Figure 4 - uploaded by Ahmad Kayello
Content may be subject to copyright.
Average Monthly Attic Air Changes Per Hour for Naturally Ventilated Cases

Average Monthly Attic Air Changes Per Hour for Naturally Ventilated Cases

Source publication
Conference Paper
Full-text available
Housing in northern Canada faces unique climatic, environmental, and social challenges. Over the past half-century, housing shortages and failures due to unsuitable design have resulted in homelessness and overcrowding. For new houses to address these issues, they must be durable, especially the building envelope. One of the main concerns for the b...

Contexts in source publication

Context 1
... the case of RH controlled ventilation, the attic ventilated at a rate of 3 ACH when the difference in humidity ratio between the attic and the outdoors is greater than 0.5 g/m 3 . For both naturally ventilated attics, the ventilation rate is wind dependent as described earlier, and the average monthly ACH values for those attics are shown in Figure 4. ...
Context 2
... mechanically ventilated case performs about as well as the naturally ventilated cases. This makes sense since the average natural ventilation rate, shown in Figure 4, is close to 3 ACH, which is the ventilation rate of the mechanically ventilated attic. BIPV/T mechanical ventilation provides the lowest RH levels of all cases by far. Figure 6 shows the RH levels in the attics with a low amount of air leakage. ...


... Further research is needed to develop solutions to meet the need for snow protection and ventilation in extremely cold regions. Strategies that allow drying of the sealed attic through diffusion to outdoor such as diffusion vent in unvented roof [30], which showed some success in hot and humid climates, or to indoors using smart vapour retarder [31], or adaptive ventilation to allow ventilation during spring and summer time while sealed during winter [32,33] could be considered. ...
Attic ventilation is typically recommended for the removal of moisture build-up caused by air leakage from indoors in cold climates, however, it may also increase the amount of snow and rain penetration into the attic, especially in the extremely cold climates. In northern regions, extremely cold temperatures can cause snow particles to become very fine, which will penetrate vents or unsealed openings. The snow accumulated in the attic would melt at temperatures above zero and penetrate to indoors through the ceiling and cause moisture problems. One of the solutions is to add filter membranes along a ventilation cavity behind the façade to prevent snow from entering the attic. The ventilated attics with filter membrane had some success but there were instances with reported water leakages and moisture damages. There have been also attempts to use un-ventilated cold roofs. Un-ventilated attics prevent snow accumulation but do not allow for effective removal of moisture, which could be risky and prone to moisture damages. This paper presents a field study of the hygrothermal performance of three attic venting systems. Three houses with different attic designs built in Canadian North were monitored: two with a ventilated attic but different strategies controlling snow entry and one with an un-ventilated attic. Measurements show that the ventilated attics had acceptable conditions. The moisture content of wood structure at most monitored locations in the un-ventilated attic remained above 20% through the summer, which indicates that without ventilation the construction moisture and moisture accumulated through winter cannot be effectively removed.
Attic ventilation is a commonly used method for the removal of moisture build-up in attics through air leakage from indoors, which is generally applied to cold climates. Un-ventilated attic is a fully sealed construction, which can prevent wind-driven rain penetration or snow accumulation. However, it does not provide an effective path for moisture removal, which may cause moisture-related issues. Northern Canada suffers from extremely cold conditions and very fine snow particles caused by extremely low temperature have high chances of penetrating into attics through vents or un-intentional openings during periods of high winds. To provide adequate attic design recommendations for this climate, hygrothermal models of ventilated and un-ventilated attics were firstly validated by comparing with field measurements and then used for a parametric study. The attic ventilation rate, ceiling air leakage rate and un-intentional air infiltration rate are set as variables for the parametric study to evaluate their effect on the hygrothermal performance of attics. Mold growth index (MGI) on sheathing is used as the performance indicator. For the ventilated attic, there is no mold growth risk, while for the un-ventilated attic, the MGI increases with the increase of ceiling air leakage rates and the decrease of un-intentional air infiltration rates. North-facing roof sheathing has higher MGI compared to south-facing sheathing. The parametric study results show that attic ventilation is required for the extremely cold climate and an un-ventilated attic has greater risks of moisture problems unless the ceiling air leakage from indoors is well controlled.
The reliable operation of Heat Recovery Ventilator (HRV) is critical for maintaining a healthy indoor environment to remove contaminants and moisture, however, it remains a challenge in the Northern Canada due to the frequent frosting under the extreme cold conditions. The heat generated by a building-integrated photovoltaic/thermal (BIPV/T) system can be used to pre-heat the incoming fresh air in HRV in order to reduce its defrost cycle, therefore, improving the reliability of HRV to provide adequate ventilation required. In this case, the BIPV/T needs to be designed for higher air temperature rise, which may not be optimum for the thermal energy and PV power generation. Therefore, system integration and optimization for coupling BIVP/T with HRVs is required. Depending on the level of thermal energy available and the outlet air temperature from the BIPV/T system, a control strategy needs to be developed to optimize the operation of HRVs. This paper presents the analysis of four different BIPV/T configurations and their integration with HRVs for a 120 m² house located in Iqaluit, NU, Canada through modelling. Results show that the outlet air of a BIPV/T façade installation can be 14.8 °C higher than outdoor air on a clear sky winter day and that the defrost cycle can be reduced by 13%, up to 619 h annually.