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At what level of airtightness will I have problems w/ my gas furnace & water heater backdrafting while exhaust fans are running?

Midwest - Missouri | Posted in Energy Efficiency and Durability on

I am in Climate Zone, 5 northwest Missouri. I have a house built in 1958. While solid, it only has 1 1/2″ batts in the 2X4 walls and I can feel drafts in the winter. It does have tar paper as a water barrier underneath the siding.

I am wanting to remove the siding and replace and add some amount of foamboard (1″ to 2″) and tape to seal joints and windows to create the air barrier, and caulk / foam the holes in the ceiling.

When do I have to worry about backdrafting from my gas furnace and water heater when they are running with an exhaust fan? I plan to get a new stove hood that is exhuasted to outside also but not a big commerical one– perhaps 250 cfm max.

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  1. user-2310254 | | #1

    Jeremiah. Check this article for methods you can use to improve your insulation.

    You also should consider a ventilation strategy:

    It's possible you will need additional makeup air for the range hood. See this article for more info:

  2. GBA Editor
    Martin Holladay | | #2

    The approach you are suggesting -- limiting your air sealing measures out of a worry that you will make your house "too tight" -- is an obsolete method. Some weatherization agencies adopted this approach in the 1990s, but the approach is far inferior to today's preferred approach: namely, doing the best possible job of air sealing, and replacing old atmospherically vented appliances with new sealed-combustion appliances (or providing ducted outdoor air to the mechanical room if the atmospherically vented appliances aren't replaced).

    All that said, an answer to your question was provided in a March 1993 article in Home Energy magazine. Here is the link: Building Tightness Guidelines: When Is a House Too Tight?

    Regardless of which approach you ultimately decide to take, you should hire a home performance contractor or an energy rater to perform a worst-case depressurization test when your air sealing efforts are completed. A worst-case depressurization test will reveal whether there are any combustion safety issues in your home.

  3. Midwest - Missouri | | #3

    I am looking for a baseline ACH at which I need to be worried about back drafting. I plan on blower door testing before starting this and would like to get an idea of how tight a house has to be to for the backdrafting to occur. I need to know if I need to add furnace and water heater in the equation. I did this exact same thing to my sister and brother in laws house when a hail storn went through in 2011 and had the opportunity. They already had furnace that was closed combustion and decided to take their water heater out so as not to have a vent thru the metal roof we put on. I am have to decide what to do as I have a brand new roof and do not really need to change the furnace it was new in 2004 and and the water heater may be old but work great.

  4. Midwest - Missouri | | #4

    Thank you, that is what I was looking for. I don't want to skimp on the air sealing but find what i could do to utilize all the air sealing without having to replace the furnace and water heater until they actually need it. I have plenty of space in the basement and could make the furnace room air tight with an intake from the outside. And I plan on hiring someone to do the blower door and worse case depressurization after done.

  5. Expert Member
    Dana Dorsett | | #5

    The backdrafting issue can only be determined empirically, unless you have well modeled flow on the exhaust fans at different pressure gradients, which gets really complicated really fast. Close all the windows, turn on all the exhaust fans, and use a smoke pencil or something to test for backdrafting at the dilution hood on the water heater. Test it both with the air handler on the furnace or AC running, and with it off.

    If you air-seal to the point where you can prove that it can backdraft, it's not a crime to cut in a vent to the mechanical room until it's time to replace the equipment. The size of the hole needed won't be any bigger than the combined air leakage you would otherwise have, but unlike a leaky house, you'll know exactly where that air is entering, and it won't be in your living space. The new forced draft or sealed combustion can usually be side vented rather than vented up the roof, but it's also often possible to use right-sized flue liners inside the old chimney flue if for some reason side venting is not an option.

    Since you're pulling the siding it will be worth drilling & filling the 2x4 cavities with fiber insulation. Even if it proves too difficult to insert a dense packing hose, drilling multiple holes per bay and blowing to known pressure does a lot for clogging up the air leaks with insulation, in addition to more than doubling the R-value. Leaving 2" of cavity empty in the stud bays would only be providing thermal bypass paths for any air leaks that remained.

    In a zone 5 location it's also worthwhile to insulate the basement walls, even in the era of cheap natural gas. Once you have tightened the place up, filled the wall cavities and added exterior foam, a foot or two of uninsulated foundation wall could easily be more than 20% of the total heating load. IRC 2015 code min would be R15 continuous insulation (3" of polyiso foam board) but you can get the same performance out of R4-R6 foam board trapped to the wall with a 2x4 fiber-insulated studwall. (There are issues with the studwall approach that need to be sorted out before going that route, to ensure it doesn't end up being a mold farm.)

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