The type of roof deck selected by the engineer and or existing roof deck is a consideration as well.
Vapor retarder over concrete roof deck.
A vapor retarder is a material or system that is designed as part of the roof system to substantially reduce the movement of water vapor into the roof system where it can condense.
A smart vapor retarder is a vapor retarder material with the unique ability to react to changes in relative humidity by altering its physical structure.
If a roof system is installed over a concrete deck without a vapor barrier the moisture in the concrete will likely.
The midwest roofing contractors association mrca announced it is necessary to include a vapor retarder of less than a 0 01 perm rating class i over new concrete roof decks.
This method is shown in the image above.
Also in reroofing situations where the existing roof deck is known to be lightweight structural concrete or there is evidence of concrete deck related moisture problems nrca recommended a high bond strength vapor retarder be adhered directly to the deck followed by the installation of an adhered or loosely laid ballasted roof system.
Exceptions to this idea include roofs over indoor swimming pools or other high humidity producing activities and over new concrete roof decks to prevent the moisture in the concrete from drying into the roof system.
If wind uplift analysis is severe a vapor retarder will assist in wind uplift resistance.
In a low slope roof assembly the vapor retarder is normally installed under the primary roof insulation.
When we use a vapor retarder in a roof system it will also act as an air barrier as long as it is sealed at all perimeters and.
If the building is considered an essential facility such as a hospital federal building data center or bank to name a few consider a vapor retarder.
Vapor retarders on the top surface of the concrete deck in its may 2017 technology research update the midwest roofing contractors association mrca advises a vapor retarder of less than 0 01 perm is necessary over new concrete roof decks.
Typically a vapor retarder has to have a perm rating of 1 0 or less to be successful.
Roofs over concrete decks should always have a vapor barrier installed.
From a practical standpoint a vapor retarder of less than 0 01 perm is effectively a vapor barrier almost no moisture passes through to the roof system above.
A smart vapor retarder is able to adapt its permeability to allow the wall to dry should moisture get trapped in the wall cavity.
This class i rating essentially acts as a vapor barrier which prevents moisture from permeating the roofing system.
The midwest roofing contractors association mrca is advising a vapor retarder of less than 0 01 perm is necessary over new concrete roof decks.
Therefore one will often see it installed directly on a roof deck such as a concrete or wood deck or on a continuous substrate such as gypsum board or wood panels that is installed directly over a metal deck.
Everyone knows that water in roof systems is never a positive.
