Roof and Attic Ventilation

Roof Vent Solid State Home Inspections

Most home owners will never stick their heads in their home's attic which makes it important for home inspectors to investigate during a home inspection. There are many potential defects in attic spaces but many of the most expensive problems will happen if a roof or attic space does not have correct ventilation, commonly through roof vents.

Why Roof Vents are Needed

Roof ventilation allows the outside air to circulate into the attic or roof space and to encourage less of a temperature difference between the outside air and the air in the attic space. This may seem counterintuitive when it comes to insulating our homes however the thermal insulation for the occupied space of the home is on the 'floor' of the roof area. This means although the attic or roof area is under the same roof covering as the occupied home, it is actually an 'un-conditioned' space relative to the home.

Here are the top reasons why we are concerned with managing the temperature differential between the attic and the outside air:

  • Life of Materials - Many manufactures of roof covering materials like shingles will only provide full warranties if the roof is correctly ventilated. Poorly ventilated attic spaces can reach 60C or more under the sun and this could expose the roof material to temperature differentials of 60C on the underside and a 25C air temperature on the top side. As materials expand and contract with temperature, this uneven top and bottom temperature will create stresses on the roofing material causing premature ageing. A well ventilated attic exposes the roof materials to less thermal stresses.
  • Condensation - Warm air can hold more moisture as a vapour than cold air. For example, when warm moist air touches a glass of cold water, the air cools and cannot hold the water vapour which it deposits on the glass as condensation. Our lifestyles in our home constantly generate moisture from cooking, showering, and breathing. During colder weather, moisture that slips into a warmer attic will condensate on the roof material. This moisture can cause rot damage to structure, water drips onto interior finishes, sagging in roof materials, rusting of nails and mechanical systems, and organic growth like mildews and moulds. A well ventilated attic carries away moist air before it condenses.
  • Ice Dams - In winter of cold weather climates, warm attics can melt snow sitting on the roof causing water to flow along the roof surface to the edge of the roof where the attic stops. When the attic stops, the water re-freezes typically at the eaves before it leaves the roof. Over time, the re-freezing water forms an ice dam which starts to back water up the roof area, under shingles, and allows water to penetrate the structure causing hidden water damages and interior finish damages. A well ventilated attic keep the attic space cold enough to prevent melting of water on the roof surface.
  • Keeping the House Cool - Lowering the temperature of the attic space in warm weather will assist the insulation and any air conditioning in keeping the occupant area of the home cool. In winter, we still want a cold attic space as it reduces the risks of ice dams and condensation.

Principles of Venting a Roof

A well ventilated roof is about creating an even air circulation pattern in the attic or roof space. Air will naturally circulate in an attic as warm air will rise. Good roof ventilation will use this air behaviour by having air intakes at the bottom of the roof, commonly in the eaves as soffit vents, and roof vents high on the roof to exhaust warm air.

As there is a 'draw' effect of the warm air pulling air from below, it is important that the ceiling of the home be completely air tight with drywall and paint. If they are not well seals, the attic will 'pull' warm, moist, and possibly air conditioned air from the occupants space for attic ventilation which is an energy waste. Any gaps in the ceiling, such as attic inspection accesses, should be sealed with weatherstripping to prevent air leakage into the attic.

A good air flow in the attic will also prevent any 'hot spots' from occurring where air may not be circulating. Occupant belongings can restrict air movement in the attic as can poorly placed roof vents or blocked soffit vents.

Lastly, there is a rule of thumb for roof ventilation which is 1:300 or one square foot of vent size for every 300 square feet of attic floor area. While rules of thumb are good, home inspectors are looking for the performance of the system as well ventilated roofs can perform poorly if the vents are in bad spots and poorly ventilated roofs can perform well if the roof is 'leaky' for air movement.

Types of Roof Vents

There are many different types of roof vents available and there is some disagreement in roof experts over the best designs and use of these products.

  • Continuous Soffit Venting - Screens or grills on the underside of the soffits all along the perimeter of the home. This is considered to be the best system for lower (cool) air supply for attic venting. Home inspectors should be cautious that the vents are not blocked at the soffit or by insulation in the attic (baffles can be added to direct air past insulation in the attic and should be between every truss).
  • Soffit Vents - Many older homes either did not have soffit venting or there were only a few vents on each side. This system is not as good as continuous soffit venting but it may not be practical to install full continuous venting in an older system. 
  • Roof Vents - Roof vents are typically placed near the top of the roof. They are commonly square in shape standing only 4-5" above the roof material with a rain cap on them. Roof vents are very common. Roof vents should only be on one side of the roof peak to encourage rising air to exhaust and prevent breezes from only ventilating air from vents on one side of the roof to the other.
  • Continuous Roof Peak Vents - Continuous roof peak vents have become more popular as they place venting slots all along the roof ridges under the roof material caps. As even circulation of air is a goal of roof ventilation, continuous soffit and roof peak vents allow even air flow through the whole attic.
  • Gable Vents - In some homes, vents are in the side walls near the peaks of the roof. These vents work well if there is a breeze horizontally through the attic which will carry warm attic air away. In still conditions, warm air near the edges may exhaust however there could be 'hot spots' in the middle of the roof area further from the gable vents.
  • Turbine Vents - These are also sometimes called whirlybirds as they spin in the wind and with rising air. Turbine vents are not well liked by many experts as they offer the same ventilation value as a standard roof vent but they have both moving parts that can fail, and they can create a negative pressure in the attic which may draw air from the home through leaks into the attic wasting energy.
  • Power Attic Vents - There are large electric motored air-handler power vents which are very effective at blowing hot air out of the attic. These, like turbine vents, are not well liked by many experts as they create a negative pressure in the attic and can suck household air into the attic wasting energy. These power vents are often controlled by a thermostat for summer heat and may not help with effective winter air circulation.

Vaulted Ceilings

Vaulted ceilings provide a challenge for air circulation and inspection of the attic space. With vaulted ceilings, there is very little gap between the drywall, insulation, and roof sheathing for an air circulation path. Worse yet, as there is no attic space, there is are no attic accesses for inspection meaning home inspectors do not have any way to determine the performance of the roof venting system.

There are methods to provide some ventilation to these spaces and builders should be providing some room for roof ventilation however once these systems are closed up, only signs of failure are visible in home inspections

No Storage in the Attic

Attics should not be used for storage of things for three primary reasons:

  • Fire Risk - The attic does not have any fire resistant materials like drywall to slow a fire. Once a fire starts in an attic space, the entire structure of the home will be on fire very quickly risking occupants lives.
  • Weight - Most buildings since the 1970's have been built with engineered trusses to hold the roof weights. These trusses are designed for a load on the outside of the span and not on the supports inside the span. When the next 200 year snow fall hits your home, the added weight of the snow and belongings in the attic may overwhelm the engineers design for the roof supports.
  • Air Circulation - Boxes and stored items will impede the air circulation that is needed to correctly ventilate the roof area shortening the life span of roof coverings and possibly contributing to condensation, rot, and mould growth.

Final Thoughts

The roof our wooden homes is a key part of the structure and waterproof 'envelope' of the home. Protecting this structure requires good ventilation of the attic and roof space saving the home owner potentially thousands of dollars in premature repairs. Combined with the potential health risks of mould and mildew growth, home owners or buyers should always call a professional home inspector to review their roof performance for the safety and security of their family.

By James Bell - Owner/Operator of Solid State Inspections Inc.