Dense-Pack vs. Exterior Foam Insulation

Aaron writes: I own a post-and-beam house built in Portland, OR, in the 1950’s with no insulation. The entire house has an exposed beam ceiling that we do not want to cover up, so we plan to put rigid foam above the roof sheathing.

On the walls, however, we are getting conflicting advice and strong arguments for each option — drill-and-fill cellulose vs. exterior foam. I do not know whom to trust or where to get a definitive answer on the best way to insulate the walls without creating moisture problems. I would like to make my house comfortable and energy-efficient, but am concerned about moisture. The walls have 2×4 framing between the timber posts.

I have read and heard that dense-pack, drill-and-fill insulation in the walls can let the moisture build up in the cavity. I’m also told that rigid insulation on the exterior under the siding will not let the house breathe and moisture will get trapped inside the wall cavities.

Steve Bliss of BuildingAdvisor.com responds: No construction topic has been studied and debated more than insulation and moisture, so it’s not surprising that you are getting conflicting advice. Unfortunately, the building codes in this area are also confusing and frequently revised from one version of the code to the next. So your local rules will depend on what version of the national code (IRC) your city or town has adopted.

There is pretty good agreement, however, by the building scientists who have been working on this problem since the 1980s. The current thinking by the people I trust the most is as follows:

You are in the US Climate Zone Marine 4 (under the system used by the building codes and the US Dept. of Energy). In that region, an interior vapor retarder is no longer required by code in an insulated wall. You can use a vapor retarder if you like, but it should not be an impermeable “vapor barrier” such as polyethylene. You could use Kraft paper, a vapor retarding paint, MemBrain, or just plain old painted drywall. Nothing to worry about here.

You can fill the cavities with whatever you like, dense-pack cellulose or fiberglass are your most economical options. There are various types of foams if you want higher R-value and have a lot of money to spend on insulation. Adding insulation to the cavity will not cause moisture problems unless you have water leaking into the cavity – more on this below.
 
Exterior Foam. Adding foam on the exterior (in addition to filling the cavity), is the simplest way to boost your wall R-value beyond what you can get with dense-pack. Adding foam on the exterior will not cause moisture problems as long as you add enough foam. An adequate layer of exterior foam will warm the wall cavity sufficiently that water vapor in the wall will not condense into liquid water. In your climate zone, the recommended minimum R-value for foam sheathing over an insulated cavity is R-2.5 for 2×4 walls and R-3.75 for 2×6 walls. You can use whatever foam insulation you like, but never, never, put an impermeable vapor barrier such as poly on the interior. The walls need to ability to dry to the interior should moisture (or water from a leak) build up in the cavity.

For other climate zones, see the table below:

Minimum R-Value for Foam Sheathing
Climate Zone   R-Value of Foam Sheathing
 2X4 walls*  2X6 walls*
Marine  Zone 4  R-2.5  R-3.75
Zone 5  R-5  R-7.5
Zone 6  R-7.5  R-11.25
Zone 7 -8  R-10  R-15
  Source: IRC Table R702.7.1
*Assumes cavity insulated with fiberglass or cellulose

With exterior foam insulation, the best approach is to leave a vented air space between the siding and foam to provides an extra layer of protection against moisture build-up or water intrusion from the exterior. The air space also helps the siding dry out when it gets wet.

Adding foam to the exterior is not for everyone, however. With the thickened wall, you will need to build out door, window, and other trim to accommodate the thickened wall, adding nailers to provide solid backing for all exterior moldings. Window and door flashings also need to installed properly to work with the new exterior foam.

Regardless of what type of insulation you use, it’s important to have a continuous air barrier on the interior of the building to keep interior moisture from leaking into the wall cavity. This is also important for energy efficiency. Unlike a vapor barrier/retarder, which does not need to be continuous (without holes or unsealed joints), an air barrier must be continuous to be effective. A material such as polyethylene could serve as both a vapor barrier and air barrier if all seams and joints are sealed. However, a simple approach is to simply use your drywall as the air barrier, by caulking or gasketing the edges where it meets the floor, ceiling or door/window jambs.

Finally, here’s the most important issue that is often lost in these discussions: In the vast majority of cases, moisture problems in walls and cathedral ceilings are not caused by the presence or absence of vapor retarders. They are caused by water leakage at joints, corners, flashings, and especially around windows (or skylights and chimneys on roofs). Uninsulated walls are much more forgiving of water leaks than insulated walls. Insulation tends to trap and hold the water. If you have a water leak, combined with an impervious vapor barrier like poly on the interior, and warm temperatures and high humidity (like your area) you can definitely rot out your walls.

The moral of this story is to make sure that there are no current water leaks into the walls and make sure that none are created by the insulation job. Pay special attention to detailing around windows. The biggest problem with retrofitting exterior foam is the detailing around window and door openings. The casings must be extended outward and then flashed to the foam. If you go this route, make sure the contractor has a lot of experience with this issue and can show you successful jobs they have completed.

Interior moisture condensing in the wall cavities in quantities large enough to cause problems is rare except in homes that run very high indoor humidity in the wintertime. Rooms with spas, indoor pools, etc., are especially vulnerable, but an uncovered crawlspace, firewood drying in the basement, an unvented dryer or lots and lots of plants can also lead to moisture problems in walls and ceilings without a good interior air barrier.

Case Study:  A Timber Frame Insulation Disaster

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