A Computation of U-Factor for an Entire Vented Attic Assembly using a 2D Model

The overall U-factor values for an attic assembly are usually computed with the ANSI/ASHRAE/IES based R-value (thermal resistance) conversion. In the ANSI/ASHRAE/IES Standard 90.1 (2010), the effects of attic air resistance, roof pitch and attic width are not taken into account while calculating the...

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Bibliographic Details
Published in:Energy procedia 2015-11, Vol.78, p.3198-3203
Main Authors: Iffa, Emishaw, Tariku, Fitsum
Format: Article
Language:English
Subjects:
Heat flux
Insulation
R value
Taper angle
U value
Ventilation
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Summary:The overall U-factor values for an attic assembly are usually computed with the ANSI/ASHRAE/IES based R-value (thermal resistance) conversion. In the ANSI/ASHRAE/IES Standard 90.1 (2010), the effects of attic air resistance, roof pitch and attic width are not taken into account while calculating the U-Factor values. In addition, the R-value is estimated using a one dimensional thermal resistance model. In ventilated attics, where the insulation near the roof sheathing is tapered, it is difficult to find the correct R-value of the attic system as the heat transfer becomes two dimensional. In this paper, a 2-dimensional CFD model is developed for various insulation R-values and insulation taper angles near roof decks. COMSOL Multiphysics 4.4 is used to model and analyse the attic structure. Results show that a discrepancy in overall U-factor for entire attic assembly between the developed model and the existing standard estimation. These results are pronounced for lower slope roofs with high insulation thickness.
ISSN:1876-6102
1876-6102
DOI:10.1016/j.egypro.2015.11.780