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Buyer and Seller Beware!*
It happens to countless homeowners, the condition was discovered during the course of a real estate transaction by the buyers home inspector. An inspection of the attic revealed spots and blotches covering the bottom of the roof sheathing. Worse yet - it turns out to be attic mould!

What does energy conservation have to do with mould in the attic? Well if you take a step back and consider how the house behaves as a system, they are often directly related.

Building science experts have long been using the "house as a system" approach to diagnose the cause and origin of building defects.

For example, ice dams. These are often caused by warm air seeping into the attic which causes the snow and ice on the roof to melt. The water drains to the edge of the roof (which is colder than the rest of the roof because it is an overhang and not warmed by the attic), freezes and creates an ice dam. As this process is repeated daily, the ice dam grows larger. Eventually water is forced under a shingle where it can seep into the house.

Understanding how the house behaves as a system and the various causes and effects is necessary todiagnose most building related problems. But how about that attic mould? How did it get there?

Mould requires chronic moisture to form and to thrive, so source(s) of moisture must be present. Possibly the moisture came from outdoors. The roof is newer and a quick check of the roof shows no obvious damage or leaks.

Possibly the moisture came from indoors. During the heating season, the interior of the house frequently has high moisture levels, especially bathrooms and kitchens. A quick check shows that all bathroom fans, kitchen vents, etc. are properly ducted completely outdoors and not into the attic. The amount of insulation looks good and the attic is well ventilated.

Don't give up - you are almost there! Remember the house as a system? You know that warm, moist air is in the house, but how is it getting into the attic?

By air leaks! Air leaks are the leading source of energy loss in most houses, and a frequent source of chronic moisture that can cause attic mould. Most homeowners are well aware of air leaks around windows and doors (especially old ones), but many overlook the numerous gaps leading directly into the attic!

Have a look around the attic and you may find large gaps around recessed lights and fans, holes where wires or pipes are installed, even large gaps around the chimney. And don't overlook the whole house fan and especially the folding attic stair - a big, un-insulated hole in your ceiling that is often overlooked!

These gaps can add up to a large hole that allows warm, moist air from the house to flow right into the cold attic. The warm moist air condenses on the cold roof sheathing, creating chronically damp conditions that can lead to attic mould growth. And the energy loss - it can be like leaving a window open all winter long!

"Your roof sheathing needs to be replaced."
Unless your sheathing exhibits dry-rot or shows signs of de-lamination it likely does not need to be replaced. A new roof is an extraordinarily expensive way to eradicate mould growth in attics. The vast majority of attics can be remediated without having to replace the attic sheathing.

"How does one determine if the roof should come off?" When the roof sheathing is rotted or damaged, or when other structural repairs are needed, a tear-off is unavoidable. When attic surfaces or insulation contain a large reservoir of toxic or allergenic mould, AND if the area were inaccessible, say because the space is too small to enter, then it may be necessary to remove some portion of roofing to give access for remediation, particularly if there is evidence of transmission of problem mould from that space into the living area.

"Bleach and water will fix the problem." Chlorine Bleach (sodium hypochlorite 6%) does not kill mould. Mould's hyphae (root structures) actually grow into wood and drywall like roots. The hyphae are not killed by bleach because bleach's ion structure prevents chlorine from penetrating into porous materials such as dry wall and wood. It stays on the outside surface, whereas mould has protected enzyme roots growing inside the porous construction materials. When you spray porous surface moulds with bleach, the water part of the solution soaks into the wood while the bleach chemical sits atop the surface, gasses off, and thus only partially kills the surface layer of mould while the water penetration of the building materials fosters further mould growth. Chlorine bleach causes long term breakdown of wood products like studs, sheathing, plywood, OSB, and other building materials over time.

Chlorine Bleach is NOT a registered EPA mould killing product. You can verify it yourself when you are unable to find an EPA registration number for killing mould on the label of any brand of chlorine bleach. Why not? Because it is not effective at killing mould as other EPA approved chemicals.

"What does the EPA have to say about using bleach to kill mould?" The use of chlorine bleach is not recommended as a routine practice during mould cleanup."


Some mould removal contractors include a process they call "encapsulating" or "encapsulation". Plainly stated, it means they paint over mould, (often with a stain killing
paint called KILZ, sold in most Home Depot stores). This practice is not recognized by the EPA or any other legitimate authority on mould remediation. The EPA guidelines for mould abatement are very clear, "REMOVE IT".

If the mould is removed, there is no need for encapsulation. Unless mould is removed, it is still there.

"Encapsulating" mould by painting over it is just a way to cover up any mould that was not removed.


Get the area as dry as possible before starting your remediation project or it will not be a successful remediation.

Start the process of drying out the space by using at least two fans. One to circulate the air inside the area (more fans may be requiredfor larger areas), and another fan to pump inside air outside of the attic.

Bring in a HEPA filter, or positive air machine during this process to prevent mould spores from moving inside the home and outside of the attic. Some attics will require the use of a dehumidifier to remove excess water from the space,but this is rare as attics are generally the warmest place in the home. You want the space as dry as possible (below 30% relative humidity) before starting the work, so that the a fungicide registered with EPA for this application can penetrate deeply into the treatment surfaces and kill the roots of the mould effectively. Some attics may need to have mouldy insulation removed before the remediation process is completed.

When HVAC systems and ducting are run in the attic, it is possible that the mould may have penetrated into the home.The home and HVAC should be thoroughly tested for mould.

All areas of the attic must be treated with an anti-microbial to kill the mould even if the mould is just in one area of the floor joists. You must treat the entire rafters, plywood sheathing and in some cases other parts of the attic with a minimum of two full treatments allowing them to dry in between.

There are two ways to treat the attic. Fogging, and pump spraying.

Fogging is a method of dispersing atomized product into the attic area without having to go through the entire attic and pump spray everything which is a very uncomfortable and tedious task with Personal Protective Equipment (PPE) and respirators on.

Pump spraying is also a way to accomplish this task, keeping in mind that you must do at least two treatments soaking all of the rafters, plywood sheathing in the attic. You should wear an P100 rated respirator while pump spraying in confined areas.

HEPA VAC any visible mould areas used to vacuum away dead mould spores. To remove stains from mould on your rafters and plywood sheathing "brush" or sand the stains out. This is a tedious task that remediators get huge sums of money to do, but it does remove the stains.


Dry ice blasting is a process similar to sand blasting, plastic bead blasting, or soda blasting where a medium is accelerated in a pressurized air stream to impact a surface to be cleaned or prepared. However, instead of using hard abrasive media to grind on a surface (and damage it), dry ice blasting uses rice grain-sized dry ice (solid CO2) pellets as the blasting medium. The pellets are accelerated at supersonic speeds and create mini-explosions on the surface (due in part to the sublimation process which expands the dry ice volume 800x upon impact) to lift the undesirable item off the underlying substrate.

Dry ice is non-conductive, chemically inert, non-poisonous and nonflammable. The key advantages of this technology are its ability to clean sensitive surfaces and the fact that there are no residues of the blasting medium after the blasting process since dry ice sublimes (turns from solid to gas) at room temperature.

Dry ice blasting is also known as:

  •  Dry Ice Cleaning
  •  Dry Ice Blast Cleaning
  •  CO2 Blasting
  •  CO2 Blast Cleaning
  •  Cryogenic Blast Cleaning
  •  Cold Jetting



Due to the type of technology, areas can be cleaned faster and more effectively cutting down labour costs.This method of cleaning is about 75% of the cost of more conventional methods.

In the case of attic mould, this method as been very successful at removing the staining and making the roof sheathing almost new looking!

It's important to correctly identify the source of the problem before you try to remove the mould contamination.

In an "ideal" attic design, the air in the attic should be completely isolated from the conditioned air of the home. As a result, the temperature and humidity of the attic space would be very close to ambient outdoor conditions. If you have mould on the attic sheathing, you would then also typically find mould on the outside of the home.

Most homes aren't perfect, however: They have air-leakage pathways that allow indoor air to pass into the attic, bringing household moisture along for the ride. If the attic air is cooler than the air that eventually reaches the attic, the relative humidity (RH) of the attic air will rise and you run the risk that mould will grow n the attic. Attic ventilation is installed with the goal of removing moisture from the attic. But if the outdoor air is damp, ventilation is not as effective.

The remedy is to find and seal the air leaks. Go up into the attic, lift up all the insulation, and seal any penetrations with expanding foam and caulking. Seal all pipes, wires, junction boxes, fans, ducts, recessed lights, and chimney penetrations. Also pay attention to the places where interior wall plates intersect the home's ceiling. Indoor air enters the wall cavities, rises up within the wall, and passes into the attic through the gap that exists between the wall's dry wall and the top plate that it's nailed to.

Adding more roof vents is a major consideration.

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