Basement Insulation Detail
Written by Todd Fratzel.
Basement Wall Insulation
Lately I’ve been swamped with questions about insulating basement walls properly. So I’ve written this article to share with you a Basement Insulation detail that I hope will clarify some issues.
Before you choose an insulation method for your basement it’s important to understand the moisture problems associated with basements. Whether your foundation walls are concrete or masonry the moisture problems are really the same.
Concrete and masonry walls have a significant amount of moisture (water) contained within the material. The moisture will be present in the foundation materials for years and will never fully dry out. The foundation wall will go through drying cycles over time. The upper portion of the wall will dry to the outside and inside while the lower portion will dry to the inside of the house.
The drying cycle releases moisture which can get trapped in adjacent materials causing moisture problems such as mold, mildew and rotting. Knowing that this cycle will continue year after year it’s important to choose an insulation detail that prevents that drying moisture from entering vulnerable materials such as wood framing, fiberglass insulation, drywall and other moisture sensitive materials.
We also need to prevent water vapor from the finished space from coming in contact with the cold foundation walls and forming condensation. Unlike traditional walls where we have vapor barriers on the outside and inside foundation walls present some challenges. In the proposed basement wall insulation detail we’ll provide a moisture/insulation barrier along the face of the foundation wall and then a vapor barrier on the warm side of the framing of the basement walls.
Insulation Detail – Insulating Basement Walls
The insulation detail shown in the figure is a detail that I’ve used several times with great success. This detail is my preferred method for insulating basement walls short of having the walls sprayed with foam. There are several variations of this detail that can be used to achieve the same result.
- Install rigid expanded polystyrene insulation from the slab up to the top of concrete (masonry) wall. Install the same insulation along the top of the exposed foundation wall and then up along the rim joist to the bottom of the sub-floor.
- The vertical expanded polystyrene insulation can be attached to the wall with a multi-purpose adhesive designed for foam. The top portions along the top of wall and rim joist can be installed using spray foam such as “Great Stuff”.
- Be sure to seal ALL the joints in the foam with tape. The best tape I’ve found for adhering to foam board is Tyvek tape.
- Frame a wall in front of the expanded polystyrene insulation. The wall can be as simple as strapping shot through the foam into the concrete or a traditional framed wall. If you frame a wall I recommend you install a piece of 1x composite decking below the bottom plate. This will prevent moisture from wicking up into the framing and finished wall.
- Optional – Install insulation in the stud cavities. Depending on the thickness of the foam you can save some money and supplement insulation in the stud cavities, it’s fine to use fiberglass or cellulose in this step. See Insulating A Basement with Cellulose and Basement Storage Room for more details.
- Install a vapor barrier. Either use faced insulation in the wall cavity or install a vapor barrier (if you use Spray-In-Place Cellulose Insulation you may omit the vapor barrier).
- Be sure to leave a minimum 1/2″ gap between the slab and finished wall board.
Once you’ve effectively sealed off the drying cycle from entering the house side the drying cycle will have to happen to the portion of the wall above grade on the outside. As i said there are several alternative approaches to this method that will work. The key is to cut off drying moisture from the foundation wall and prevent it from entering the finished wall cavity.
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