Rim Joist Insulation vs Sill Plate Insulation

Insulating the crawl space properly is a great way to not only save money but to make your home more comfortable. Many crawl space encapsulation contractors refuse to install any crawl space insulation much less go as far as air sealing the crawl space and insulating the rim joist and sill plate of the crawl space. Rim joist insulation (or band insulation or band board insulation) is different from the sill plate insulation but both are needed to properly insulate your home and control moisture.

What is the Rim Joist?

The rim joist id a board that is at the end or caps the floor joists of the home. It is located above the cinder block or concrete foundation wall and creates the outer box of the floor assembly. The rim joist sits directly on top of the sill plate. Unfortunately, rim joists are not typically built with pressure treated wood so they can grow mold and fungus when they become damp.

What is the Sill Plate?

The sill plate is normally the only pressure treated wood found in the crawl space or basement. Because it comes in direct contact with the masonry wall, code requires it to be pressure treated in the event moisture is transferred from the foundation wall. Today, some homes are being built with a foam material called the sill plate gasket, that is laid on the foundation wall and the sill plate is placed over the foam.

This is a great idea because this material is moisture resistant but also can naturally air seal the gaps often found between the sill plate and the cinder block wall. This is a fairly new process and not required in most areas so not all new homes have the sill plate gasket. Older homes never have it but sometimes have a piece of metal instead that was thought to deter termites known as a termite shield.

Termite shields have pretty much been abandoned in the south so they must have not been very effective.

How does the Rim Joist Get Wet and Grow Mold?

The rim joists only protection from outside elements comes from the siding, brick, stone or other materials used on the exterior structure of your home. These exterior materials offer very little R-value to protect the rim joist from extreme cold and extreme heat.

If the home’s HVAC ducts run through the crawl space and warm air is leaking from the duct work (as mush as 25% or more), that warm air meets the cold rim joist and condensation begins. Condensation can also take place in the summer when the cool air conditioning leaks from the duct work and meets the hot improperly insulated rim joist. Once the rim joist moisture content gets above 15%, molds and fungi can grow.

Damaged rim joists can cost big money to fix properly so investing in rim joist insulation can keep condensation from happening.

It is Critical to Air Seal and Insulate the Rim Joist

New home construction will normally involve insulating the rim joists with a batt insulation. In East Tennessee, code requires an R-19 batt insulation which is normally fiberglass insulation with a moisture barrier. The problem with this type of insulation is it can still allow condensation and does not air seal the rim joist in any way.

Condensation can build from high humidity that can certainly get behind the moisture barrier and over time create mold problems.

When installing spray foam insulation or our preferred method, combination of foam board and spray foam, this seals the rim joist and gives it the proper R-value per residential building code.

Building Science Backs Up Rim Joist Insulation

BuildingScience.com calls air sealing the rim joists a critical seal and I would say is second only to properly air sealing the attic followed closely by properly air sealing the sub-floor between the crawl space and living space.

3 Vital Crawl Space Air Seals Video

4 Crucial Attic Air Seals Video

In building air barriers, the field of the opaque wall typically does not contribute strongly to the building’s overall air leakage.  Instead, details that connect building components are often the cause of much of the air leakage, such as the roof-to-wall interface, wall-to-foundation interface, and other details (e.g., bathtubs, fireplaces, service penetrations).

The rim joist or band joist is a particularly troublesome detail.  This condition occurs at the basement- (or conditioned crawlspace) to-first-floor interface, and between floors. The concrete foundation-to-wood sill plate connection is often the source of significant air leakage. BuildingScience.com

Call Crawl Space Ninja to Inspect Your Joists and Sill Plates

The professionals of Crawl Space Ninja have the experience to inspect your homes joists and sill plates. Our team can recommend options that can resolve your foundation issues.  Complete the form for a free consultation for crawl space and foundation repair.


10 thoughts on “Rim Joist Insulation vs Sill Plate Insulation”

  1. Mid-Hudson Valley, NY and a 30-year old house: I have 10″ block foundation with 8″ pressure treated sill plates. That leaves about a 1-inch gap on the basement side of the block, the rest covered by the sill plate. Should I seal these gaps? I could use spray foam or a sealing tape.

    This would deter potential rodents and insects using those gaps. Sealed gaps would also limit some basement moisture through the top blocks. And radon gas? I am not sure of all the variables of doing this project, but would love some advice.

    1. Hi David, if your pest control company does not void a termite warranty I would seal. You can use pre cut foamboard to seal major gaps and spray foam to air seal and lock in place. Hope that helps.

  2. I’d like to air seal and insulate my rim joists but after calling my local Terminix they said they don’t want the band joist insulated and want 8″ below the sill plate for a termite gap. He suggested I scrap the wall insulation plan and insulate the subfloor. I’ve been planning to encapsulate and insulate the walls of my crawlspace as well as add a dehumidifier. My crawlspace is level with the ground and I have a rough stone veneer wall covering a steel pier foundation so spray foam seems to be the logical and easiest way to air seal and insulate walls above the vapor barrier on the floor. I live in Western NC and called my local inspector who said I was only required to have a 2″ Termite gap and am allowed to insulate the band joist. My first winter in the house last year was pretty cold and I’d like to solve that but don’t want to run into issues with the termite inspections when I want to sell the house (Last year we bought it, they did inspect and nothing was found). Any experience or advice for my situation? Should I get another termite inspector’s opinion?

    1. Hi Jamin, we have local pest control companies that try to override the termite gap recommendation and go as far as void any warranties if the crawl space is not done to their specifications. There are other termite companies that are completely fine with implementing a warranty so we tend to steer people towards the ones that understand encapsulation and are easier to work with. I don’t want to tell you who to use but I believe there are others that would be fine with what the inspector recommends. Hope that helps.

  3. Hi! I’m gradually chipping away at encapsulating my crawl space and have about 75% of my rim joists insulated with EPS foamboard, foil side facing the crawl space, and spray foam. However, the other side of the foambord has a layer of clear plastic on it, and I’m concerned about the possibility of moisture build up on that side causing damage to the interior side of the rim joist. I’m wondering if I should remove the EPS and replace with XPS foamboard instead. What do you recommend?
    Thanks so much and I really appreciate all your wonderfully educational videos and articles!

    1. I understand your concern but both EPS and XPS foamboard could trap moisture. So the best solution is to ensure the outside is not allowing any moisture to enter. If fiber glass insulation can trap moisture then any type of foamboard or spray from can trap moisture as well.

  4. Hi Michael! I’m curious about your opinion on this. I have a 2005 house, no termite shield or sill plate gasket. Treated wood sill plate. And what appears to be something like a double rim joist (in places I see a cutout for an anchor bolt head and another board behind it. One side of my house faces the ground and we’ve had water issues there. I plan to rectify the surface water to the best of my ability and should be able to mitigate nearly all of it but my question is if there’s damp conditions on that side of the house, would that work it’s way up the block, through and into the sill plate, possible the rim joist(s), and damage them prematurely if I seal them from the vapor barrier to the floor deck with foam board and spray foam? I’ve had an interior perimeter drain installed and holes drilled in the block so water can make it’s way to a sump pump (and does when we get days of rain). Checking it today, it was pretty dang dry, but I have a dehumidifier running and that area’s down to 40%’s, but pre-sealing. No idea what it would look like post-sealing.

    I’ve wondered it if would be better to use something like a caulk for these joints, and then fiberglass batting against the rim joist, thought being that if it does wick up moisture, it would wick it out and the dehumidifier could assist in keeping the wood dry. I’m all over the map, as you can tell!

    1. Hi Austin, if the rim joists and sill plate are damp or risk becoming damp in the future the best option would be no insulation. Even fiberglass can trap moisture long enough to cause issues even with your perimeter trench and dehumidifier installed. If your goal is to air seal, then fiberglass alone won’t accomplish that either. Caulking with fiberglass would air seal and give you a thermal break but you still run the risk of trapping moisture if the block walls are wet enough to allow moisture to travel up them. However, with your perimeter trench and holes you mention plus the dehumidifier, I doubt that will be the case. I hope that helps.

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