WRB for a fiberboard wall assembly?

astroskidder | Posted in GBA Pro Help on July 1, 2014 12:33pm

I’m building a house where the exterior walls have been sheathed with a bitumen treated structural fiberboard. I have two questions:

1. Should I tape the seams of the sheathing on the exterior side of the wall? If so, with what?

2. What is the best WRB for this type of wall assembly… building paper or Tyvek/Typar etc.?

Thanks.

Answer

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GBA Editor

Martin Holladay | Jul 01, 2014 07:55am | #1
Daryl,
Asphalt-impregnated fiberboard is hard to tape, as far as I know. I don't know of any tape that would work, but if I had to experiment, I would start by trying Siga Wigluv tape.

Asphalt felt, building paper, or a plastic housewrap like Tyvek or Typar all work well as WRBs. The most important thing is getting the installation details right (lapping the WRB properly, and integrating the WRB with flashing at the window penetrations). These installation details matter more than the material you choose. For more information on this issue, see All About Water-Resistive Barriers.
astroskidder | Jul 01, 2014 10:49am | #2
Thanks Martin.

We live in what can be a wet, hot climate and I'm concerned about vapour drive through the WRB on the exposed sunny sides of the cladding. I've read some reports about some housewraps actually making the matter worse by trapping water behind the WRB and causing problems with moisture sensitive assemblies like fiberboard, especially in cold climates (zone 6 for us).
GBA Editor

Martin Holladay | Jul 01, 2014 10:58am | #3
Daryl,
If you are worried about inward solar vapor drive:
(a) It's best to avoid so-called reservoir claddings.
(b) Make sure that your walls don't include interior polyethylene.
(c) Include a ventilated rainscreen gap between your siding and the WRB.

For more information on this issue, see When Sunshine Drives Moisture Into Walls.