Drying out capacity and snow melting risk for ventilated wooden roofs - a parameter study

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Brice Hunt
5 years ago
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1 Drying out capacity and snow melting risk for ventilated wooden roofs - a parameter study Sivert Uvsløkk SINTEF Byggforsk 9th Symposium on Building Physics in the Nordic countries Thursday Tampere SINTEF Byggforsk 1

2 Snow melting and ice production at the cold roof eaves may lead to trouble: Leakage of trapped water Damaged tiles and gutters Serious accidents caused by falling roof ice hitting people on the ground SINTEF Byggforsk 2

3 Calculation model, snow melting Airflow through the ventilation gap system Driving forces: p v - wind pressure difference p t - stack effect pressure difference Heat balance, section 1, t air flow < 0 o C: q roof1 = q snow1 + q airflow Heat balance section 2, (snow melting section), t air flow = 0 o C : q roof2 = q snow2 + q snow melting SINTEF Byggforsk 3

have been varied: Wind speed Outdoor temperature Snow layer thickness

4 Some results for ventilated shed roofs Dimensions and design of the ventilation gap system has been kept constant The following parameters have been varied: Wind speed Outdoor temperature Snow layer thickness Roof angle Thickness of insulation Length from inlet to outlet SINTEF Byggforsk 4

5 Example of pressure drop distribution through the ventilation gap system Flow path length: 10 m, counter batten: 36 mm, batten: 23 mm, roof angle: 15, gap inlet: 36 mm, gap outlet: 36 mm, mesh with opening area: 50 %, wind coefficient difference inletoutlet: 0,5, wind speed: 1 m/s, outdoor temp.: -2 C Pressure drop, Pa. 1,0 0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0 Inlet, gap opening Inlet, mesh Inlet, felt Battens, singular loss Air gap, friction loss Battens, friction loss Pressure drop location Outlet, felt Outlet, mesh Outlet, gap opening Sum pressure drop Thermal buoyancy Wind pressure difference SINTEF Byggforsk 5

6 Maximum snow melting and ice production at outdoor temperatures close below 0 C Possible ice production, kg/m mnd Outdoor temperature, C Wind m/s 0,1 0,5 1,0 1,5 2,0 3,0 4,0 Snow: 0,5 m, roof angle: 1,4, roof length: 10 m, insulation: 300 mm, counter battens: 36 mm, battens: 23 mm, gap inlet: 36 mm, gap outlet: 36 mm, mesh with opening area: 50 % SINTEF Byggforsk 6

7 At outdoor temperatures close to 0 C the roof angle has low influence as the p from buoyancy also is close to zero Possible ice production, kg/m mnd Outdoor temperature, C Roof angle, 0,0 1,4 15,0 30,0 45,0 Snow: 0,5 m, wind: 1 m/s, roof length: 10 m, insulation: 300 mm, counter battens: 36 mm, battens: 23 mm, gap inlet: 36 mm, gap outlet: 36 mm, mesh with opening area: 50 % SINTEF Byggforsk 7

8 Wind pressure difference between the inlet and the outlet of the ventilation gap system is the main driving force for the airflow Possible ice production, kg/m mnd Wind, m/s 0,1 0,5 1,0 1,5 2,0 3, Roof angle, 4,0 Snow: 0,5 m, outdoor temp.: -2 C, ci-ce: 0,5, insulation: 300 mm, roof length: 10 m, counter battens: 36 mm, battens: 23 mm, gap inlet: 36 mm, gap outlet: 36 mm, mesh with opening area: 50 % SINTEF Byggforsk 8

9 Increasing the thickness of the roof insulation layer is a very efficient measure to prevent snow melting Possible ice production, kg/m mnd Roof angle, Insulation, mm Snow: 0,5 m, outdoor temp.: -2 C, wind: 1 m/s, ci-ce: 0,5, roof length: 10 m, counter battens: 36 mm, battens: 23 mm, gap inlet: 36 mm, gap outlet: 36 mm, mesh with opening area: 50 % SINTEF Byggforsk 9

10 The calculation results seems to be in good agreement with experience form real houses Some photos from Trondheim, Norway, all taken the 6th of January 2011 SINTEF Byggforsk 10

11 Most of the pitched roofs shown on the pictures are 24 years old with 200 mm roof insulation SINTEF Byggforsk 11

12 Snow melting and ice building on new houses are mainly caused by - roof windows and - inadequate designed roof unites for exhaust air SINTEF Byggforsk 12

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15 New, well insulated, horizontal, ventilated roofs seems to perform well SINTEF Byggforsk 15

16 Horizontal, ventilated roofs 5 years old, ca. 300 mm roof insulation no roof windows no visible ice the last two winter seasons SINTEF Byggforsk 16

17 SINTEF Byggforsk 17

18 Conclusions For small roofs, < 10 m, with 300 mm roof insulation: Low risk of snow melting as the heat left for snow melting is minimal The main driving force is wind, even at low wind speeds The roof angle is of minor influence - nearly horizontal, ventilated roofs seems to perform well Snow melting and ice building on new houses are mainly caused by: - roof windows - inadequate designed roof unites for exhaust air SINTEF Byggforsk 18

Hygrothermal Performance of Ventilated Cold Roofs an Experimental Study

Hygrothermal Performance of Ventilated Cold Roofs an Experimental Study Andreas Holm ABSTRACT Kristin Lengsfeld Attics with an insulation on the ceiling plane inherently are more prone to moisture damage