Basement insulation upgrade options – GreenBuildingAdvisor

Hi All,

Long time reader but first time poster. We recently built a house in climate zone 5A (Indianapolis) and working with our builder and doing a fair amount of air sealing after we moved in I think we ended up with a very comfortable pretty good house (ACH 50 of about 2.5). The one exception to this is the only area I didn’t specifically work with our builder to specify insulation before we started building: the basement. The basement is mostly finished, but we have two large unfinished mechanical rooms. The temperature in the basement is consistently 6-10 degrees cooler than the first floor with which it shares it’s HVAC system (not zoned). With now a year of living here I can say this temperature difference is pretty consistent in a heating or cooling season. We have a heat pump water heater in the basement and I thought this might be a major contributor but we recently went on vacation for a week and I turned off the water heater and it made no measurable difference in the temperature delta between the two floors. Basement is overall very dry from a bulk water standpoint with a sump pit.

The builder’s standard practice for basements for a long time has been 2×4 studs 24″ oc built out ~1inch from the concrete foundation, and then unfaced batts in the cavities leaving an air gap to the concrete (see pictures). Foundation is  poured concrete and the top ~2ft of the foundation is exposed on the exterior. Pretty sure there is no insulation under the floor slab but there is thick rubberized carpet pad and carpet over most of the floor in the finished areas.

In reading here I realize this is not an ideal insulation strategy potentially for several reasons, but this strategy in many houses this company has built in the last 20 years has so far not led to any significant moisture/mold problems (we have several friends who live in houses built by this builder >10 years ago), and really has just left us with a cold basement (actually a shared complaint among our other friends in this companies’ houses).

I don’t currently have plans to rip out all the drywall and framing and start over, but I have considered starting with an insulation upgrade in the unfinished parts of the basement to incorporate some rigid foam against the concrete. This would be limited in thickness to about 1″ without removing framing (which while I am capable of doing, I would for obvious reasons prefer to avoid).

I have a couple ideas about how foam could be incorporated, but wanted to first get people’s thoughts on if this type of upgrade if eventually completed in the entire basement would lead to an appreciable improvement in comfort which is the ultimate goal of considering any of this work.

Thanks in advance, and happy to answer any questions or provide additional info if helpful.