Do galvanized roof vent pipes cause condensation issues, in unvented spray foam roofs?
severaltypesofnerd
| Posted in Green Building Techniques on
For a conditioned attic with spray foam on the underside of the roof deck, what about any galvanized plumbing pipes hanging down? If a plumbing pipe is on the roof, and extends down through the conditioned space, it will be nearly the temperature of the outside, maybe even freezing.
Have people encountered problems with humid indoor air condensing on those pipes? What’s a workaround? Is ABS/PVC solid or foam core better?
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Replies
Bryce,
Maybe it's a regional thing, buts haven't seen galvanized pipe used for vents in decades. What are you using for the rest of the drains?
Just about every home here has galvanized waste stacks going through the roof. I'll be adding spray foam to one of them soon, it has 5 galvanized, one 5" cast iron toilet stack, and a cellular core ABS pipe for a new laundry that's all sun damaged and brittle (I'll replace it with galvanized), plus a bonus asbestos cement pipe. The home was built in 1926, and all the pipes except the plastic one are original and in perfect condition, ready for another 90 years.
Most of those pipes are in the top plates and will be encased in foam. However, some run from the lower floors, into the to-be-conditioned attic, then out to the roof. Those are the ones of concern.
You could wrap the galvanized pipe with insulated pipe wrap. The entire pipe won’t be “near the temperature of the outside”. Wrap as much as is accessible and call it good.
Note that using PVC instead of galvanized will reduce the thermal issue you’re worried about. You can get black plastic vent boots that wrap the pipe for UV protection, or paint the PVC with a thick coat of black exterior paint. It’s really only ultraviolet light that causes the exposed plastic pipe to break down, so if you protect the pipe from UV light it should last practically forever.
Bill