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Benefits Of Breathable Underlayments For Metal Roofing

By David Nemes, President, Nemco Industries Inc.

Cross-section views of of BREA Underlayment (above) and Dry-Tech Underlayment (below) from Nemco Industries. The bottom cross-section image shows Dry-Tech used beneath metal roofing. (click images to enlarge)


Comparing European and North American roof construction, it seems rather logical that both have something to offer the other party. Europeans have built churches, official and residential buildings using steel, copper, lead and other similar materials for centuries, yet similar products are lifting their heads in North America only recently. Moisture and condensation has been dealt with by creating technically compatible buildings where air circulation has been the main feature. Designing appropriate air circulation and correctly applied roofing and wall materials, have given the total building a healthy environment where condensation and harmful bacteria are part of history.

The Europeans are now also using increasing quantities of “breather” membranes that are used within pitched roofs and are referred to as vapor permeable underlayments. “Breathability” is therefore the measurement of the amount of water vapor, or perspiration moisture, transported through the barrier layer, into the atmosphere. “Vapor permeable underlayments relate not to the passage of air, but rather to the passage of water vapor through the membrane.”

Building regulations have required increased amounts of insulation in the attic space during the last 30 years and the combination of heating and double glazed windows has increased air tightness with a consequent increase in the risk of condensation becoming one of the major problems within cold roofs.

Ongoing research and progress has resulted in the development of a range of construction membranes for use in a wide variety of wall and pitched roof systems to prevent condensation — most recently witnessed in Vancouver, B.C., Canada, where millions of dollars was been spent on roofing and wall membrane control and repair. Adequate breathability is the key to all mold and rot issues. A little moisture and no air circulation will cause insurmountable damage to buildings.

As the water vapor in a building passes through a conventional ceiling and high permeability insulation, it reaches a cold roof space. The cold reduces the air’s ability to hold vapor and consequently increases the risk of condensation forming on rafters and roof timbers, which can cause mould growth and rot. In hot and humid climates, the above applies in reverse.

Traditional systems allow water vapor to be trapped, within the roof space by roofing underlayments having low permeability such as bitumen, or synthetic roofing underlayments, making it necessary to design a means to allow the vapor to escape from cold pitched roofs. This means of escape can only be increased by ventilation at the eaves with the use of ridge ventilation.

In Europe, roofing underlayments and breathable membranes have become an important component of modern pitched roof construction. Their correct specification, detailing and installation helps to minimize the risk of water ingress into the completed building, giving the construction team greater confidence that new roofs will be reliable and robust.

There are several different test methods, which can be used to measure breathability. However, there is no global consensus that any one method can measure the conditions encountered during different end uses. It is also clear that a Universal correlation cannot be obtained using different methods to test the same product; therefore, it is apparent that a specific test is needed for a specific end use condition.

Nemco Industries Inc. has introduced, into the North American Markets, a breathable mechanically fastened roofing underlayment with the following features:

• Very strong and robust vapor permeable underlay. (146 perms)
• Waterproof, four ply material.
• Approved by: Miami Dade 09-0915.10 NOA and ICC-ESR 3017
• Suitable for Non Ventilated cold and warm roof applications.
• UV Stable for up to four months.
• Hydroponically treated.
• Self-adhesive tape on overlaps preventing water ingress.
• Produced in rolls of 808 square feet
• Must always be installed in accordance with local installation guidelines.

Great popularity has been gained by added innovation in roofing membranes. As a result, the addition of a single layer breathable membrane, a newcomer to the roofing industry, has been added, that of 3mm high polypropylene webbing attached to a breathable membrane, called Dry-Tech. This method of combining two media into one makes roofing battens unnecessary, saving costs of labor, as it incorporates breathability and the creation of an air space into one and the same installation.

Without space between deck and metal sheathing, corrosion, mold and mildew can occur as no air circulates freely. In cold regions condensation is of major concern and in hot climates the same issue in reverse. Control of moisture has also prompted metal roofing manufacturers to consider voiding warranties should no air space be incorporated into the structure.

For more information, call 1 (888) 840-3435, or visit

About Nordic Waterproofing USA and Canada

RoofAquaGuard_logoNordic Waterproofing supplies underlayment for use with metal roofs. For more information, visit

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