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New house, spray foam attic, gas heat and water heaters

I am building a new house and want to foam the roof. I also want to use gas heat and water heaters.

The HVAC rep states that they pipe air to the units to meet code. Does that sound about right? Does that majorly break the roof 'seal'? Is that different from sealed combustion?

Also, it doesn't look like I can use standard NG water heaters. From what I have read it needs to be direct-vent. Is that right?

Direct-vent water heaters are around $1k versus $400 for standard NG. Tankless gas heaters are a little cheaper and they all say direct vent. Would one or two of those be the way to go? Can the foam be sprayed directly on the direct vent pipe?

Basically trying to figure out most economical way to get both foam roof and gas appliances ...

Thanks for any tips.

Asked by C Smith
Posted Wed, 02/12/2014 - 21:46
Edited Thu, 02/13/2014 - 10:10


1 Answer

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First of all, you don't have to put your furnace and water heater in your attic. In fact, there are several good arguments for putting these appliances somewhere else -- the attic is just about the worst place in your house to put these appliances.

Better locations for these appliances include a basement, a crawl space, or a first-floor mechanical room near the center of your house.

If you insist on putting these appliances in your attic, then it's certainly a good idea to create an unvented, conditioned attic by installing spray foam insulation at the sloping roof-line. But if you do that, you need to make sure that your furnace and water heater are both sealed-combustion appliances (with ducted combustion air from the outdoors to the appliances).

If you install these appliances in a basement, you have a little more latitude -- and you can install old-fashioned atmospherically vented appliances if you want. (However, sealed combustion appliances are always preferable.)

For more information on these issues, see Creating a Conditioned Attic.

Answered by Martin Holladay, GBA Advisor
Posted Thu, 02/13/2014 - 08:23

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