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Spray foam and HRV system questions

Brian Pasold | Posted in Mechanicals on

Hi I have a raised ranch.
I am gutting the first floor and adding a 2nd story addition. With full attic.
I am going to use open cell spray foam.
All walls and attic roof lines. After the addition I will use it to finish the basement.
I am running a 90+ efficient furnace for the 1st floor and basement.
Which does draw fresh air into it
I am planning to get a similar unit for the addition
3 baths with fans, 1 600 CFM range hood
Would I need to install a HVR unit?
I live in the Midwest. I heard stories about windows icing up and having to open windows.
I would hate to have to install one later.
Would it be wise to just do it now?
I have been doing some reading and talking to people I know that have had issues and people that have the HVR unit and no problems.
Any help would be greatly appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, it's hard to imagine that a 90% efficient furnace won't be big enough to handle both floors of a ranch home that is insulated with spray foam. Whoever performed your load calculation probably made a mistake. The smallest available furnaces sold in the U.S. are almost always grossly oversized for tight, well insulated homes. What you need for your second floor is ductwork, not a second furnace.

    Second, if you plan to install a 600-cfm range hood, you need to provide a makeup air system. It would be better if you chose a much smaller range hood fan. More information here: Makeup Air for Range Hoods.

    Third, every tight house (especially every tight house that is insulated with spray foam) needs a mechanical ventilation system. That could be an exhaust-only ventilation system, a supply ventilation system, or a balanced system that includes an HRV. You need to choose one of these three types of ventilation systems, and you need to be sure it is designed properly, installed carefully, and commissioned. More information here: Designing a Good Ventilation System.

    Finally, one of your statements is somewhat unclear. You wrote, "I am running a 90+ efficient furnace for the 1st floor and basement, which does draw fresh air into it." It sounds like you may be describing a central-fan integrated supply ventilation system -- or maybe not. If this ventilation system is properly designed and commissioned, it includes a motorized damper and an AirCycler control to prevent underventilation and overventilation, and the air flow of the system has been verified to be sure it meet ASHRAE 62.2 requirements. If some of these components are missing, you don't really have a ventilation system -- just a hole in your house.

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