Interior – Attic condensation

On the main floor, ceilings are full of penetrations. Wherever a light fixture, fan, or smoke detector is located, a chase is provided for moisture laden air to enter the attic space. These areas are best sealed from the attic side. Insulation would be temporarily swept away and the penetration sealed with a caulking or spray foam product.

In every home the natural movement of air from the lowest level to the highest is referred to as the “stack effect” of the home. Essentially your house is working like a chimney as the air warms it rises. When the warmed air leaves the lowest level it creates a negative pressure in the basement (cellar or crawlspace) this happens because we should have no opening in the basement walls or foundation that would allow the movement of uncontrolled, unconditioned air to enter the home. 

As the air rises, it comes into contact with the interior side of the ceiling. At this point, we start to create a positive pressure where the conditioned air is trying to migrate into the attic space. Should this occur the warm moist air will condense on the underside of the cooler roof sheathing material.

During the winter months this causes the snow on the roof to melt, about an eighth of an inch of the snow that is in direct contact with the roof sheathing the rest of the snow above works like an insulating blanket and results in water running down the roof (in winter) until it reaches the overhanging portion of the roof or what is known as the soffit. Due to this, now being beyond the heated portion of the roof the melt water now freezes and creates what we refer to as an “ice dam”.

Another adverse effect that can occur under these conditions is a deterioration of the roof deck sheathing. As the warm moist air rises into the attic space it comes into contact with the roof deck. At this point it cools, condenses, and stains (blackens) the underside of the roof deck sheathing. Make no mistake this is a mildew that is now forming on the underside of the roof deck.

A common misconception is that this can somehow impact the indoor air quality of the home. The stack effect of the house would disagree with this assumption. As previously noted the air movement in Canadian construction only moves in one direction, that is up.

Reverse stack effects do exist but only in climates that focus on A/C rather than heating. While this does not have any impact on our indoor air quality it can start to deteriorate the structural integrity of the roof deck itself. This deterioration is more common on roof’s that have been sheathed with an OSB board (aspenite). Sheathing can start to delaminate under extreme conditions. At this point entire roof replacement may be required.

Also important to understand is that the attic space should always be atmospheric pressure, that is the same pressure that is outside is inside (the attic). 

Should an excessive amount of roof vents be installed, these vents now have the capacity to vent more air out of the attic than what the soffits can bring in. The end result of this is that instead of the attic being atmospheric pressure, it is now a negative pressure (a vacuum). 

Given this scenario, we now have a positive pressure on the interior ceiling side and a negative pressure on the attic side, thus, inducing significant air leakage into the attic. Once again contributing to a greater potential for further ice build up.

Recommended remedial actions:

  1. Remove soiled fiberglass insulation;
  2. Vertical walls in an attic are always best to be insulated with a spray foam application, thus, ensuring that gravity does not eventually displace insulation in attic walls;
  3. Ensure all ceiling penetration are sealed with either spray foam or caulking;
  4. Ensure 100% of all soffit venting is free of obstruction, install baffles to ensure insulation does not obstruct soffit air movement, DO NOT INSTALL ADDITIONAL ROOF VENTS; 
  5. Installing an application of spray foam on the top plate of the exterior walls that extends 12 inches in to the attic will greatly reduce the effects of stack effect causing heat loss;
  6. Install cellulose insulation overtop of existing insulation to bring insulation levels up to an R60;
  7. Ensure attic access is tight fitting, insulated and weather stripped.