Bay Window and Floor Deck Cantilever Insulation Details
I’ve written quite a few articles on how to insulate basement walls and floors but one more specific topic related to this subject keeps coming up and that’s how to insulate a bay window floor or cantilever floor framing. In this article I’ll share a detail for new construction and also a detail for renovating existing bay window floors and deck overhangs.
With the deck overhanging the foundation wall the area is exposed to extreme temperatures and the possibility of significant air infiltration. Even with 2×12 floor joists you’d be lucky to get a R30 insulation value with the floor perfectly sealed from air using fiberglass. That’s a big “IF” and frankly even at R30 that’s really not sufficient in cold areas of the country. With even a minimal amount of air infiltration that R value plummets to R15 or less!
The following details address both air infiltration and increased R values to promote a much warmer floor system around deck overhangs.
New Construction – Bay Window / Floor Overhang Insulation Detail
The sketch above shows one method for insulating the deck overhang at a bay window or cantilever that encloses a bump-out. Quite often the insulation in this area is dealt with long after the framing is finished and results in a very cold drafty spot in the house. Most often I see this area stuffed with fiberglass insulation after the house is built and thats just not an effective approach.
It’s important to note that this detail needs to be addressed during the framing process. The following steps will help reduce air infiltration and increase the effective R value.
- Install a layer of XPS or Polyiso foam board insulation to the bottom of the cantilevered floor joists. At a very minimum I’d recommend at least 1-1/2″ to 2″ of XPS (polyiso will achieve higher R values).
- Install exterior grade sheathing product below previously installed foam board to protect it from the elements and rodents.
- Inside the basement area, install a piece of 1-1/2″ XPS foam on top of the foundation wall tight to the sill plate (seal against the sill plate with spray foam from a can).
- From the top side, prior to sheathing the floor deck, install XPS or Polyiso foam board tight between the joists and tight against the rim joist. Seal all the edges with spray foam. Also be sure to seal the bottom of this piece of foam to the lower piece previously installed below the joists.
- Install an additional piece of XPS or Polyiso between the floor joists directly above the foundation wall (as if it were joist blocking). Again, seal this piece around the sides and bottom.
- Install fiberglass or cellulose insulation in the joist bay.
- Install a final piece of XPS or Polyiso on top of the fiberglass or cellulose. Be sure to seal this piece to all the framing and foam.
- After all those steps you can install the deck sheathing.
The real key to this detail is spending time sealing every very well. Air infiltration is the root of all evil when it comes to how cold / hot a floor feels with this sort of location.
Existing Construction – Floor Overhang Insulation Retrofit
The detail above shows one approach to retrofitting an existing floor cantilever that you might find at a bay window or fireplace bump out. In this situation, its likely that only some fiberglass was stuffed into the joist bay after the framing was complete. There are several ways to deal with this such as spray foam or blowing in cellulose. However, if you’re looking for an approach to do yourself without specialized equipment the one shown above will work pretty well.
The key to the success of this approach is having adequate access to the interior joist bays from the basement and also to the underside of the cantilever from the outside. Both locations may prove very difficult to access to this is something you need to evaluate before you begin. The following steps describe the approach shown above.
- The first step will be attaching some XPS or Polyiso foam board to the bottom side of the cantilever from outside. If you can easily remove the existing sheathing then I’d recommend that. If not, install the foam over the existing sheathing as shown above. Be sure to keep it as tight to the foundation and perimeter of the cantilever.
- Install some protective exterior grade sheathing to cover up the foam. This may require some creative trim details to hide from the front such as a decorative water table or similar detail.
- From the inside, try to install a piece of XPS or Polyiso tight against the rim joist. Be sure to seal the edges with spray foam.
- Pack the joist bay full of fiberglass or cellulose (cellulose may be difficult).
- Install a piece of XPS or Polyiso between the joists and above the foundation wall. Be sure to seal to the joists, foundation, and deck above.
This approach will significantly cut down on air infiltration and improve the overall effective R value.
Final Thoughts – Insulating Floor Deck Cantilevers (Overhangs)
They keys to remember when insulating a deck cantilever (like bay window floors, 2nd floor overhangs, and chimney chases) if providing sufficient R value and preventing air infiltration. The minimum recommended R value for Zone 6 (DOE) is R30. If you’re using fiberglass, that number is nearly impossible to attain with just fiberglass and no air sealing even in a floor with 2×12’s. So the key is getting increased R value while also stopping as much air infiltration as possible. Following these key steps will help you make your floor overhang much more comfortable in the winter and summer.