Our contractor has proposed running the heat duct to the second floor through the outside wall of the house. This would be for a new house we are building in northern Wisconsin. The duct would be insulated w/2″ foam & the wall is insulated too (6″ R-19 batt). Is this a bad idea? Because of our plan, this is not easily done another way. If we did run the duct through an inside wall, it would require a large bumpout in the wall and ceiling. Anyone have any word of wisdom?
Thanks!
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It sounds acceptable to me, but I'm just a HO amateur DIY type. I have heard of lots more hairbrained ducting before.
How big is the duct?
There will be 2 ducts. He has not specified size yet, but there's one for each bedroom. Does that help?
I think what rich meant was size in inches, say like 3" by 14"? That would be a common size and putting that in an extyerior wall framed with 2x6 means that you will only have 1-1/2" left for foam which would reduce the R-value by about 25% over what I had just stated, or increase the heat lost in transit to the upstairs, requiring a larger overal supply to make up for the loss, possibly an extra duct elsewhere, and concievably a larger furnace, though I tend to doubt that since most systems I see are overdesigned nowdays.
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That pretty much covered it. :)
While it is better not to run heat or water in an exterior wall, it can be done here, depending on the kind of foam and whether it is well fitted and sealed in.
Polyisoanurate foam would be about R7 per inch. Polyurethene foam the same, more or less. But extrruded polystyrene is only about 3.5 per inch.
The former would provide about5 R-14 just in the insulation and you can add close to another Rone for the sheathing and siding, depending again on how it is installed and what materials it is. Hardiboard has almost no insulative value while some vinyl backed by foam has a little bit.
So you could end up with R-15 resistance to heat loss outside that heating duct, which is all we have had for a generation now as a standard.
but again - it MUST be sealed in place with spray foam from a can wjhere it meets the studs or you will have a wicked convection current. seal against draft there and this R15 foam will actually perform better than the R19 of FG batts in the rest of the wall because batts alllow convection loops and heat loss that accelerates as wind speed increases and temps decrease while th efoam remains pretty much as effective in any temperature differential.
Welcome to the
Taunton University of Knowledge FHB Campus at Breaktime.
where ...
Excellence is its own reward!