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Duct insulation quiz


Jeff Remas
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You are doing phase insepctions on single family home in climate zone 5 currently under construction. During the rough electrical, plumbing and mechanical inspection you notice that the builder had the HVAC company install 1" duct board for the feed and return of the gas forced air heating system. On the 1" thick Knauf if marked R4.3. The installation is in an unfinished basement that is 75% below grade. When you question the builder about the R value and the fact that it will need to be R8, he replies to you that he will not be installing insulation in the floor framing. Instead he will be placing a pvc coated R10 blanket insulation on the basement walls from top to bottom and R19 against the rim board instead of placing R30 in the underfloor framing.

Is this an acceptacle, code compliant installation to eliminate having to use 2" R8 duct board?

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Just for fun, and as a learning exercise, I'll take a stab at it. Like a lot of nights lately, I've got too much time on my hands. But as a disclaimer, I must state the following:

1. We don't do basements around here. What VERY few I see are partial, and most are still a mess. I have no clue about how to insulate ducts in one. (Nor would I know how to assess the condition of an oil-fired furnace like you have in Yankee-land since we absolutely don't have those here either!)

2. More importantly, I am in zone 8, not 5. We don't get real cold around here. (My wife's feet are in zone 3, but that's another topic.)

I don't have a clue about the requirements for zone 5.

However, applying what little common sense I may have in assessing the overall effectiveness of your stated installation, it would seem to me that the efficiency shouldn't be significantly compromised if the proposed installation is properly done.

But then, this is a WAG and I could be totally wrong. Shouldn't the plans outline the required installation details? In any case, it seems like a call for the AHJ if there's a variance.

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See! I told ya I didn't know anything about climate zones. I was using the maps about plant growing zones, not the right ones. Seems that the numbering system is sorta inverted. To set things straight, it appears that my wife's feet would more properly be labeled as zone 7. Maybe a 6 at best. Does that make my answer more nearly correct?

I guess not.

In all seriousness, I don't do phase inspections -- and certainly not in your area. I still think it's a matter for the AHJ. As I said, I was just guessing -- although I was trying to apply some small amount of rational thought/analysis to the issue. Temps don't vary nearly enough around here for this to be an issue that the muni's look at closely.

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New Jersey Energy code:

"Ducts or portions thereof located completely inside the building thermal envelope" are exempt from the R-8 requirements.

So it all depends on what the area required.

Now, to throw another wrench into the equation. The basement does not have to be insulated IF ALL heating systems meet the following:

90 % for furnaces, 85% for boilers or 8.0 HSPF for air source heat pumps.

You may need a Masters degree to read the attached.

Download Attachment: icon_adobe.gif bulletin_07-2.pdf

287.92 KB

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In this case it would be allowable to do this because you are now making the basement part of the thermal envelope. The only catch is that the space must be able to maintain a min. 50 F temp during the heating season and less then an 85 F temp during the cooling season in order to comply with the definition of conditioned space.

This is becoming a popular method with contractors because they reported savings approximately 30% in costs vs having to insulate the underfloor and use the expensive 2" duct board.

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