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Passive solar wall design for cinder block house

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Posted by blah on September 15, 2009, 4:38 am
 
I am in the process of renovating an older cinder block house in
southern Oregon.  I wish to use the concrete slap and cinder block
walls as the thermal mass.  I'm insulating the outside of the cinder
block walls with hard foam insulation, but am unsure about the vapor
barrier.  Specifically, where exactly should it be located?  I've
search the internet and many passive solar design books, but have
found nothing about the specific location of the vapor barrier for a
cinder block home insulated on the outside.  I'm aware that cinder
block can suffer from moisture and mold, so I want to get this right.

Big thanks to anyone that can help.

Brent

Posted by Joesepi on September 16, 2009, 3:41 am
 
I doubt a vapour barrier has any function outside of the foundation. The
solid foam insulation will act as one somewhat. The foundation is usually
damproofed which means two coating or bituminous material (tar based
usually) on the outside of the concrete. Many people add Delta cloth, in
addition to this coating which is large dimpled plastic sheeting (dimples go
inside) as a moisture barrier and air pocket for any leaks to drop to bottom
to the drainage tile. Clear stone and a good landscaping cloth material
needs to on top of drainage tile. The cinder blocks have been in contact
with the soil since installation, already, anyway.  People, I have seen use
foam on the outside, usually seal the foam to the foundation and it performs
all the moisture barrier functions.

Inside the foundation (concrete) a layer of Tyvek or vapour  breathing but
not liquid passing  material is placed so that any insulation touching the
concrete will not get wet and freeze, and act as a conducting heat bridge
through the wool insulation material or mould it.

The vapour barrier is then placed on the warmside of the insulation to keep
moisture in the building from penetrating the insualtion and wood studs for
moisture damage due to condensation and rob insulation value.

With wall sealed on both sides you could have problems with trapped
moisture. I am not aware that cinder blocks get mould problems but have no
experience with them. From a neighbour doing the foam on the outside of
their concrete foundation this is quite impressive. The guy says he has to
take off his shirt when he works in the basement now...LOL




Posted by blah on September 27, 2009, 5:53 am
 
Thanks for the insightful response.  I sounds like you feel an tight
vapor barrier membrane in the walls is not needed?

A little background:
My concern about the possible need for vapor barriers became
significant as I removed the old interior wall materials from the
house I'm renovating and found a lot of mold.  Foiled covered
insulating sheathing and hard foam insulation, all of which had mold
underneath (between it and the block).  Also found mold under carpets
in the corners of rooms (old slab may not be vapor sealed).  Thought
this might be from a lack of heating in the winter and no vapor
barrier.  Now, as I read more, it sounds like a tight vapor barrier
would be detrimental as it does not allow the walls to breathe.  Not
sure what to think, just don't want to find mold inside my walls a
couple years from now.

Thanks a bunch for any further guidance.


Posted by Josepi on September 27, 2009, 12:12 pm
 The truth is not out there yet on some of these new techniques but.

Things I know.
 - there is no such thing as a vapour barrier. All things breathe vapour
somewhat...just more or less.
 - refering to the statement above means a vapour barrier is needed on one
side but air flow on the other side to keep any build-up of moisture down. A
"barrier" slows down the penetration and the ventilation makes sure it
doesn't accumulate

This is hard to apply to things in the ground. It would seam the vapour
barrier would need to be on the outside and some kind of breathing system
would need to be applied to an air cavity in the wall to the outside. Inside
air would result in condensation, assuming the inside air is more moist and
the outside is colder.

I would have thought some with more experience on this subject would have
put in their two cents worth.



Thanks for the insightful response.  I sounds like you feel an tight
vapor barrier membrane in the walls is not needed?

A little background:
My concern about the possible need for vapor barriers became
significant as I removed the old interior wall materials from the
house I'm renovating and found a lot of mold.  Foiled covered
insulating sheathing and hard foam insulation, all of which had mold
underneath (between it and the block).  Also found mold under carpets
in the corners of rooms (old slab may not be vapor sealed).  Thought
this might be from a lack of heating in the winter and no vapor
barrier.  Now, as I read more, it sounds like a tight vapor barrier
would be detrimental as it does not allow the walls to breathe.  Not
sure what to think, just don't want to find mold inside my walls a
couple years from now.

Thanks a bunch for any further guidance.



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