I was also wondering about the effects of insulating the inside of the foundation in my crawlspace, using 2inch thick polystyrene. The underside of my floor is not insulated. And the crawlspace is a dirt floor. I was specifically told not to insulate the floor so that heat could seep into the crawlspace and keep the water pipes from freezing. (the waterpipes are insulated). But last week on a very cold day I went around back of the house and opened the door into the crawlspace. It was very warm in there. So I suspect I am loosing a lot of heat thru the foundation walls. Not to mention into the ground. Would insulating the concrete block foundation walls be worth the cost (ie: ROI > cost of lost heat). Thanks. Pooh Bear

Crawl space. Use spray foam on the interior of the perimeter crawl space walls, sill plate and rim joist area. You will have a much warmer floor and your pipes won't freeze. Be sure to insulate your hot water lines to reduce wait times for hot water. PEX plumbing can also reduce the threat of broken pipes due to freezing. PEX can be chewed by rodents though. This post was edited by jakethewonderdog on Mon, Aug 18, 14 at 15:56

First of all, if the DWV(drain,waste & vent) piping is installed correctly there should be an adequate pitch to insure there is never any standing water in the pipes so freezing is basically a non-issue. The exceptions to this would be P-traps which may be below the sub floor such as under the bathtub, shower stall or laundry standpipe. In this case the P-traps will generally be well up in the joist bay and very near the underside of the sub floor. General purpose insulation can be installed in the stud bay on the underside of the trap but do not insulate the top of the trap. Keep the top area open so that residual heat from the living space above can come through the sub floor and maintain a minimal temperature in the joist bay in the proximity of the P-trap. Water supply lines are a whole different problem. We must keep in mind that insulation does not generate heat, it only retards the rate of heat loss. Even well insulated supply pipes will freeze if exposed to sub freezing to sub-zero temperature for a prolonged period of time. One method of protecting the pipe is to run it as high as possible through the joist bays and insulate the underside in the same manner as suggested for P-traps. Another method is to run the hot & cold lines very close together, then install a circulation loop on the hot water line and cover both the hot & cold lines in a common insulation. In this manner the slight amount of heat radiating from the hot water line would be retained in the common insulation and thereby protect the adjacent cold water line. The next option would be to install thermostatic controlled electric heating tapes on the pipes, then cover that with insulation to keep the heat in. In regards to what type of insulation. It really doesn't make a lot of difference. While the preformed poly-foam insulation is what the homeowner is usually most familiar with, they make both poly-foam and rigid preformed sections of fiberglass pipe insulation with either a paper or plastic covering in sizes ranging from 1/2" diameter up to 20" diameter and they also make matching preformed covers for Tee's and elbows but you would have to get those types of insulation from a local HVAC or Plumbing Supply.

Correct the water issue firstoff. Ideally, the soil surface inside the crawlspace should be higher than that outside - water WON'T flow uphill. If you can't raise the inside level or lower the outside, at least install drains or make changes in drainage patterns in the yard to diminish the possibility of water accumulating and standing in the crawlspace. Once you've remedied the water situation, I'd put down a 6-10 mil vapor barrier on the ground, lapped up 6" or so onto the perimeter walls, close the foundation vents - and never open them again - and have 2 inches or so of wet-blown cellulose blown onto the perimeter walls. That way, any leakage from your ductwork(I'm presuming it's in the crawlspace?) is contained inside the crawlspace, but is still able to help heat your floors. In a dry crawlspace, with no standing water, opening & closing foundation vents CAUSES moisture problems. You close them in the winter, to keep your pipes from freezing & your floors from getting so cold, right? But, then you open them back up in the summer? I'll bet it's nice and cool in the crawlspace in the summer, so all that hot humid air comes rolling in through those foundation vents, to condense on your floorjoists, etc., just like moisture condensing on the sides of a glass of iced tea. I'd insulate my perimeter walls, and NOT under the floor - but not until you eliminate the water in the crawlspace problem.