Insulating under basement floor
 

I'm trying to weight the advantage of insulation "under" a new basement floor in the total thermal year of a new basement space. I can understand that if only heating was considered that it would be a "no-brainer". But in my current basement it is a welcome refuge in the hot summer to work in my shop in the cooler earth conditioned space. We have about equal heating and cooling days and the basement is not living space so it being somewhat cool in winter is tolerable for me when working down there. New basement will have insulated walls and insulation between it and heated living space so any heat lose from basement should already be minimal. I hate to do anything more that will lower the overall summer cooling benefit that earth can provide. It is not always easy to build for a total balance of comfort in a space as we all have different climate and personal situations so just coping the other guys project is not always proper. Thoughts?

Drake, is your new basement a walk out or all below grade? Is your hvac system in this area,attic,or another area?

I think you're fine not insulating the floor. The energy savings is pretty small compared to insulating the walls. Most of the energy loss in a basement is to the outdoor air and the upper surface of the ground. Deep underground, the energy loss (in both summer and winter) is fairly minimal because the ground is only about 20 degrees away from the typical indoor temp. The top part of the wall is the most important to insulate. As you move down, it's less beneficial. Once you get to the floor, it becomes hard to justify economically -- although it can help keep your feet comfortable (if cold feet bother you). Logically, it would reduce your free cooling in the summertime.

INSULATE!

Your ground temperature is about 45F. If any air comes into the home, with a dew point above that, then you will have condensation.

Your location is well known for very high humidity in the summer.

You will still have a cool summer basement floor as you are only going to put in perhaps R8, but the thermal mass will help you in both summer and winter.

Remember that cold air sinks and I have seen quite a few moldy concrete floors due to the dew point exceeding the surface temp.

This advice is really specific to your area as most other areas I only suggest basement wall insulation (on the dirt side). But your hot humid summers and frigid winters allow a good return on sub-floor insulation in your area (especially a do it yourself project).

Steve



The cost is small, but the lack of mold, the ability to use that large thermal mass and the foot comfort (in winter) are all factors to insulate.

I agree. In most houses the walls are mostly underground.
In my home, it's those upper 3 or 4 feet of basement wall that get the coldest during winter.
(I've done a few measurements. I've seen upper walls get down to 25F).
If the walls were mostly exposed outdoors, you really need insulation!

Moisture can be handled by one of these gismos.. :D

http://www.johnnyonenergy.com/wp-con...ter-Heater.gif

I'm using a heat pump hotwater heater. And, I have a theory that having a slab that
is a bit warmer than the ice-cold air from the heat-pump, makes the basement air temp
recovery time a lot faster.


One other thing I was wondering about.. If you have under-slab insulation,
will the load-bearing of the slab be less?

Will it support a heavy milling machine? Or big hot water tank?

More details - only top 12" of any wall exposed, one existing block wall other three will be PW foundation(which won't have condensation), all walls will be R-20+ top half R-15 to slab. Heat will be in slab hydronic(first for 4", second thin slab), R-15 between heated floor and basement. 45 degree ground temp seems a little cool my well water is mid 50's but yes high humidity is prominent here in summer. It will be just a basement space, shop storage, utility, extreme weather protection, etc(no finished living space). Will have a HPDWH to help remove some humidity. I plan a very frugal consumptive life style in retirement so if water heater draws too much much heat from basement in winter I will supplement with some simple solar heat for extra BTU's in winter.

In my experience with the A7 ASHP waterheater, that ice cold air goes right to the basement floor.
That area of the basement has no floor covering at all.
Right after a heat pump run, the floor is slightly colder than other areas in the basement.
But, within a few hours, the floor surface temperature is back to normal..

My slab is a Geo-source of "heat".. :D

And of course during August, that slab is my Geo-cooling source.. :p

This is not a easy task before taking any step you should must meet with expert builders who will guide you in a good way according to your budget and condition.

richard8,

We actually have amongst us in this forum a high degree of expertise, and budget and condition issues are a regular part of the conversation.

-AC

A well insulated basement won't cost much to cool in the summer. It still benefits from the ground because the delta T on the floor will be very small during summer. The difference is that you will control the temperature with your HVAC system, instead of being at the whims of the earth (as much). Condensation issues will also be eliminated, as noted previously. I suspect that even if you do pay more to cool it in the summer, it will be far outweighed by the winter savings on heating. But that is just my hunch.

You can't really add the insulation later (without great cost and disruption), so I'd think twice before missing out on this opportunity. Then again, it's been a few months since the original post...