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Community and Q&A

Should we actively condition our unfinished basement?

user-6356169 | Posted in Mechanicals on

We’re in SW Virginia, building a 2 story, 1900 sq ft home with a full basement. 2 inches of XPS foam under the slab, and 2 inches of polyiso on the interior of basement walls. Heat pump/forced air and woodstove will heat upper floors. Future plans include doubling the thickness of polyiso all around in the basement. Was also thinking that if we had cold floors on the first floor, I could add some insulation under the floor.

We also could potentially be finishing about 600 to 700 sq ft of the basement, but that might be a few years off or might never happen. For that finished room, which might be used infrequently, was thinking we’d make it superinsulated, heat with either a baseboard heater or gas appliance, and ventilate with a small “spot ERV”.

I liked the idea of keeping the rest of the basement on the cool side for food storage and running freezers.

Now the HVAC contractor is telling us that by code we’d either need to condition the basement or insulate under the floors from the get go. (Our builder didn’t seem to know this, or just overlooked it.) He’s suggesting we force some air down their, but at amounts to keep temperature about 10 degrees F cooler than upper floors in heating season. A return would be installed as well, but he recommends to cover it up with cardboard during heating season, and open it up during cooling season. I’m wondering about pressure buildup if we do that.

Also, wouldn’t we need to increase our ventilation rate? We might often have the ERV running but not forced air heat or cooling (high solar gain days, or days when we’re neither heating nor cooling but are running dehumidifiers.) But when forced air is running, wouldn’t the basement air be mixed with the air from the upper floors? Shouldn’t we add the additional 950 sq ft in basement to our required ventilation calculations? We might have to get a higher capacity ERV in that case.

Any thoughts, suggestions? Does our HVAC guy have it right about the code requirements? Should I take the easy route and just condition the basement?

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  1. GBA Editor
    Martin Holladay | | #1

    Your HVAC contractor is wrong. There is no need to actively condition an unfinished basement. If your basement already has ducts and a water heater -- even if all it has is ducts -- it will be plenty warm.

  2. Expert Member
    Dana Dorsett | | #2

    Where are you located in VA?

    Your HVAC contractor is mistaken, and your builder probably knows it.

    The generic code reads:


    N1102.2.9 (R402.2.9) Basement Walls

    Walls associated with conditioned basements shall be insulated from the top of the basement wall down to 10 feet (3048 mm) below grade or to the basement floor, whichever is less. Walls associated with unconditioned basements shall meet this requirement unless the floor overhead is insulated in accordance with Sections N1102.1.2 and N1102.2.8.


    This is normally interpreted that walls of unconditioned basements have to be insulated ONLY IF the floor above the basement is not insulated, and conversely, the floor above an unconditioned basement needs to be insulate ONLY IF the basement walls are NOT insulated.

    Your basement walls are insulated to better than code-min, which renders the floor insulation question moot.

    Unless there is a local exception peculiar to VA or local town codes, as long as the basement walls are insulated there is no code requirement to insulate the subfloor above an unfinished basement, even if the basement has no heating equipment. Even the foam under the slab is not a code requirement.

    Run an aggressive load calculation on the basement as-is. The design heat loads of basements are so small with even R10 on the walls and no slab insulation the cost of adding an electric baseboard capable over covering that load would be cheaper than insulating between the joists, in the event that your local codes vary, or the inspector interprets it differently.

  3. user-6356169 | | #3

    We're in Montgomery County, VA, just outside of Blacskburg.

    Thank you Dana and Martin. I'm going to ask my builder to check with the county building inspector and get a definitive answer on their interpretation.

    If it were unconditioned, I believe we would need to insulate the galvanized trunk ducts with 3 inch wrap. They've already installed it with bubble wrap, which won't suffice for unconditioned, and they'd have to go back and rectify this.

  4. user-6356169 | | #4

    The HVAC contractor told me that the basement "loaded out" at about a fifth of a ton.

  5. Expert Member
    Dana Dorsett | | #5

    A fifth of a ton is 12,000/5= 2400 BTU/hr, (British Thermal Units per hour) which is about 700 watts, if covering it with a baseboard heater.

    If it's the HVAC contractor who did the load calculations, 19 out of 20 times those numbers are padded to the high side by a hefty fraction. That's why hiring a qualified engineer is more appropriate, ESPECIALLY when the house is being built to a higher standard than code-minimum.

    Montgomery County residential codes are based based on ICC (International Code Council) 2015, and +13F is the generic outside design temperature (probably the 99.6th percentile temperature bin rather than the 99th.)

    If the HVAC contractor used a heating indoor temperature higher than 68F or an outdoor temperature lower than the County required +13F, that alone is padding out the load numbers. More common padding would be to use code-minimum R & U numbers, code-max air infiltration, and ASHRAE 62.2 ventilation rates without benefit of heat recovery.

    County amendments to the 2015 code (if any) would be found here:

    Unfortunately it's a scanned document, which would require a tool that does character recognition to search for "basement".

    But I sincerely doubt Montgomery County regulators went out of their way to insist that floors above insulated unheated basements be insulated, which would add construction expense for nearly zero performance gain.

    The relevant generic 2015 code sections can be found here:

    ...and here:

    You are in zone 4A, thus R10 is code-min for basement walls, the floor would only need the R19 if your basement walls were less than R10.

  6. GBA Editor
    Martin Holladay | | #6

    A few observations on duct insulation.

    Bubble wrap is close to useless. It has negligible R-value, and most marketers of bubble-wrap products are deceptive.

    That said, you don't gain much advantage by insulating the ducts in a basement. The idea is (a) to do a good job of air sealing your basement, and (b) to insulate your rim joist and basement walls. If you do that, your basement is part of your interior space. If your ducts leak a little bit of heat, no harm occurs. The heat is inside your home's thermal envelope.

    There are a few exceptions to this general rule -- especially if you have an undersized duct serving a remote room that barely gets enough heat. If you have this kind of duct, it makes sense to insulate the duct. But if your duct system is well designed, just make sure that the duct seams are sealed -- you can probably skip the insulation.

  7. user-6356169 | | #7

    Thanks so much Martin and Dana. I feel like I have much better understanding going into meeting with HVAC contractor tomorrow. I was thinking that decent duct insulation would be really important, but maybe I won't insist on that. We did our best keeping duct layout efficient, locating air handler centrally.

    I'm wondering if maybe I should let them install a few registers and a return in the part of the basement that we might eventually finish. I could keep them closed up until that time, but I'm not sure of the impact that would have on the balance of the rest of the system. I really don't like the idea of closing up the return but leaving supplies open in winter, as HVAC contractor suggests.

    I still have the issue of our ERV capacity. We had calculated required ventilation rate of 80 cfm for the top two floors. (0.03 * sq ft + 7.5 * # of occupants). If we start mixing up basement air with the upper floors, that comes out to 104 cfm. Our ERV (Renewaire EV90P) will never deliver that much air unless we have close to no ductwork at all. I understand that some people believe that the ASHRAE formula might lead to overventilation. But I think we're building a pretty tight house, so we might need it. On the other hand, hate to increase ventilation capacity much, because we're hoping for a lot of days where we won't be running forced air (and basement air won't get mixed in), but will have windows closed and need ventilation.

    Builder now agrees that HVAC contractor is probably mistaken about under floor insulation requirement. Waiting for definitive word from county building inspector. BTW, we're in Montgomery County, VA, not MD.

  8. GBA Editor
    Martin Holladay | | #8

    Will your ERV have dedicated ventilation ductwork? Or were you thinking of trying to distribute ventilation air through the ducts installed for your heating system?

    If there is a good possibility that you will eventually create a finished room in your basement, you should include the area of that future finished room when performing your air flow calculation for the ventilation system (using the ASHRAE 62.2 formula). You should purchase and install an HRV or ERV capable of ventilating at the ASHRAE 62.2 rate -- but you don't need to run your HRV or ERV at the maximum rate for 24 hours per day.

    Most HRVs and ERVs have more than one speed, so you can ramp it down. Moreover, you can always put the HRV or ERV on a 24-hour timer, so that it doesn't run for 24 hours per day.

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