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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 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.

About Michael Church

Michael Church has been involved with indoor air quality since 2005 and feels the unhealthy crawl space is one of the major problems causing poor indoor air quality.

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