Construction Materials-A Less Common Sheathing

This post first appeared on the Green Building Advisor’s website.

When I first started working in the trades as an electrician back in the mid 1990’s, we worked for a couple contractors that liked to use buffalo board sheathing.  I suspect the product was given this name because of its resemblance to buffalo chips.  (If you don’t know what a buffalo chip is, you’ll have to look it up, it’s not the kind of chip you eat.)  I’ve heard it called several other names, bildrite, beaver board, brickboard, bagasse, but it is best known as fiberboard sheathing.

Fiberboard sheathing is primarily made of wood fibers that are combined with a binder.  (Don’t confuse fiberboard sheathing with fiberboard insulation, such as Gutex.)  Other additives improve moisture and fire resistance.  It does not have the density of other sheathing products, this gives fiberboard a higher R-value, approximately R-2.5 per inch.  Another feature, a perm rating of 5 to more than 20 depending on the manufacturer.  Being it’s a lower density sheathing, it does not have good shear strength and usually requires some additional form of bracing, that being said, there are some fiberboard products on the market that are rated as structural sheathing.  When installing new, manufacturers require a house wrap or other water resistive barrier be installed as soon as possible.

Fiberboard has been around for more than 100 years.  It became popular in the 1950’s in pockets around North America.  I haven’t seen it used in new construction in my market since the late 1990’s, though the product is in stock at my local lumberyard.  I suspect they purchased a bunch recently when lumber prices spiked as an alternative to plywood and OSB.  I bought one 4 x 8 sheet to test for $12.

I remember working on a home several years ago where the cladding was installed over the fiberboard without any type of house wrap.  More recently, I was involved with a couple older homes where fiberboard was present.  The photo to the left was one of those projects.  An air leak on an exterior wall caused the fiberboard to become a condensing surface during the winter.  Though I didn’t have to work with the sheathing on those projects, I got to thinking about how someone would install a new window, or tie in an addition when fiberboard is present.  What if there was no house wrap?  What do we need to do to make the water and air control layers continuous?

What sticks to it?  To answer this question, I decided to conduct a little wing-nut test.  If anything had a chance, it would be one of the better tapes currently on the market.  I built a small mock-up wall with 2x framing and attached a piece of fiberboard sheathing.  I ran three of the best tapes I have from the fiberboard to the wood framing, rolled, and then let set for a few hours to achieve a good bond between the tape and these two surfaces.  The three tapes were Siga’s Wigluv, Pro Clima’s Tescon Vanna and 3M’s 8067.  I then tried to pull the tape off. 

All the tapes easily pulled away from the fiberboard but stuck tenaciously to the 2x wood framing.  If the tape was never disturbed and never got wet, it might stay in place, but I wouldn’t guarantee it.  Next, I tried using a primer designed to be used with a different type of tape, Henry Blueskin.  I sprayed the primer on the fiberboard, allowed it to dry for a few minutes, then applied a piece of 3M’s 8067.  The primer increased the hold substantially.  The two photos show the two pieces of 3M tape, the left piece was adhered to the primer, right had no primer.  Much less fiber was pulled off on the primed piece.  I’m assuming the primer locked the fibers in place.  I have much more confidence in the primed area holding the tape over time.

Another Option.  A fluid applied membrane may also work.   Products like Zip’s Liquid Flash or Prosoco’s Fast Flash are basically a liquid tape.  I tried Zip’s product on part of the unprimed fiberboard.  It held better than the tape, but not as good as the primed tape area.

The takeaway.  My preference is not to use fiberboard sheathing, just my opinion.  There are a lot of older houses in my market with the product installed.  My goal has always been to leave a house I work on better than I found it.  Coming up with a few strategies when working with uncommon building materials can go a long way towards achieving that goal.