on Sat Dec 28, 2002 7:39 pm
"Chuck & Linda" <clearned@...> wrote:

>...Wisconsin ... tamarack perlons on our curvy
> Cherry Beams.
> ... lay up oak planking milled 7/16" to ride the
> tamarack wave.
> on SunRays wavy delights he lapped the planks
> over each other. I was planning on doing this but
> since it leaves a 7/16" rise every 5" or so inches I am
> wondering about whether the friction at that point
> where the wood laps each other, will wear down
> the vapor barrier.

> My other option is to not lap them and accept that
> there will be a crack between each board where vapor
>can rise up to meet the Tu-Tuff. What I am wondering
> about is whether this would create any moisture problems
> between the cracks?

>So to sumarize:
>1. will the laps wear down the vapor barrier?
>2. lap the planks or not?
>3. and yes or no on vapor barrier?

Chuck & Linda;

#1.
From past observation of cabinetwork (ie made with lumber dried to
less than 7% moisture content) which has been sealed with a finish on
all sides & ends, a generalisation was made that one could expect to
see 1/32" of movement across the grain for every 2 inches in width,
due to seasonal changes in humidity. ie for a 5 inch wide piece of dry,
sealed, wood, one could expect the wood to regularly expand and
contract at least 3/32".

For boards used in a ceiling, I'm guessing that if they are sealed with a
finish, it would only be on one face and if some forethought is
exercised, possibly the end grain, leaving almost 50% of the board
surface to do what it is that hygroscopic materials do (albeit the end-
grain is where most moisture gets sucked up and released).

So will movement due to the seasonal humdity cycles eventually
abrade a hole into the Vapour Diffusion Retarder (VDR) sheet
membrane ? I suppose the answer would depend on how long one
expects the building to last.
25 yrs ? Maybe not.
50 yrs. Maybe yes.
100 years. Probably.

However, wood movement due to humidity changes probably isn't the
only thing that will stress the membrane. Every time the wind sucks,
there will be movement and abrasion. Every time the temperature
changes. Long-term creep of the membrane itself. etc. One or
combinations of the preceding can result in failure of the membrane in
a few short years if not sooner.

#2.
It's not clear from your description what the lapped joints will
look like but I'm guessing that you're referring to laps along the
longitudinal edges of the boards, in clapboard fashion.

In high humidity situations, wherever there is wood tight against
another material, the joint holds moisture, usually leading to mould
growth and possibly eventual rot.

In Chuck & Linda's scenario, drying would have to occur towards the
inside since there is a VDR on the backside ostensibly preventing
vapour movement towards the outside.

I can think of 2.5 ways that that drying would occur:
(1) temperature on the backside of the planking higher than the temp
on the roomside and
(2) The temp on the roomside being really hot at the same time that
the humidity is really low.

Another issue is that ship-lapping provides support for the board only
at small areas at the edges. It sound like Chuck & Linda intend to
load the boards with bales-and-who-knows-what. This would be
like a slow-motion karate-punch to the board wouldn't it, leading to
the board likely cracking along its length if unsupported across it's
width ?

And, if fastened improperly (ie nails or screws at both edges) the
boards would split even without being loaded.

The botom line, IMO is that lapping them a la SunRay (although I've
never seen what SunRay actually does, I'm going by Chuck's
description only) would seem to be bad detail on a number of fronts.

It might be better to leave a gap as a reveal to create a shadow line
for visual interest and spline it with thin strips of 1/8" Masonite or
somesuch (salvaged doorskins from discarded slab doors etc. ?)
Such a detail would mean that each board would yield more
coverage, ultimately leading to reduced wood usage.

#3.

It should be obvious that some strategy will be needed to minimise
moisture movement at the ceiling area in a ColdClimate building
where vapour pressure will be highest.

A sheet membrane is usually the easiest solution in that it acts as an air
barrier and a VDR. In buildings where drywall is used on the ceiling ,
often taping/caulking/paint is effective enough that a sheet membrane
becomes redundant.

Since Chuck & Linda's ceiling will be comprised of boards with no
sheet membrane, the boards would have to be assembled/sealed like
the hull of a boat in order to be effective at minimising moisture
movement into the insulation where it will reduce its insulative
properties, likely result in mould & fungal growth and possibly
subsequent rot.

Which brings up a point that Bruce, King of Sausalito mentioned on
the GSBN List before he skeedaddled of to Australia to el Lupo's
Wino Festival:
" No one has ever died from exposure to moulds"
a statement which should be good for a head-bonking thread here in
the future.
~~~ * ~~~
Rob Tom
Kanata, Ontario, Canada
<ArchiLogic@...>
(winnow the "chaff" from my edress in your reply)

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