[Greenbuilding] Ceiling Air Barrier

[RT]

On Sat, 10 Dec 2011 17:52:27 -0500, Peter/Pam Martin <p2j2 at shaw.ca> wrote:

>  southern Vancouver Island,would like to add an air barrier to an  
> existing ceiling
> exposed ceiling joists with t&g above, then 1x6 boards, then R20  
> fibreglass batts with vented attic above.

> was thinking of a vapour permeable or semi-permeable air barrier between  
> the 1x6 and the fibreglass, perhaps 1" EPS or house wrap or building  
> paper or ? What think ye?

Pam/Peter;

I really don't anything about the climate of southern Vancouver Island  
other than it being bizarrely un-Canadian-like.

But the detail of t&g ceiling (ie air-leaky) with 1x6 boards directly on  
top did make the hairs on the back of my head perk up a bit.

The wood-on-wood contact combined with relatively uncontrolled air (and  
piggy-backed moisture) leakage from below sounds like a potential moisture  
trap with good possibilities for mould & mildew formation and subsequent  
wood rot.

I think (again from the perspective of a clueless-about-the-We(s)t-Coast  
Easterner) that I'd look at inserting the air barrier on top of the t&g  
possibly eliminating the second layer of wood and replacing it with rigid  
insulation (maybe Roxul rather than EPS . ie the Roxul will not  
burn/melt/drip in a fire situation (don't know how much of an issue that  
would be since thickness of the t&g wood ceiling is not mentioned... big  
issue if the t&g is 1", not so much if it's 2")  if the 1x6 is there to  
provide some additional rigidity for the t&g ceiling... notwithstanding  
Roxul's other outstanding capabilities WRT moisture/drainage .

The thought of a thin layer of straw/clay on top of a sheet membrane air  
barrier to provide some moisture handling capacity and protection/support  
for the membrane cross my mind but if rigidity of the t&g ceiling is an  
issue, then it probably would not be worth considering.

As for the membrane itself, I'd be inclined to use something like the 6-10  
mil black poly that farm supply outlets sell. It comes in 20 to 100 foot  
widths (ie potentially no seams to deal with), is good and tough,  
inherently UV resistant and a lot cheaper than the clear poly that is  
typically sold at building supply outlets.

I realise that spun-plastic fibre housewrap (ie Tyvek et al) would seem to  
be preferable to a vapour-impermeable poly membrane but I do have to  
wonder how much drying to the indoors would really be possible with R-30  
or more of insulation on top of the membrane, on the south end of  
Vancouver Island (ie does it ever get really hot ... hot enough to convert  
condensed moisture into vapour and drive it through the vapour-permeable  
membrane ?)

For the air barrier,



-- 
=== * ===
Rob Tom
Kanata, Ontario, Canada

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