) and can double as an air barrier if detailed as such.
It's also watertight, so often leaks that get behind the WRB are blamed on the WRB because depending on conditions it can take a long time for liquid water to vaporize and flow through the Typar to the exterior.
Would a higher perm be better in a cold climate though?
ReneK,
One way of looking at it is whether the amount of drying the wall needs to stay safe is met by the perm rating of any given WRB - and whether it's realistic to expect walls to allow large amounts of drying to perform well. If something like Tyvek Commercial's 28 perms is too low, my feeling would be that the wall design relies too heavily on drying to the outside.