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Vented Townhouse Attics: A Basic Guide

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AtticAttics: the last frontier. This unexplored area of the house tends to go about unnoticed by most homeowners. Generally hidden, attics can largely impact the roof and living space below, with hidden problems causing discomfort and costing you money. 

Typical Construction and Function of Vented Attics

The attic is the space between the roof and the ceiling of the upper floor of the house. Based on Southern Ontario’s climate (warm to hot, humid summers and cold winters), most townhouses have vented attics, which bring outside air into the attic near the eavestroughs and then exits the roof near the peak. During cold seasons, attic ventilation helps vent moisture that enters the attic from the interior space, which reduces moisture problems such as condensation, ice forming on the attics' sheathing, and ice damming from occurring. During warm seasons, ventilation reduces the hot air in the attic which decreases the cooling load required inside. In attics, insulation is installed over the ceiling to minimize the temperature fluctuation between the exterior environment in the attic and the living space below. 

Factors Influencing Vented Attic Performance
Although vented attics are common, some details tend to be overlooked in their construction, leading to decreased performance. The three main factors of performance are air-leakage, ventilation, and insulation. 

1. Air-Leakage: No matter how well the house is insulated, if all the windows are open, the heated or air-conditioned air in your home will leave. Similarly, the ceiling of the upper floor has several penetrations going up into the attic. These penetrations are generally found near the following locations:

Attic 3

– Around the attic access hatch
– Around framing
– 
Around mechanical units such as bathroom fans
– Around plumbing equipment such as a plumbing stack
– A
round electrical equipment such as wires and electrical boxes.

If unsealed, the heated or air-conditioned air inside the home escapes through these openings into the attic or the exterior air in the attic enters the house. In an ideal situation, the interior air should not mix with the exterior air in the attic; however, vented attics are designed to address minor air leakage between the two spaces. Sources of air-leakage should generally be identified and sealed to improve the performance of the HVAC equipment, shingle life, and occupant comfort. To identify locations of air leakage, a thermal camera can be used to identify warm and cold spots. Air leakage can also be identified through black stains on attic insulation, caused by moisture staining or filtration of debris as air passes through.

2. Ventilation: The ventilation area required in a vented attic is specified in the Ontario Building Code to be no less than 1/300 of the area of the space. This ventilation can be achieved using mechanical systems and/or the stack effect. Typically, air enters through openings in the soffit near the eavestrough and exits through openings near the peak of the roof. To maintain the desired ventilation these openings and the path between them need to remain unobstructed by wood blocking, insulation, etc. 

3. Insulation:  Insulation in the attic provides thermal resistance between the interior space and the exterior environment. For attics, batt or blown insulation is typically used as it is generally cheaper, easier to install, and can fully cover a complex horizontal space. The specified attic insulation for new homes in the Ontario Building Code was updated in 2019, which increased the required R-Value from R-50 to R-60. Depending on the insulation type used, different thicknesses may be needed to meet this requirement. The chart below is a general summary of the thickness required to achieve R-60 in the attic. Distribution of the insulation is also important – thin spots in the insulation coverage can result in cold spots in your home. 

 

   

Thickness Required to Achieve R-60

Insulation

Approximate R-Value per Inch

Inch

mm

Blown in Fibreglass

3.0 to 3.7

16.2 to 20

412 to 508

Blown in Cellulose

3.0 to 3.7

16.2 to 20

412 to 508

Blown in Mineral Fibre

2.8 to 3.7

16.2 to 21.4

412 to 544

Fibreglass Batt

3.0 to 3.7

16.2 to 20

412 to 508

 
How Does Your Home Compare?
Based on this data, how does your condominium hold up? Taking a good look at your air seal, ventilation, and insulation (or having a professional do it for you) can not only make your home more comfortable and durable, but it can also save you money on your heating and air conditioning bills. 

 

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Robert Dziurdzik, B.A.Sc., EIT, BSS

Project Coordinator, Pretium Engineering Inc.

www.pretiumengineering.com

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