I’m remodeling my house in SF Bay Area climate zone 3C. Fujitsu ductless minisplit downstairs and ducted minisplit in attic, with my HVAC contractor supposed to deeply bury the ducts per our contract (though I notice now it states to R20, not R25 as one might expect). Cellulose insulation to be installed to base depth of R49. Ducts with R8 insulation, reflective vapor barrier, to 20 cfm25 leakage.
We have completed air sealing house and it is very tight (excluding windows and doors which are still taped, 0.22 ACH 50)
The boots of the ducts are bare metal, haven’t been foamed over from attic side.
I contacted my HVAC contractor in preparation for insulation install, and because he is subbing this out, asked him to ensure his guy to cover all the ducts (except right next to air handler) with 3.5″ of insulation over the top per definition of deeply buried ducts.
He is from a reputable company who has a great reputation and has been recommended on this website, and he says the following:
“Just FYI there is often not 3 1/2 inches over all the ducts This is based in the world of averages. Fortifying the duct R-value to avg 20 is our goal The load calculations are based on R 16. The sides and bottom of the duct are infinite R. The larger ducts such as large returns will crown above the insulation level. R44 Total depth is the goal. That’s R14 above normal R30.
If a small amount of the duct sticks up on larger ducts above that is a very small percentage of the overall duct surface area. With R8 Exceeding the R 20 goal easily
The risk of condensation in California is dramatically low compared to most of the rest of the country. However raw metal in the attic will condense.
Duct work that has R8 insulation And a vapor barrier will not.
Generally the boots are taped off at the time of installing the register so we know the registers will fit.
We do things a little bit different in California because of the dry climate aspect. It changes a lot of those concerns. I understand you want to get this done the best but I wouldn’t worry too much about condensation.
It is possible with the tightening of the building envelope that some moisture will stay inside the shell but that’s what the ERV is for.”
Doesn’t seem like he is planning on spraying the exposed boots with closed cell foam, nor completely covering the ducts. To me, the former seems risky, and the latter seems to not meet the terms of the contract and silly not to do.
Any thoughts or advice? If he doesn’t want to do it, I could attach some fiberglass batts to the crowning ducts myself as well. I think he does want me to be satisfied with install.
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