Roof and knee wall venting for vaulted ceilings
Last Post 06 Apr 2015 11:31 AM by Dana1. 23 Replies.
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jchristofUser is Offline
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02 Apr 2015 12:04 PM
With ANY 2-part expanding foam if the chemical mix proportions or tempertature is off during installation there can be long term outgassing relevant to the chemically sensitive (or even the rest of us.) This is not a rare phenomenon, even though most installations are fine.


Do you happen to have any good links to general info on this issue. I realize that each manufacturer will publish specifics but I'd like to read a general assessment. I spoke to several reps at the local EPA office here in KC and both of them down-played the air-quality risks of closed cell installation and long-term hazards. Hard to tell how up-to-date anybody is on the specifics thoough.
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03 Apr 2015 03:13 PM
Posted By BadgerBoilerMN on 26 Mar 2015 04:39 PM
Is this still the case if the remainder of the cavity is filled with glass or cellulose?

If you fill the rest of the cavity with fiber insulation, the R-value of the thermal bridge rises too, reducing the heat transfer through the wood.  Any time a wall cavity is only partially filled with foam, it's well-worth completing the fill with the cheap stuff to reduce the performance lost to thermal bridging.
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06 Apr 2015 07:35 AM
I still use fiberglass in my walls and I spray 8” of foam in my roofs. But you won’t see me using cellulose or blue-jean insulation in my homes.
Just because it's recycled doesn't mean it's green.
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06 Apr 2015 11:31 AM
Posted By David_lee on 06 Apr 2015 07:35 AM
I still use fiberglass in my walls and I spray 8” of foam in my roofs. But you won’t see me using cellulose or blue-jean insulation in my homes.
Just because it's recycled doesn't mean it's green.

8" of closed cell spray polurethane foam (ccSPF) meets code min, but it in almost all instances the amount of HFC245fa blowing agent used would have a higher lifecycle environmental impact than the energy use it offsets, due to it's ~1000x CO2 global warming potential (GWP).    With most conventional heating fuels it would take well over a century's fuel use offsetting to break even on just the climate impact aspect.  It would probably break even sooner than that in a colder climate if the heating was resistance electricity on a 90%+  coal fired grid (higher elevations in West Virginia, mayhaps?). But over the next century it's unlikely that any US local grids will be as high-carb as that in the post-2030 time frame, and very few houses are heated with resistance electricity.  Until the foam vendors move over to the low GWP HFO1234_ _ blowing agents (only recently commercially available in the US), with closed cell foam "Less is more".

And you can get away with substantially less.  In BadgerBoilerMN's US climate zone 6  4" of ccSPF on the underside of the roof deck plus 6" of Spider / Optima /L77 or 7" of cellulose would hit code min without condensation & mold issue.  Any of those fit nicely in a 2x12 rafter cavity.

In zone 5 it only takes 40% foam to 60% fiber, in zone 4 it takes 30% (20% for zone 4C), etc. ,  See the prescriptives in IRC 2012 Chapter 8 which are predicated on R49 (zones 3 & higher) for the center-cavity R of R49.  But it's the foam/fiber ratio that is most critical, since that is what defines the average temperature at the foam/fiber boundary. If the R-value is higher than code-min, the minimum foam-R has to grow proportionally with the fiber-R to keep the warm side of the foam sufficiently above the dew point of the conditioned space air to be able to use latex-paint (or other class-III vapor retarders) as the interior side vapor barrier.

Just curious, what's the verditude beef with cellulose (or scrap cotton)?
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