Renovation & Design

ASK THE INSPECTOR: Knee wall attic poses moisture problem

QUESTION: We moved into a 1922 River Heights home at the end of May 2009. We had an Energy Audit done on our home about a year later and are interested in upgrading our attic and knee wall spaces.

The home is 2 1/2 storeys and the knee wall surrounds three sides of the upper story. The two-by-four rafters are insulated with wool and Zonolite, which is held in place with fir boards. The attic side of the knee wall is insulated with pink fibreglass batts between the studs, and the floor of the knee wall is insulated with wool and Zonolite just to the top of the floor joists.

There are no soffits to allow air into this space. Part of the space has wood flooring that allows access for storage purposes. There is also a single-pane window in this space, with a screen that can be opened in the summer. In this space there are three exhaust fan vents that run to three separate rooftop exhaust vent hoods. The exhaust hoses are not insulated. There is also a small closet door to this space that is not insulated.

How should we insulate this space to improve our energy savings? Should we add more batt insulation to the back side of the existing wall insulation and also add more fiberglass batts to the existing floor insulation, along with insulating the door to the space? Also, should we change the exhaust fan hose to an insulated one with vapour barrier? Should we look at adding roof vents as well? We are a bit confused as to how we should proceed.

If we open the space to the outside, we should technically remove the insulation in the roof rafters. As this insulation contains Zonolite, this would be a costly endeavor, far exceeding the energy savings gained. At a minimum, I believe I should insulate the exhaust-fan hosing and perhaps add something to prevent backdrafts down the pipe into our home.

In addition, we are now getting ice dams and icicles along the south side of our roof along the eavestrough outside the knee wall area. This, I believe, is an indicator of heat loss through the roof from the knee wall area.

Any help would be appreciated. Thank you. Dave Mainprize

ANSWER: You appear to already be well on the way to understanding the issues of improperly insulated and vented knee wall attics in contributing to ice damming and other issues in your home. You appear to have a few remaining items to address before having a significant improvement in heat retention and prevention of moisture issues.

One of the biggest issues I see during inspections is related to improper insulation and ventilation in older 1 1/2- and 2 1/2-storey homes. As you've observed, one of the main problems is improper insulation of the area between the rafters in the small knee wall attic spaces. This is the primary area of concern in your home, as you already have some insulation in the proper locations -- the attic floor and knee walls.

Removal of the old rock wool from between the rafters is normally a straightforward job, but yours is complicated by the presence of vermiculite, which may contain asbestos. While disturbing this type of insulation in a sealed attic is not recommended, you have little choice. Since this potentially hazardous material is already in the knee wall attic floor, and you do not have a well-sealed door, dealing with it is imperative.

It's probably better to leave this insulation in the knee wall attic than try to remove it. I suspect that there is a minimal amount of vermiculite in between the rafters in your knee wall attic, due to the typical method of installation.

This material was normally installed by pouring into cavities from large bags. These cavities are between ceiling joists in the bottom of attics, or between studs in walls. To access the area between the rafters in your knee wall attics, this material would have to have been poured in from the main attic, above the visible sloped ceilings in the upper floor rooms.

To get this material all the way into these cavities would have required the old insulation to be removed from between the rafters inside the knee wall attics and the sloped bedroom ceilings. It appears that this has not been done in your home, as there is still older rock wool present.

It's my guess that the vermiculite you are seeing may have been installed in the upper attic and is only a small amount that may have slid down the sloped ceilings through small gaps in the old insulation. If this is the case, there should be minimal amounts disturbed when you remove the old fir boards and rock wool from the rafters.

Regardless of the amount, you must take special precautions to prevent breathing in any of this insulation and protect the living area from contamination. Breathing protection should be a respirator rated for asbestos and your body should be covered from head to toe, preferably with disposable coveralls and gloves.

The first order of business is to properly insulate and seal the knee wall access hatch, probably with extruded polystyrene sheathing and weatherstripping. After insulating the exhaust-fan ducts and sealing the knee wall attic floor where these pipes come through, the boards and loose insulation can be removed from between the rafters. It may be most practical to leave the insulation that falls from this area on the attic floor, to prevent possible contamination of the home if you try to remove it. The old window may come in handy to dispose of the old fir boards without taking them through the house.

Once this is all out, covering the entire knee wall attic floor with a thick layer of new cellulose fibre insulation will be your best option. This should cover any loose vermiculite and prevent future friability of any asbestos fibres.

Adding more insulation to the knee walls themselves would help with heat loss, but may not be necessary and too difficult to accomplish in the small space. Installation of several roof vents, near the top of the knee wall attics, will finish the job, preventing moisture issues and further ice damming.

Ari Marantz is the owner of Trained Eye Home Inspection Ltd. and the president of the Canadian Association of Home & Property Inspectors - Manitoba ( Questions can be e-mailed to the address below. Ari can be reached at (204) 291-5358 or check out his website at



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