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

[Rob Tom]

On Mon, 12 Mar 2007 19:30:31 -0400, Frank Tettemer <frankt at webhart.net>  
wrote:

>
> But cracks are cracks, and in plaster renders, very hard to prepare for
> or plan for. Or prevent.

Frank;

I think that the strategy is to assume that cracks will occur at the  
wall-ceiling junction and having anticipated them, devise a means to  
ensure that continuity of the air barrier at that crack.

That is the basic princsiple behind the use of control joints.


> I visualize the home, with it's inner air pressures, and think about
> this whole question from that avenue of exploration.  And "typical house
> stack-affect" makes me tend believe that ( on a non-windy day), internal
> house pressures from stack effect (in a cold climate)
> are busy moving *hot air* and moisture vapour *outwards*, through any
> passage available, towards the great outdoors, about anywhere on a wall
> from the neutral plane, on up.
>
> And from *below* the neutral plane, *cold, dry air* spends its time,
> slipping *inward* through any cracks.

That is what physics tells us to be true.

>  I'm not so sure that what
> you've said is true, Monsieur Tom.
>
> Rob Tom wrote:
>> "In predominantly cooling climates, air leakage would tend to
>> be from the outside-in at the top plate wall/ceiling junction butthe  
>> outside/inside differences in temp & humidity  (and hence the concerns  
>> WRT)
>> aren't as great in that situation."
>
> I'm thinking: "airleakage goes from inside to outside, near the top of a
> wall assembly, and then one can address airleakage on that basis".

Assuming that the perceived un-truth is the reference to the reversal of  
infiltration/exfiltration locations WRT to the neutral plane in cooling  
situations, I would refer interested parties to:

--> the ASHRAE Handbook of Fundamentals,
---> the chapter on "Ventilation and Infiltration"
----> "Thermal Buoyancy, Stack Effect"
-----> "Cooling Season"

Basically, indoors is cooler than outdoors. Coolth tends to drop and the  
warmer air replacing it infiltrates above the neutral plane.

> But replacing these plaster coats with tyvec or tarred felt is not a
> good option, to me.

I haven't looked at the book to which Duck Foo'd was referring so I don't  
know what it suggested but I wouldn't imagine that they're suggesting that  
plaster be replaced with Tyvek or tarred felt.

For one thing, the Fire Marshall wouldn't like it one bit.


=== * ===
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