Masonry Magazine February 1987 Page. 62
Letters
More on Efflorescence
Editor:
The recommendations in Michael Merrigan's article on efflorescence (Masonry, September/October, 1986) are, in my opinion, cause for concern. He suggests that sandblasting should be used with caution. Given the number of problems that he identifies with sandblasting, I am surprised that his recommendation is not simply "do not sandblast."
Secondly, he recommends that after sandblasting or acid cleaning the masonry should be sealed. This may be appropriate advice for a southern climate, a modern building or for a building in which the cause of efflorescence has been corrected, but our experience with sealants is that they may not be appropriate for use on all buildings.
As an example, I am sending to you the advice of Martin Weaver of Heritage Canada, which was recently printed in their magazine Canadian Heritage.
William N. Greer, Architect
Historical Preservation Section
Toronto Historical Board
Toronto, Ontario
The following is the material to which Mr. Greer refers.-Ed.
Q-The Canadian Opera Company's Opera Centre on Front Street, built in 1887, has just been chemically cleaned and repointed. Constructed primarily of John Price brick, with sandstone for the rusticated bases, sills and lintels, the building experiences interior relative humidity levels of 20 to 40 per cent at some times of year, but has no vapor barrier or insulation. The contractor suggests we should apply a silicone sealant to prevent deterioration of the freshly cleaned masonry. We have reservations. What would you advise? Stephen Wells. Arcop Associates, Toronto, Ontario.
A-We have reservations, too. According to current internationally accepted conservation practice, in the conditions existing at the Opera Centre it is not recommended to apply any transparent, so-called water-proofing or moisture repellent coating to the exterior surfaces of sandstone or brick. They aren't really needed, in the first place.
With no vapor barrier inside the building and potential relative humidity levels as high as 40 per cent, the application of exterior waterproofing would pose a totally unacceptable risk of condensation forming within the wall and being trapped behind the coating. No matter how many claims are made for the ability of such substances to "breathe," continuous performance cannot be guaranteed. This could produce surface exfoliation and other deterioration of the masonry you've just paid to have cleaned.
Of course, the application of transparent coatings is frequently-and correctly-specified for new or relatively-new masonry with its associated rain screen construction, often in
supaflu
Join the leaders in the "Cast-in-place" Chimney Lining Business!
-Dealership Packages tailored to your business
-Very high profit potential
-Financing available
-New 20 Year Guarantee.
For more information contact: Wayne M. Hubbard
National Supaflu Systems
P.O. Box 89 Walton, NY 13856
(607) 865-7636
62 MASONRY-JANUARY/FEBRUARY, 1987