Insulating Wood Framed Walk-Out Basement Walls
Walk-Out foundations create an interesting dilemma in when considering wall insulating details. I’ve written extensively on the subject of how to properly insulate basement walls which deals with concrete (and block) walls but I’ve never addressed the occasional wood framed walk-out walls that exist in some many homes today. In this article I want to discuss the options most suitable for insulating these wood framed exterior walls that exist in homes with walk-out basements.
Typical Wood Framed Walk-Out Basement Walls
Typically we see most wood framed walk-out basement exterior walls insulated with fiberglass insulation and some type of vapor barrier. This detail is used in millions of homes in the exterior walls above grade with some success (the industry is slowly moving away from this detail, but it’s still the most widely used today). So if this detail works ok above grade, why might it not be the most suitable solution in a basement? The answer really revolves around the high humidity levels that typically exist in a basement regardless if there’s a walk-out wall or some other means of egress like a bulkhead.
Many times when we remove fiberglass insulation from a wood framed basement wall we find signs of mold and moisture. In most situations we find frost (during the winter) building up behind the fiberglass, on the surface of the exterior sheathing, around nails (from siding and framing). Moisture from the basement penetrates the wall system, hits the back side of the sheathing and exposed fasteners and condenses and forms frost on cold days. This cycle goes back and forth through the seasons and helps promote mold and mildew growth.
Improved Wood Framed Walk-Out Basement Wall Insulation Details
If you’re going to take the time and money to properly insulate the rest of the conventional foundation walls it’s definitely worth spending a bit more money and time on the framed walk-out walls as well. There are several options to consider including:
- Spray Foam – One of the best options is to have the framed walk-out walls spray foamed with closed cell foam. The spray foam will not only provide the highest R value but it will also create the tightest wall system preventing air infiltration and moisture movement. This option is the most expensive and also requires a professional installation in most cases.
- Flash & Batt (Spray Foam & Fiberglass Combination) – A process that’s getting lots of attention and I’m seeing more on job sites is called “Flash & Batt”. In this detail the stud bays are “flashed” with spray foam, typically about 2″ of closed cell foam which creates both a good starting layer of insulation value (higher R value per inch than fiberglass) and it seals the wall against air infiltration which drastically improves the efficiency of fiberglass insulation (fiberglass significantly loses R value when air infiltrates the wall section). This option is more cost effective than the one above but typically involves a spray foam contractor plus yourself or additional labor for the fiberglass.
- Foam Board & Fiberglass – Another option is to use a combination of closed cell foam board (XPS Foam Board or Polyiso Foam Board) and fiberglass insulation. This detail is a great option for DIY’ers and general contractors that don’t want to involve a specialized contractor like spray foam applicators. In this approach a layer of XPS or Polyiso foam board are cut to fit in the stud cavity tight against the exterior sheathing. Then the foam is sealed to the framing using canned spray foam. This effectively seals the stud bay from air infiltration and vapor transmission. In order for this to work effectively the foam board should be a minimum of 1-1/2″ thick (2″ preferably). Finally the stud bay can be filled with unfaced fiberglass insulation. It’s VERY important not to install a vapor barrier over the fiberglass as this would create a double barrier trapping any moisture between the insulation and vapor barrier.
Example Basement Walk-Out Framed Wall Insulated with Foam Board and Fiberglass Insulation
In the picture above you can see a fairly typical walk-out basement wall. The wall has two features that are fairly common, on the left side of the photo is a portion of the concrete foundation wall that sits about 4′ above the slab due to the changing grade along the side of the house. On top of the wall is a wood framed wall which transitions to a full height completely wood framed wall at the rear of the house. For this walk-out wall there are several details to consider:
- The exposed concrete foundation wall (above left, and lower) is covered in closed cell foam insulation board. The next step will be to frame a short wall in front of it (it will have a decorative cap on it after, instead of framing the new wall all the way to the ceiling). In this case the lower portion of the wall is insulated exactly as I’ve discussed in my Basement Insulation Article.
- Each stud bay was insulated first with 2″ of DOW XPS foam board. The pieces were cut so they fit easily into the bay. Then a can of spray foam was used to seal the foam board to the framing. It’s best to do the spray foam last after all the foam is fit so you can use an entire can before it clogs up.
- Lastly, the remaining stud bay is filled with unfaced fiberglass insulation. It’s extremely important that a vapor barrier is NOT installed over the fiberglass with this detail. It will trap moisture in the stud bay leading to serious problems.