We own a house that was built in 1929. Someone who owned the house before us insulated the attic with vermiculite. I am going increase the insulation levels a lot. So I went up there to measure the area for the amount of fiberglass batts I will need. What I discovered is a lot of white mould on the roof boards. I think I should clean it up before I re-insulate, but I’m not sure. Is there another way to deal with it?

Attics in older homes often have problems with mould and mildew. Back in the day, when there was no insulation in your attic, this attic space probably functioned quite well. That’s not to say that the insulation is entirely to blame for the problem. Originally though, the attic was not isolated from the interior air so it was part of one contiguous breathing space that aspirated to the outdoors. The rising hot air from the building pushed through the attic and roofing to the outside and all was well (except the heating costs).

Once insulation was introduced to this space, it sealed and separated the attic from the living area below. Nobody considered that the attic had to breathe, so it morphed into a hot and often humid area, full of trapped dead air. To compound the problem, a vapour barrier is often mistakenly omitted when older attics are insulated. So, the humidity from the hot humid interior air condenses on the roof boards because it is not prevented from entering the attic by a vapour barrier. Fortunately, sometimes the ceilings of older houses are well sealed with layers of old oil based paints. These layers of paint often save old attics that have been retrofitted with insulation, from the mould you are now dealing with.

The key to the whole problem is to introduce lots of fresh circulating outdoor air through your attic. Unfortunately, most older homes do not have soffit vents. However, you can easily install manufactured round soffit vents in drilled holes. These soffit vent plugs are readily available at you local building supply store. Install 38mm vent plugs on 300 mm centers. More is always better than less when it comes to attic venting. Before you are install the new insulation, ensure these vent plugs allow air flow into the attic by installing insulation baffles (“mor vents”) between each rafter bay in the attic. Then, install large gable end vents. These vents allow the fresh air drawn through the soffits to escape and carry off any heat and moisture that could provide an environment for mould growth in your attic. Then, it may not be critical to attempt a lengthy clean up of the existing mould. Without a hot humid environment the existing mould will no longer flourish.

When working in your attic, be careful not to disturb the vermiculite insulation because it may contain traces of asbestos. Wear a good quality respirator and leave your shoes and coveralls in the attic before you come back into the living area to avoid the potential for transfer of contaminants to your living quarters.

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This article was written by Steven Cannon, a Registered Building Official and partner in Lynch Building Inspection Services Ltd. of Nelson, British Columbia. It originally appeared in The Nelson Express.

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