Thoughts against notching a joist for a dryer vent exit?

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dsmith060

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I'll soon be closing up my basement ceiling with drywall and wanted to replace my dryer vent cover along with the 10" or so line connection. The place where the cover attaches is pretty mangled from a former owner jamming the cover back on, so I want to get a better seal while I still have easy access.

Anyway, the current setup doesn't strike me as a particularly great, so while I had things disassembled, I thought it might work to notch into the last joist and start a gentle slope up to the exit in the rim joist from the 2nd to last joist, as opposed to the 2 sharp angles. (Photos attached)

I know I can only notch outside the middle third, and only 1-1/4" for the 2x8 joist, but beyond that, is there anything I should be concerned about here or is this a sound approach? Or is there not a lot to be gained by changing the current setup?

Thanks for any feedback!
 

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Dana

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Or is there not a lot to be gained by changing the current setup?

Thanks for any feedback!

There is very little to be gained by changing it up.

That looks like a pair of 45s as-installed, which is not exactly "...2 sharp angles...". The "equivalent length" of a pair of 45s is worth about 5' of straight pipe. A pair of slip jointed full 90s is worth about 10'. Replacing the mitered slip adjustment ells with a pair of smooth 8" radius 45 degree ells would reduce it to about 2 equivalent feet, still probably not "worth it".

What is worth it is to seal up all the slip joints with duct mastic rather than relying on tape.

The other thing that is worth it prior to building out the ceiling is to meticulously air seal the band joist to the subfloor above and foundation sill, and the foundation sill to the foundation. It looks like somebody sealed the foamboard to the foundation which is probably fine, but it's not a very reliable way to air seal the sill to the concrete, or the wood elements to each other. Polyurethane caulk works far better in this application.

The other thing that is "worth it' in this application is to pull out the wadded up mis-installed fiberglass batts and either re-install them correctly, carefully sculpting the fit with a batt knife (or an 8-10" bread knife), tucking them into the corners where band joist meets subfloor, floor joists, and foundation sill. Better still, use high density R15 rock wool (which is very air retardent & fireproof relative to R19 or R13 fiberglass.)

Going one better would be to install 1.5" cut'n'cobbled polyiso foam board in there next to the foundation sill (followed by well fitted R13-R15 batt. The foam board a half inch narrower than the joist space in both dimensions for ease of installation- tack it in place with either one 10 penny nail or blobs of foam board construction adhesive on the band joist, then foam-seal it in place with can foam. This is not a substitute for air sealing the band joist with caulk first. With R9-ish foam on the exterior of the R12-R15 the average temp at the facer stays warm enough throughout the winter that it won't puddle an accumulate in the foundation sill, or even enough adsorb wicking to raise the foundation sill to risky levels. Don't go thinner than 1.5" on the band joist foam or higher than R15 on the batts or the moisture content of the foundation sill can rise over a winter.

Better still would be to use 1" foam unfaced EPS board on top of the foundation sill prior to the band joist foam board. At 1" EPS is still sufficiently vapor open to allows any moisture wicking into the foundation sill from the concrete to dry toward the interior, but any faced foam (or even XPS) puts it at risk unless there is a good capillary break between the sill & concrete (which becomes harder to retrofit now that there is XPS foam on the walls.)

Unless the XPS is 3" thick on the walls it doesn't meet IRC code thermal performance even on a labeled-R basis, let alone derated for loss of performance over time. XPS is blown with a mixture of HFC blowing agents (all of which are extreme greenhouse gases currently banned under the Kigali Amendment to the Montreal Protocol, an amendment team has yet to sign up for.) Over a few decades the actual performance drops toward R4.2/inch, not the labeled R5/inch. At 20 years it's only warranteed to R4.5 inch, but it doesn't magically stop there. From a design point of view it's prudent to assume R4.2/inch, which is the fully-depleted performance (and the same as EPS of similar density).

With at least 1" of XPS on the walls it's safe to snug up a non-structural 2x4 studwall insulated with R13 - R15 batts, which would meet IRC 2018 performance. The risk of moisture accumulation over the winter would be significant if left exposed, but with standard interior latex on gypsum board the accumulation is slow enough in your local climate to not present a problem. Do not install sheet polyethylene, vinyl or foil wallpaper on the interior side or it becomes a moisture trap for any ground moisture finding it's way in through the XPS (either as leakage at the seams or vapor diffusion through the foam.)

As a general rule it's better to design XPS out of the stackup, at least until HFO or CO2 blown XPS becomes available. Owens Corning "Foamular NGX" is the only HFO blown foam I'm aware of currently on the US market and it's not yet available at box stores (or even most distributor warehouses). The HFCs are NOT a subtle problem- it makes XPS by far the least environmentally friendly insulation in common use today, with nearly order of magnitude higher CO2e footprint relative to EPS, (same polymer, different blowing agent) R for R.

CSMP-Insulation_090919-01.png
 

Fitter30

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Avoid cutting a joist in the middle 1/3 it will weaken it. Most drier vents can't be over 35' with 45* is 2.5' and elbows 5'. Check with your install manual.
 

dsmith060

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Thanks for the feedback!

It sounds like it's really not worth cutting into the joist to save a couple feet on the run. I was already planning to better insulate and air seal the joist bay while I had easier access, so I appreciate the tips there. So I'll plan for that and also go with the mastic instead of the tape -- great idea. Thanks again!
 
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