[GBlist] tar-paper for roofing

[Richard Kadulski <kadulski@direct.ca>]

The reason for the roofing felt ("tar paper") under metal roofing is to
provide a secondary weather barrier.  However, and more importantly, it
is important to remember that there will always be some water under the
metal, and not necessarily because of leakage, but because of
condensation on the underside.  

Roofing sees very wide temperature fluctuations between day and night,
especially on clear nights (even in summer).  With nightime cooling,
there will be condensation of moisture out of the ambient air. Warm
moist air contains a lot of moisture, so there is a lot of potential for
condensation.  Since metal roofing is not airtight, there will always be
a source of moisture due to natural air circulation on both sides of the
roofing. 

-- 
Richard Kadulski Architect
204 - 1037 West Broadway
Vancouver, BC V6H 1E3

Tel/fax 604-689-1841

Climate adapted, energy sensitive, sustainable and healthy design
Editor Solplan Review: the independent journal of energy conservation,
building science & 
construction practice


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