Cold floors and drafty baseboards usually start at the rim joist and the crawl space below. Closed-cell foam closes both in one visit.
Tell us what the Des Moines property is doing and we'll take a look.
Walk the perimeter of an unfinished Des Moines basement in January and put your hand up where the floor joists meet the outside wall. That band, the rim or band joist sitting on the sill plate, is where the framing meets the foundation, and it is rarely sealed. Wood shrinks, the sill sealer (if any) was a strip of foam under the plate, and every joist bay is a separate pocket with gaps at the top and bottom. Cold air is pulled in there to replace the warm air leaving the attic.
Fiberglass batts stuffed in the bays do almost nothing about that airflow, and they hold moisture against the wood. Two inches of closed-cell spray foam across the rim, the sill and the top of the foundation wall seals the whole band and insulates it at the same time. It is one of the most cost-effective places in the house to use foam, and it is where we start on many jobs.
A lot of Des Moines-area houses have a crawl space under an addition, a room over a former porch, a split-level's lower wing or the whole footprint of a slab-less ranch. Most were built vented, with fiberglass batts hanging between the floor joists above bare dirt or a thin plastic sheet. In our climate that arrangement fails two ways: the batts sag and get wet from summer humidity coming through the vents, and in winter the floor above is cold because the vents are pulling in 14 F air.
The better approach in a cold, humid climate is to treat the crawl space as part of the house: put a sealed vapor retarder on the ground, insulate the walls rather than the floor above, seal the vents and the rim, and let the space stay close to indoor temperature. Closed-cell foam on the crawl space walls does the insulation and air sealing in one step and is not damaged by occasional damp. Iowa's energy code benchmark for crawl space walls in new construction is R-19 or R-15 continuous, which is a good target for a retrofit too.
We will not foam a crawl space that has standing water or active bulk water intrusion. Drainage has to be handled first, and the scope will say so if that is what we find.
The crawl is cleared of debris and old batts. Any fallen insulation and the old ground plastic come out. A heavy polyethylene vapor retarder goes down over the ground, lapped up the walls and sealed at seams and to the wall. Vents are blocked from the inside with rigid foam sealed at the edges. Then closed-cell foam is sprayed onto the foundation walls and up over the sill and rim joist to the specified thickness, checked with a depth gauge.
Because a crawl space is accessed only for service, the code allows foam there with an ignition barrier rather than a full thermal barrier, and some products carry approvals to remain exposed in that setting. The scope names the product, the coverage requirement and how it will be met. If there is a furnace or water heater in the crawl, combustion air and clearances are part of the plan.
The rim and crawl are the bottom of the stack effect. Sealing them cuts off the supply of cold replacement air that feeds drafts at floor level, which makes the whole first floor feel warmer at the same thermostat setting. It also stops the humid summer air that has been condensing on cold ductwork and pipes in the crawl, which is where a lot of the musty smell in older ranches comes from.
Together with attic air sealing, closing the rim and crawl handles the two ends of the pressure path. That pairing is the core of most of the retrofit scopes we write in Des Moines.
Questions
In Central Iowa, no. Vents bring in humid air in summer and freezing air in winter and do little to dry the space. Sealing the vents, covering the ground and insulating the walls is the approach that keeps the crawl dry and the floor above warm. If the space has bulk water, that is a drainage problem to solve first.
Insulating the floor leaves the crawl cold and damp, exposes ducts and pipes to freezing, and the batts eventually fall. Insulating the walls brings the crawl inside the thermal envelope so everything in it stays near room temperature and dry.
Only where the rim is accessible. If the basement ceiling and walls are finished, we can often reach the rim from an unfinished utility area or from outside details, but a fully finished basement may need small access openings. We will look and tell you what is realistic.
The residential code allows certain spray foam products to remain exposed at the rim joist within stated thickness and density limits; others need an ignition barrier. The scope specifies which product is being used and whether any covering is required.
Tell us what the house is doing and we will come look at where the heat is going.
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