Rim Joist Insulation

Three inches of closed cell foam in your rim joists is the cheapest meaningful energy improvement available in a Minnesota basement, and it usually runs $1,200 to $2,500. It is also the fix for a moisture problem most homeowners do not know they have.

Call (218) 514-6594.

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Where the rim joist is and why it leaks

Walk into your basement and look up at where the concrete wall ends. Sitting on top of the foundation there is a horizontal board, the sill plate. On top of that, the floor joists run across the house, and closing off the end of each of those joists is a board running around the perimeter of the building. That is the rim joist, sometimes called the band joist or the band board.

On the other side of it is your siding. That is often the entire assembly. A single board, sheathing, siding.

Now count the joints. Foundation to sill plate. Sill plate to rim joist. Rim joist to subfloor. Rim joist to every joist end. Joists only run one direction, so on a typical Duluth house you are looking at 40 to 60 joist bays along the two walls the joists land on, plus a continuous rim seam along the other two. Every one of those joints has been shrinking and moving since the day the house was built.

None of those joints were sealed. Wood framing was never intended to be airtight. Add the stack effect, where warm air rising through the house pulls cold air in at the bottom, and the rim joist becomes one of the largest continuous air leaks in the building.

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The fiberglass problem

Most Duluth basements have fiberglass batts jammed in the rim joist bays. Some were put in by a builder, some by a previous owner on a Saturday. Either way, that assembly is not just ineffective. It can cause damage.

Here is the sequence:

1. Fiberglass does not stop air movement. It slows conduction and that is all.
2. Basement air in winter is warmer and more humid than outdoors. It moves through the batt.
3. The rim joist board is close to outdoor temperature, because there is nothing outboard of it but sheathing and siding.
4. Warm humid air meets a below-freezing surface. The moisture condenses, or frosts.
5. The batt sits against the wood, holding the moisture in place, and slows drying.
6. Repeat every winter.

Pull a batt off a north or west facing rim joist in a Duluth house in late winter and you may find frost, dark staining, or damp wood. Over years that becomes rot in the sill plate, which is a structural repair rather than an insulation project.

Closed cell foam breaks the cycle at step two. Air never reaches the wood, so there is nothing to condense.

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What the job looks like

1. Pull out what is in there. Every batt comes out. All of it. If there is damage behind it, you want to see it now.

2. Look at the wood. We check the rim and the sill for staining, softness, and rot. If there is real damage, you get told about it before we spray, not after we have covered it up. That is a carpentry repair and it comes first.

3. Prep. Mask everything nearby. Overspray does not come off, so this step matters more than it sounds. Ductwork, the water heater, the panel, and anything stored against the wall gets covered or moved.

4. Spray. Three inches of closed cell in every bay, wrapping the ends of the joists, tight to the sill plate, and lapping down onto the top of the foundation wall to seal that joint too. Applied in two inch lifts, because closed cell put on too thick in one pass can scorch internally.

5. Trim and clean. Any foam that expanded past where it belongs gets cut back. The job should look neat, not like a cave.

6. Cover if required. In an unfinished basement, whether exposed foam needs a thermal barrier or ignition barrier depends on the AHJ and the product. That goes in the written scope.

Most houses are half a day.

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Why closed cell and not open cell

The rim joist is one of the few places where the material choice is not close.

• Vapor control. Closed cell reaches Class II vapor retarder status at roughly 1.5 to 2 inches. Open cell is vapor open, which puts you right back at the condensation problem.
• R-value in a shallow space. Rim joist depth is limited by the joist depth and by what you can reach. At R-6.5 per inch, closed cell fits R-20 into three inches. Open cell would need five and a half.
• Water. If a pipe ever leaks or wind driven rain finds a way in, closed cell does not absorb it.
• Adhesion and rigidity. Closed cell bonds hard to the framing and stays put. Nothing to sag out and land on the floor in ten years.

More on the material on the closed cell page.

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What it costs

Job	Typical range
Rim joist perimeter, average house, 3 inches closed cell	$1,200 to $2,500
Rim joist perimeter, larger or multi-level home	$2,000 to $3,500
Rim joist added to a crawl space or basement wall job	$600 to $1,200 incremental
Partial, a few problem bays only	subject to minimum charge

About the minimum charge, said plainly. The board foot math on a rim joist job comes out well below $1,200. The reason the price is what it is: the rig, the hose, the crew, the masking, and the cleanup cost the same on a 450 board foot job as on a 4,500 board foot job. Nobody in this trade prices small jobs off the board foot rate alone.

Which leads to the useful advice. If you are going to do a crawl space, a basement wall, or an attic in the next year or two, do the rim joists in the same visit. You pay the setup once. The incremental cost of adding rim joists to a job that is already happening is a fraction of doing it standalone.

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What you should actually feel afterward

This trade oversells results, so here is the honest version.

Things people consistently notice: the basement is warmer and less drafty, the floor above the basement is warmer, especially near exterior walls, and the cold draft along the floor at the bottom of the stairs goes away.

Things you will probably not notice immediately: a dramatic drop in the heating bill from this job alone. The EPA’s Energy Star program estimates that a homeowner who air seals and insulates the attic, floors over crawl spaces, and basement rim joists saves an average of around 15 percent on heating and cooling costs. That is the whole package, not the rim joists by themselves. Anyone quoting you a specific dollar savings for rim joist foam alone is making it up.

The thing you will not see at all: the sill plate not rotting. That is the real return and it does not show up on a utility bill.

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Book it with something else if you can

Call (218) 514-6594 or send the form. Tell us roughly the perimeter of the house and whether the basement is finished, and mention anything else you are thinking about. Bundling is where the savings are on a job this size.

See also: crawl space insulation, closed cell spray foam, retrofit insulation.