A Computation of U-Factor for an Entire Vented Attic Assembly using a 2D Model. (November 2015)

• Record Type: Journal Article
• Authors: Iffa, Emishaw; Tariku, Fitsum
• Is Part Of: Energy procedia. Volume 78(2015)
• Page Start: 3198
• Page End: 3203
• Publication Date: 2015-11
• Subjects: Heat flux -- Insulation -- R value -- Taper angle -- U value -- Ventilation.; Power resources -- Congresses; Power resources -- Periodicals; Power resources; Conference proceedings; Periodicals; 333.7905
• DOI: 10.1016/j.egypro.2015.11.780
• Languages: English
• ISSNs: 1876-6102

Abstract

Abstract: 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.