Is it OK to run PEX thru attic?

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Mark Mathe

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I am currently designing a post-frame home and am wondering if it is safe, and will meet code, to run PEX through the attic to the different bathrooms? All exterior walls and bottom side of roof will be sprayed with 2" of closed cell foam insulation. The PEX, full lengths with no connections, will be installed through ceiling joists and have about 12" to 16" blow in insulation on top of them.
 

Dana

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I am currently designing a post-frame home and am wondering if it is safe, and will meet code, to run PEX through the attic to the different bathrooms? All exterior walls and bottom side of roof will be sprayed with 2" of closed cell foam insulation. The PEX, full lengths with no connections, will be installed through ceiling joists and have about 12" to 16" blow in insulation on top of them.

Yes, installing PEX in the attic meets code, and is relatively safe from freeze-up if fully inside the insulation envelope of the building. Putting it at the bottom of 12-16" of blown insulation is a good strategy.

Can I convince you to NOT use 2" closed cell foam on the walls and roof deck? There are multiple reasons for this, starting with...

...1: Closed cell foam between framing is a waste, since it's thermal performance gets cut off at the knees by the thermal bridging of the framing. Do the math.

cell-foam-chart-1200x1020.jpg


With a 2" shot of closed cell foam the thermal bridge through the framing is only 2", which is no more than R2.5. At any framing fraction it's going to matter from a whole-assembly performance point of view.

...2: Closed cell foam has a high environmental impact, particularly closed cell foam blown with industry standard HFC245fa, a powerful greenhouse gas (about 1000x CO2 @ 100 years), and would be contraband if the US had signed the Kigali Amendment to the Montreal Protocol. There are slightly higher performance and more expensive versions blown with HFO1234ze, which has a much lower impact, but it still has high polymer per R. A 2" layer of 2lbs closed cell foam (R12-R14) uses the same amount of polyurethane as 8" of half-pound open cell foam (R30).

...3: At about R14 even HFO blown closed cell foam would not meet IRC 2018 code minimum R20 for walls or roofs in US climate zone 3A (all of OK except the panhandle is in 3A.) At 3" it would meet code for walls on an R-value basis, but would underperform and R20 5.5" of open cell foam or cellulose/fiberglass in 2x6 stud bays. (See reason #1).

...4: Site installed foam that is installed at the wrong mix or wrong temperatures or even the wrong moisture content of wood surfaces can outgas isocyanates for years. The quality of the installation is highly dependent on the attention to detail by the installer.

Since you're planning on blowing 12-16" of insulation on the attic floor, what's the point of the 2" foam at the roof deck? The 2" closed cell foam at the roof deck is going to cost more than R49 cellulose (preferable to fiberglass in your climate) on the attic floor!

Installing even 2" of exterior polyisocyanurate foam sheathing on the exterior of the wall sheathing and using 3.5" of fluff (any type) in the wall cavities would meet or beat code minimum. The details of installing foam that thick or thicker has a learning curve to it, which may be a show stopper. But installing 0.75" -1" foil faced foam on the exterior of a 2x6/R20 wall is dead-easy, and most types of siding can be simply long-nailed through the foam. Foil faced polyisocynurate is about half the density of closed cell spray polyurethane, and uses a very similar chemistry, but it is blown with low-impact hydrocarbons rather than high impact HFC (or high cost HFOs), and under well controlled factory conditions. But even a no-foam 2x6/R20 wall meets code minimum in zone 3.
 

JohnCT

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My concern would be more about mice chewing on the PEX. Every fall, I get mice in my attic, and this on a two story Colonial. That means they are climbing inside full insulated walls to get there.

John
 

Dana

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My concern would be more about mice chewing on the PEX. Every fall, I get mice in my attic, and this on a two story Colonial. That means they are climbing inside full insulated walls to get there.

John

If they're climbing through insulated walls they're not exactly "...full insulated walls...", and if there are holes in the top plates big enough to pass a mouse those walls are WAY leaky, enough to create localized attic insulation wetting issue in winter. Are you sure they're climbing up inside walls, rather than getting in via sidewall or soffit holes into the attic? Blower door + IR imaging can usually find mouse-sized holes pretty quickly.

Running the PEX in walls wouldn't be much better protected from rodent damage than running it in the attic.
 
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