[Strawbale] How to run chminey through strawbale wall?

RT ArchiLogic at yahoo...
Sun Sep 26 20:43:21 CEST 2010

On Fri, 17 Sep 2010 16:04:38 -0400, dirk witvrouwen  
<dirk.witvrouwen at hotmail...> wrote:

> I'd like to know a safe way to run a woodstove chimney horizontally  
> through a strawbale wall.  I'm using a double walled stainless steel  
> chimney (3cm mineral insulation between the two shells).

I'd say "Just don't (run a chimney through the wall).

I would say:

"Relocate the woodstove to a spot that is as close to the geometric centre  
of the floor plan as is possible and run the chimney straight up through  
the roof exiting as close to the ridge of the roof as is possible."

Keeping most of the chimney inside of the house minimises heat loss from  
chimney to the cold outdoors, hence minimises the cooling that the chimney  
will experience and that will minimise the opportunity for creosote  
formation (assuming proper burning habits) and the risk of chimney fires.

Also, most of the heat loss through the chimney walls will be beneficial  
to the conditioned interior living volume.

Also, if exiting through the wall results in the chimney being at the  
eaves of a sloped roof and if the house is in a climate that receives snow  
and if the roof cladding is metal, the potential for the chimmey being  
knocked over by sliding ice and slow is created, not to mention the  
increased potential for roof leaks around the chimney flashing, assuming  
that the house is provided with reasonably wide overhangs to keep the SB  
walls dry.

I could go on but basic point is "Why create potential problems ?"  
assuming that the building is still either in the design stage or  
construction stage.

But if the chimney *must* go through the wall, then I would cut a round  
hole through the straw and then simply install wire mesh  and a 50 mm  
thickness of concrete (aka "Portland cement plaster) over the straw,  
taking care to properly embed the mesh in the concrete to ensure that the  
mesh will do its job of providing the tensile reinforcement necessary to  
prevent cracking due to the thermal stresses to which the plaster chimney  
hole lining will be subjected.

If necessary, I would install pieces of 11 mm thick OSB or plywood on the  
inside and outside surfaces of the SB wall with matching holes cut in the  
OSB to serve as temporary formwork, using an F-clamp or such-like to keep  
the formwork in place. I would of course first cover the OSB or plywood  
with salvaged polyethylene sheeting to facilitate easy form removal and to  
help keep the mixing water in the concrete for as long as possible to  
ensure full hydration of the cement during the curing process.  I would  
also insert a few strategically-placed anchor sleeves into the wet mud to  
provide points of attachment in the hardened concrete for the sheet metal  
cover/air barrier that will eventually go over the penetration.

Then I would install a sheet metal sleeve inside that plastered opening,  
using non-combustible spacers to create an airspace of 50 mm or more  
between the plaster and the sheet metal sleeve.

Then I would install more non-combustible spacers between the sheet metal  
sleeve and the insulated chimney to create the manufacturer's specified  

The sheet metal sleeve mentioned above functions as a radiant barrier and  
the 50 mm air space behind it will help to minimise heat transfer to the  
50mm thick concrete hole lining so that the potential for pyrolisis of the  
straw adjacent to the concrete lining will be minimised.

Just in case it's not obvious, all of the sheet metal mentioned above  
should be cut from corrosion-resistant (ie hot-dipped galvanised or  
galvalume or stainless steel) flat stock of 26 gauge or thicker.

For the sheet metal cover/air barrier on the inside/outside of the  
penetration, I would cut the hole about 12 mm smaller than the outside  
diameter of chimney and peen over the 6 mm excess to the inside of the  
hole. That peened-over edge will help to stiffen the opening in the sheet  
metal cover and also provide a caulking groove (using a fire-rated  
caulking of course ie "Dap Fire Stop" silicone.)

=== * ===
Rob Tom
Kanata, Ontario, Canada
< A r c h i L o g i c  at  Y a h o o  dot  c a >
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