To Vent or Not to Vent, That is the Question

I received an email today from a homeowner whose architect and GC wanted justification from a SIP expert regarding the question of roof venting on a SIP structure. The roof detail that provides “back-venting” is often called a “cold-roof”. However, in the context of SIPs, it would be far more accurate to refer to the detail as a “dry-roof”. The concept of a cold-roof is more applicable when we build up North and are concerned about ice-dams. The ventilation under the sheathing helps prevent the melting of snow and refreezing when the run-off reaches the overhang.

Whenever we strive to increase energy efficiency within a structure, it should be agreed that improving airtightness is as (if not more) important than increasing Thermal Resistance (R-Value). When we increase airtightness, we simultaneously decrease the building’s ability to dry.

Think of the 100+ year old barns that are still standing  despite routinely getting wet. Without insulation, a structure is actually far more durable because it has a greater capacity to dry. The fastest way to do harm to an old timber frame barn is to pack it with insulation and fail to ensure a good means of drying. It will get wet, it will stay wet, it will rot, and it will fail!

The diagonal furring strips define the air channel and receive the vertical board siding. Notice the polypropylene black mesh at bottom to allow air to keep pests out and let air in.

All super-insulated and airtight structures need detailing that promotes a robust capacity to dry. This detail is often used behind siding and is known as a rain screen. The gap allows air to freely move in and out which also helps to dry the cladding and help dry any unintended wetting of the wall assembly’s exterior sheathing. The added benefit to this detail is it reduces thermal drive which causes premature paint failure. An additional benefit is increased sound resistance. The air gap helps to reduce the transfer of sound vibration through the assembly.

All the benefits of back-ventilating your siding are applicable to your roof. It will improve the durability of your structure by increasing the roof’s ability to dry when it gets wet. And, remember that Murphy says it will always get wet. Back-ventilating will improve the life of your cladding and it will help improve your STC performance (sound transmission class).

It is important to note that many types of wall and roof claddings are “self-ventilating”. This is where a material choice can make a big difference. If you opt to specify a slate, tile, or multi-crimp metal roof, your roof cladding is doing double duty as a finish and as a self-venting system. If you chose a standing seam or asphalt shingle roof, your decision to add venting could be one that increases the durability of the structure.

The location of your structure must also be considered!!! I tell builders that if they want to be crappy at their trade, they should go to Phoenix and build where the climate is more forgiving with stupid mistakes. If you’re in an arid climate or one that has very little rainfall, the decision to ventilate would likely be a waste of money. However, if you were in New England, skipping a ventilating detail could spell disaster.

The decision to ventilate or not should be based on

1.   climate/location

2.  cladding type

3.  the building’s type of assembly.

Your building will always be more durable if you add ventilation!

Back ventilated metal roofing. The herringbone pattern is a good detail when your roofing runs from eave to ridge.