Model of thermal buoyancy in cavity-ventilated roof constructions
Artikel i vetenskaplig tidskrift, 2021

Timber roof constructions are commonly ventilated through an air cavity beneath the roof sheathing in order to remove heat and moisture from the construction. The driving forces for this ventilation are wind pressure and thermal buoyancy. The wind driven ventilation has been studied extensively, while models for predicting buoyant flow are less developed. In the present study, a novel analytical model is presented to predict the air flow caused by thermal buoyancy in a ventilated roof construction. The model provides means to calculate the cavity Rayleigh number for the roof construction, which is then correlated with the air flow rate. The model predictions are compared to the results of an experimental and a numerical study examining the effect of different cavity designs and inclinations on the air flow rate in a ventilated roof subjected to varying heat loads. Over 80 different test set-ups, the analytical model was found to replicate both experimental and numerical results within an acceptable margin. The effect of an increased total roof height, air cavity height and solar heat load for a given construction is an increased air flow rate through the air cavity. On average, the analytical model predicts a 3% higher air flow rate than found in the numerical study, and a 20% lower air flow rate than found in the experimental study, for comparable test set-ups. The model provided can be used to predict the air flow rate in cavities of varying design, and to quantify the impact of suggested roof design changes. The result can be used as a basis for estimating the moisture safety of a roof construction.

analytical model

cavity ventilation

roof construction

Air flow

building envelope

Författare

Toivo Säwén

Chalmers, Arkitektur och samhällsbyggnadsteknik, Byggnadsteknologi

Martina Stockhaus

Student vid Chalmers

Carl-Eric Hagentoft

Chalmers, Arkitektur och samhällsbyggnadsteknik, Byggnadsteknologi

Nora Schjøth Bunkholt

Sintef Foundation for Scientific and Industrial Research At the Norwegian Institute of Technology

Paula Wahlgren

Chalmers, Arkitektur och samhällsbyggnadsteknik, Byggnadsteknologi

Journal of Building Physics

1744-2591 (ISSN)

Vol. In Press

Ämneskategorier

Annan samhällsbyggnadsteknik

Strömningsmekanik och akustik

Husbyggnad

DOI

10.1177/1744259120984189

Mer information

Senast uppdaterat

2021-03-18