ASSESSMENT OF SNOW EFFECTS ON HEATING LOADS FOR BUILDINGS WITH GREEN ROOFS

Open Access
Author:
Zhao, Mingjie
Graduate Program:
Architectural Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
July 07, 2011
Committee Members:
  • Dr Jelena Srebric, Thesis Advisor
  • Jelena Srebric, Thesis Advisor
  • Robert Berghage Jr., Thesis Advisor
Keywords:
  • HEATING LOADS
  • GREEN ROOFS
  • HVAC SYSTEMS
  • SNOW EFFECTS
Abstract:
A green roof is a specialized roofing system that supports vegetation growth on rooftops. Due to benefits to the environment and energy savings, green roof technology is rapidly gaining popularity in North America as a sustainable design option for buildings. Many previous studies have demonstrated that the addition of green roofs could save energy for space cooling during the summer. However, there are limited studies focused on the performance of green roofs during the heating season, especially under snow. In order to contribute to green roof buildings in regions with cold winter and snow, this current thesis presents an on-site experimental investigation and focuses on the assessment of snow effects on heating loads calculation for green roof buildings during the winter. This field experiment took place during the winter of 2010-2011, monitored three green roof buildings, two roof reference buildings and one bare soil roof building. The six buildings were constructed identically and instrumented with a network of sensors. To describe the indoor and outdoor environment of the buildings, a weather station collected meteorological data; the sensor network was able to collect the data related to the building parameters. Manual measurements were conducted during the snow period to obtain snow properties. The manual data of three types of buildings were compared to each other. In addition, a heat balance model for roofs with snow layer was set based on the collected data. In order to calculate the heating loads and roof resistance better, a list of equations for snow conductivity calculation was given. By regarding the measured heat flux through the roofing system as the main indicator to show the roofs performance, an analysis was made to compare the different types of roof. The comparison was made for one continuous week-time when there was snow on the roofs, and for another continuous week-time during the no-snow period. The measured heat flux data were analyzed statically by setting three null hypotheses; the results showed all the calculated p-values were less than 0.05 and the hypotheses were rejected, indicating that the roof type was a significant factor affecting the heat loss through the roof from the building interior. The comparison of mean heat flux showed the green roof building had the least heat loss among the three types of buildings, regardless of the accumulated snow layer. In addition, the results indicated that the green roof buildings had larger heat loss reduction when there was no snow on the roofs. This may be because the existence of the snow layer affected the R-value of the entire roofing system and the heat transfer process through the roofs, thus reducing the thermal performance of the green roof. Finally, the relationship between snow properties and heat loss was presented for each type of buildings. The changes of daily snow properties, including snow depth, snow water equivalent (SWE) and snow density were presented with the change of heat loss during the snow period. The results indicated that the green roof buildings had different relationships compared with other buildings.