[Strawbale] 5 perms / no ventilation; and top-of-wall (Mark Piepkorn)

[Rob Tom]

On Mon, 12 Mar 2007 14:55:03 -0400, John Swearingen  
<john.skillfulmeans at gmail.com> wrote:

> On 3/12/07, David Neeley <dbneeley at gmail.com> wrote:
>>
>> John,
>>
>> In the original post, Dr. Straube was quoted as saying: "The top seal
>> of the wall needs to be an air barrier, and not a vapor barrier."
>>
>> Does this mean you disagree?

>>
>> My question, based upon the original Strabe comment, was whether Tyvek
>> or one of its competitors might actually be useful to give such an air
>> but not moisture barrier used in that position.

In the early days of superinsulated wood-framed construction (here in  
Canada at least) (like a quarter of a century ago ?) when people took  
great pains to ensure the continuity of the polyethylene sheeting that was  
typically used at that time as the "continuous air/vapour barrier", it was  
necessary to devise some means of connecting the polyethylene used over  
the basement wall insulation and the poly sheet used over the main storey  
wall insulation and the easiest way to accomplish that connection while  
maintaining air-tightness over the rim joist/mudsill was to place a strip  
of the barrier material under the mudsill, leaving flaps on both sides to  
enable joining once the rest of the framing had been erected and the walls  
had been insulated.

This of course meant that if one used the same material (ie polyethlene)  
it would mean having a vapour-impermeable membrane on the cold side of the  
insulation that would be used to fill the ends of the joist cavities,  
thereby contravening the "no vapour barrier on the cold side of the  
insulation" rule of thumb so the early SI builders used vapour permeable  
Tyvek for wrapping the rim joist.

But the same situation shouldn't exist at the top plate of a SB wall  
assembly.

I understand that there might be concern about inablity to dry inwards but  
the concern at that point is more about bulk moisture transport via  
air-leakage from conditioned interiors than it is about wet walls needing  
to be allowed to dry out.

Other than roof leaks, chances are pretty slim that that portion of the  
wall right up against the eaves would be subjected to severe rain wetting  
and the consequences of inward vapour drive and hence the need for drying  
to the interior.

Any moisture in the wall as a result of roof leaks would tend to move down  
and accumulate in the lower portions of the wall.

=== * ===
Rob Tom
Kanata, Ontario, Canada
<A r c h i L o g i c   at  c h a f f y a h o o   dot   c a >
winnow the chaff from my edress in your reply