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How should I redo my Zone 5 crawl space, please?

user-726861 | Posted in Green Building Techniques on

I recently purchased a weekend home (soon to be a weekday vacation rental) in the foothills of Colorado’s Front Range. It’s in region 5. It has a crawlspace with uninsulated concrete block walls that are an average height of five feet. One end is about six feet and the other is four, if that matters. There are a few cracks in the mortar that I can see daylight through. The floor joists are insulated with fiberglass batts and have been a home to mice for years I believe. There’s a bit of an odor, to say the least. The propane furnace is down there, too, and the ducts are not insulated. The furnace draws its combustion air from the crawlspace, not from outside. The crawlspace is not vented, but it does have five 1’x2’ single pane aluminum frame windows. I had a radon mitigation system installed because the levels were quite high in the living area, so the floor of the crawlspace is covered with a sealed sheet of white poly and there’s a fan that draws air from underneath and sends it out above the roofline.

The current conventional fix for this crawlspace is to remove the floor joist insulation, seal the crawlspace walls and rim joists, and install rigid foam boards on the walls as are outlined in “Building an Unvented Crawl Space” ( The counterintuitive exception to this convention you cite is from the study done in Flagstaff, AZ ( My house is at nearly the same elevation as Flagstaff, and the historical weather in the two places is quite similar. My location is a bit dryer and not as cold though.

Do you foresee any problems if I redo my crawlspace like the best performing Flagstaff model, using vapor barrier on the walls and rigid foam board insulation over the floor joists? In Flagstaff they insulated the HVAC ducts with R-6 paper backed fiberglass. That won’t work in my case with the history of mice. I’ve had a wildlife specialist find and fill the rodent entrances, and I’ll have him come by again for another round of exclusion after I’ve had the fiberglass removed from the floor joists. Is there another way to insulate the ducts? I learned here that the shiny foil bubblewrap doesn’t work. Will I negate the heat savings by having uninsulated ducts? In Flagstaff the counterintuitive approach used 20% less natural gas than the vented crawlspace with the insulated floor and the current closed crawlspace technique used 53% more.

Thank you for the help!

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  1. GBA Editor
    Martin Holladay | | #1

    In spite of the counterintuitive data from Flagstaff, I usually recommend bringing a crawl space inside a home's thermal envelope. You have several complicating issues in your case, however, including the radon issue and the atmospherically vented furnace.

    In the long run, you might consider switching to a heating and cooling system that doesn't involve crawl space ducts. When your furnace finally conks out, it might be a good time to install a few ductless minisplits so you can abandon the ductwork.

    If you choose to keep the crawl space outside of your home's thermal envelope, you need to follow these steps:

    1. Do a good job insulating the floor assembly with a method that includes a continuous layer of rigid foam on the underside of the joists. You might consider protecting this rigid foam with a layer of taped drywall, especially in light of your mouse problem.

    2. Make sure that your atmospherically vented furnace has enough combustion air. This might require an outdoor air duct that terminates near the furnace -- a duct that is a potential mouse highway. Needless to say, if a combustion specialist recommends an outdoor air duct, make sure that the exterior termination includes rodent screening.

    3. I don't know any good substitute for fiberglass duct insulation. Make sure that your insulated ducts are protected by a thick vinyl or polyethylene jacket. Tape all seams in the jacket that covers the duct insulation, and inspect regularly for rodent damage.

  2. user-726861 | | #2


    Thank you for the quick, informative reply.

  3. Expert Member
    Dana Dorsett | | #3

    Many/most furnaces can be re-configured to draw ducted combustion air directly to the from the outdoors to the furnace enclosure, not just a proximity vent/critter highway into the crawl space.

    What's the make & model #?

    How many years has it been in serivce?

    Is it right sized for the load (kinda, sorta) or ridiculously oversized? To get a handle on that, see:

    The amount of material required to insulate the foundation walls of a crawlspace is often less than it takes to do the complete underside of the floor. The crawlspace is not currently vented, but drawing combustion air from the crawlspace depressurizes it, probably increasing the radon levels. Insulating and better air sealing the crawlspace isn't likely to affect the radon levels, but in the event that it does, a small amount of active exhaust ventilation or a single-room style ceramic core HRV can more reliably control the ventilation rates than the current random air leakage situation.

    This outfit in Henderson is always advertising reclaimed roofing foam at a fraction or virgin-stock retail:

    To hit IRC 2015 code min takes 3" of roofing polyiso (R17-ish) or 2.5" of foil-faced (R15) goods.

    There may be others:

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