Blown Insulations

By: Home Comfort Svc  09-12-2011

In our climate with relatively long heating seasons anywhere between 10% and 15% of a building's heat loss can occur through the ceiling into the attic. Half (50%) of this heat loss is by conduction, i.e. heat flowing through the insulation and framing. Air leakage through openings and cracks account for the remaining 50% of heat loss.

There are four main factors to be considered when insulating new attics or roofs:

  • Framing structures must allow for adequate insulation plus room for ventilation above the insulation where possible.
  • Framing should allow for the full thickness of insulation to be applied over the exterior wall top plates.
  • The insulated ceilings need a properly installed air/vapour barrier under the insulation which must be sealed airtight.
  • Proper ventilation is required to remove any moisture build-up and help to reduce summer heat build-up.

Attic Applications

There are several types of loose fill insulation that can be used for the insulation of your attic. All types should only be installed by a qualified contractor.

Loose fill insulation should be blown to a uniform depth to achieve the desired R-value. At the eaves, care should be taken to keep the insulation from blocking the ventilation or from disappearing into the eave space. Pieces of batt insulation, or a wood baffle should be installed before the work begins. Building supply stores now sell cardboard or foam plastic baffles that can be stapled between the rafters. In any case, be sure that the insulation extends out far enough to cover the top of the exterior wall.

If your loose fill is deeper that the joists, build a crib around the attic hatch so that it can be filled to the edge. Fill the space between a masonry chimney and the wood from around it with non-combustible insulation which must be certified to the appropriate CSA standard.

Do not cover recessed light fixtures and be careful not to insulate too closely around flue pipes and gas vents. Insulation will tend to cause a heat build-up and create a potential fire hazard.

Wall Applications

Blown or loose fill insulations can be injected in the exterior or interior walls of existing homes by drilling small openings from the inside or outside.

The insulation is densely packed, then plugs are used to close openings for a smooth finish on drywall, wood, or siding, or colour matched mortar for brick.

For construction of new homes, a membrane is fastened to the dry exterior walls which creates a cavity to inject insulation.

Things to Consider: Ventilation

Houses with peaked roofs and accessible attic spaces are the simplest to vent. The amount of attic ventilation is directly related to the size of the ceiling area in the building. In most cases, the ratio of unobstructed, free ventilation area to ceiling area should be approximately 1 to 300. Because these ratios refer to unobstructed ventilation area, the area must be increased because the vents are covered with screening (to keep out insects, etc.) and with baffles (to keep out rain and snow).

Ideally, locate vents to allow for good cross ventilation from end to end and from top to bottom. This means placing vents at the eaves and at the peak. Any of these are adequate when used in conjunction with under-eave (soffit) vents.

Fifty percent of the ventilation area should be continuous soffit vents and the other 50 per cent gable, ridge, or roof vents. Ridge vents are preferable where practical, but they must be equipped with baffles to deflect wind blowing up the roof and to prevent the penetration of water and snow.

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