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CFD modelling and thermal performance analysis of a wooden ventilated roof structure

  • Research Article/Architecture and Human Behavior
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Abstract

Thermal comfort and energy saving are objectives of key significance that building design must meet. Since a low energy building can be obtained as a result of the good realization of all its components, roofs call for particular attention as they represent a large part of a building’s total surface area. In this paper the benefit of using ventilated roofs for reducing summer cooling load is investigated. The investigation has been conducted comparing a ventilated roof assembly with different channel heights (3 cm, 5 cm, and 10 cm) to the same non ventilated structure, assuming buoyancy-driven airflow. Direct comparison between the open and the closed roof structures as a function of different cavity heights and outside environmental conditions is presented. To provide fundamental information about the thermal performance of these building envelope components, the computational fluid dynamics (CFD) model has been used to develop correlations for the characterization of the airflow and heat transfer phenomena in the ventilation cavity which have been implemented in a whole year energy simulation software. The present analysis shows a conflicting discrepancy among the indexes of performance describing the actual energy saving potential of a ventilated roof.

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Abbreviations

a :

solar absorptance

c p :

specific heat (J/(kg·K))

Gr :

Grashof number

h :

overall heat transfer coefficient (W/(m2·K))

I :

solar radiation (W/m2)

k :

thermal conductivity (W/(m·K))

ṁ :

mass flow rate (kg/s)

Nu :

Nusselt number

Ra :

Rayleigh number

Re :

Renolds number

p :

pressure (Pa)

Q :

heat flux crossing the ventilated roof (W/m2)

Q 0 :

resulting heat flux when the ventilation channel is kept closed (W/m2)

Q 1 :

resulting heat flux for the case with thickened insulation (W/m2)

Q air :

air heat gain (W)

S :

dimensionless performance index

T :

temperature (K)

u i,j,k :

components of the velocity according to i, j, k (m/s)

V :

volumetric flow rate (m3/h)

cav:

cavity air bulk conditions

conv:

convective

ext:

external conditions

in:

inlet conditions

L:

cavity length

out:

outlet conditions

s:

cavity spacing

sa:

sol-air

w:

wall

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Authors and Affiliations

  1. Dipartimento di Fisica Tecnica (dFT), Università di Padova, Via Venezia 1, 35131, Padova, Italy

    Giacomo Villi, Wilmer Pasut & Michele De Carli

Authors
  1. Giacomo Villi
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  2. Wilmer Pasut
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  3. Michele De Carli
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Correspondence to Giacomo Villi.

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Villi, G., Pasut, W. & Carli, M.D. CFD modelling and thermal performance analysis of a wooden ventilated roof structure. Build. Simul. 2, 215–228 (2009). https://doi.org/10.1007/s12273-009-9414-7

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  • DOI: https://doi.org/10.1007/s12273-009-9414-7

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