Home Project FYI.

Help and Ideas For Your Home Remodeling Project

Home Project FYI.  - Help and Ideas For Your Home Remodeling Project

Attic Ventilation Systems

Ventilation For Attics and Roof Structures

Well-ventilated roof structures use air movement to exhaust heat from the attic or roof structure to the outside. Poor roof ventilation can shorten the long-term service life of certain types of roofing materials, especially those which contain asphalt, such as black felt underlayment and asphalt shingles. Ventilation also influences moisture levels in the attic and comfort levels in the home.

Roofing materials absorb sunlight as heat. This heat is then released both upward into the open air and downward into the roof structure.

Cools the Roof


Keeping the attic space or rafter bays cooler helps keep roofing and underlayment cooler and extends their long-term service life. Loss of volatiles, which help keep shingles flexible and waterproof, is accelerated by poor ventilation. Loss of volatiles by poor ventilation contributes to the distortion you see in the organic shingles in the photo above.

Cools the Living Space

Evacuating heat before it reaches the living space helps keep the home cooler and more comfortable, and reduces cooling costs.

Proper Ventilation Removes Moisture

The third benefit of roof ventilation is that it can remove excessive moisture vapor. Excessive moisture vapor can cause problems such as mold, decay, corrosion, and roof sheathing expansion.

The roof sheathing in this new home expanded after the shingles were installed, resulting in buckling of the shingles around the panel edges. Oriented strand board, commonly known as OSB, is typically used for roof sheathing, and leaves the mill with a 3% or 4% moisture content, so it’s very dry.

Installing sheathing and roofing it over too soon in a climate with high humidity can cause this problem. Roof sheathing needs time to adjust to local humidity levels. Another way to say this is that roof sheathing needs to reach “equilibrium moisture content” (EMC) with the home site’s environment before roofing materials are installed.

Roof structure ventilation systems can be divided into two basic types:  active ventilation  and passive ventilation.


Thermostatically Controlled Fan

An active ventilation system requires some kind of power to operate.

Two types of powered ventilators are common: attic fans and whole-house fans.

Attic fans are usually automatically controlled by a thermostat.

These ventilation systems are often installed in a gabled end, as you see here. That’s the adjustable thermostat circled in yellow.

You may also see these ventilation systems installed between rafters, as you see here. The white conductor should be stapled to the framing member nearest to the fan.

This is what a roof-mounted ventilation fan looks like from the roof.

Whole-House Fan Ventilation Systems

Attic ventilation system

Whole-house fans are large fans, usually 24 to 30 inches, installed in the ceiling of the top-most floor in a central part of the home. The hallway ceiling is a common location. The fan may be controlled by a thermostat, a timer, or a manual switch.

Whole-house fans pull hot air from the living space and exhaust it to the outside through the attic space. They’re typically used with a downstairs window open or with a window-mounted cooling appliance, such as an evaporative cooler, to create a flow of cool air through the home.


If they’re used improperly, whole-house fans can cause dangerous backdrafting.

“Backdrafting” is a term used to describe the conditions that result when air is pulled into the home through an exhaust flue.

Instead of flowing up into the exhaust flue and to the outside, exhaust gasses are pushed into the living space by air being pulled down the flue.

This can happen when no fresh air is supplied to replace air pulled out of the living space by the whole-house fan. If no window is left open, replacement air will be pulled into the home through the path of least resistance. If the power of the whole-house fan exceeds the forces propelling the byproducts of combustion to the outside, backdrafting will occur.

Backdrafting is more likely to occur in homes with atmospheric furnaces, which are furnaces that have no exhaust fans.

It’s also more likely to happen in tightly built homes that have no system for providing makeup air. An example would be a heat-recovery ventilator, also known as an HRV.

It’s a good idea to know the potential dangers associated with whole-house fans, and providing ventilation through a source of outdoor air, such as an open window or an evaporative cooler that is installed in an open window.

An air conditioner will not provide outdoor air since all it does is cool the air that re-circulates through the living space.


Finding Air Leaks in Your Home

Air Leaks: How an Infrared Camera Can Help You Find Themair leaks

Infrared (IR) cameras are quickly becoming an indispensable tool for home inspectors.  Reading the thermal images produced by an infrared camera during an inspection allows for quick and accurate identification of defects and air leaks that may not always be immediately apparent to the naked eye.

Infrared camera imaging is especially useful when looking for air leaks, including insulation defects, during an energy audit because it allows the inspector to actually view the apparent temperatures in a given area.

By purposely controlling the temperature and air pressure in the interior of a house, air can be forced inside through cracks and holes.  Using an infrared camera, the sources of these air leaks can be quickly located and visually documented.  Areas of insufficient insulation also become more apparent when viewed through an infrared camera and can be visually documented, as well.

How Infrared Cameras Work In Finding Air Leaks in Your Home

While there are other infrared tools available, such as spot radiometer and thermal line scanners, a thermal imaging camera is the most accurate device to use for energy-audit inspections and to help locate air leaks.  The infrared camera reads infrared radiation in order to express heat differences and temperature signatures.

The infrared camera sees light that is within the heat spectrum that exists just beyond the spectrum that can be seen with the naked eye.  Differing heat signatures are displayed in the camera’s viewfinder as a gradient color scheme, with hotter areas displayed as brighter colors, and cooler areas as darker colors.


An inspector can view this information on the infrared camera in order to make observations and find defects and air leaks.  By viewing the hottest and coldest areas, inspectors can collect valuable data about the building envelope.  Images taken with the infrared camera can be included alongside digital photos of the same problem area in the inspection report.

Before Looking For Air Leaks

Equipment should always be checked before the inspection.  Make sure batteries are charged on the infrared camera (as well as the laptop, digital camera, and any other equipment that will be used).  Find places to charge equipment on site, if possible, in order to avoid any delays associated with dead batteries.  If the inspection is likely to take a while, such as for a very large property, it may be a good idea to have extra batteries charging while performing the first part of the inspection.  This way, batteries can be switched out if they die while gathering data.

Any digital storage media should also be checked for adequate memory so as not to run out of room while saving images, which are important as the visual documentation of the problem areas.  Digital images of problem areas should be saved next to thermal images for comparison, so it is important to have plenty of memory in cameras and on data cards for both types of cameras.

Setting Up for an IR Thermal Imaging Inspection

Because the infrared camera shows differences in heat signatures for a given area, it is important to set up the testing conditions such that the temperature difference between the interior of the house and the air outside is as large as possible.  The peaks of the heating and cooling seasons for any region are generally optimal times to gather thermal imaging data, since heat or air conditioning can be run in order to maximize the temperature difference.

All windows and exterior doors should be closed during testing.  It is also helpful to move furniture away from walls so that they don’t block baseboards, and to remove curtains and blinds (or secure them out-of-the-way) so that accurate readings can be taken at areas typical for leaking air, such as at floor-wall joints and window frames.

Achieving a 15°  to 20º difference in temperature is ideal.  The heat or air conditioning should then be turned off, and the inspector should wait at least 15 minutes before commencing with the IR inspection.

Air Leaks From Insulation Defects

Once a solid difference in temperature has been established between the interior and exterior of the house, insulation defects can be viewed by the camera.  By looking at the difference in apparent temperatures, hot and cold spots can be identified as areas that may have missing or inadequate insulation.  If a potential problem area is pinpointed using the infrared camera, the insulation in that spot should be examined to verify that it is an issue and to gather more details on the exact nature of the insulation problem.  Was insulation moved during a fixture installation and not properly replaced?  Is the thickness of the insulation inadequate for the application?  These are the types of details inspectors can gather after locating an issue with the insulation.

Finding Air Leaks

Finding the sources of air leaks using thermal imaging requires some additional setup beyond what is needed to find insulation problems.  By changing the air pressure of the interior in relation to the exterior, air flow can be increased to force air through cracks and holes.  With the warmer or cooler air from the outside flowing into the house through the cracks and holes, inspectors can use thermal imaging to locate the sources of these air leaks.

The best way to pull air inside through cracks and holes is by using blower door equipment.  The blower door test creates ideal conditions for pulling air in through leaky spots, and these spots are then visible in the thermal image.  If blower door equipment is not available, a house’s exhaust fans and ventilation system can be used to create similar (though less controlled) conditions, allowing useful data to be gathered in the hunt for air leaks.

Areas to Check For Air Leaks

Knowing what areas to examine for air leaks and insulation issues will speed up the process of finding problems.  Infrared equipment is extremely useful for pinpointing the locations of air leaks and specific spots where insulation is inadequate, but it is also useful to know where to start looking for such defects.  Air leaks can often be felt with the hand during a blower door test.  Also, during cold months, areas of insufficient insulation may be apparent due to the change in temperature in specific spots.

Inspectors can start with the following areas, or any area that is already suspected of air leaks or lacking inadequate insulation:

  • air leaks at light fixtures;
  • electrical receptacles;
  • windows and doors;
  • attic access;
  • attic insulation;
  • basement rim joists;
  • cellar door air leaks;
  • crawlspace air leaks;
  • ductwork;
  • plumbing penetrations, and traps under tubs and showers;
  • air leaks around plumbing vent pipe penetrations;
  • chimney flue and fireplace surround air leaks;
  • dropped soffits;
  • dropped ceilings;
  • air leaks in kitchen soffits;
  • air handlers;
  • air leaks at cracks between partition top plates and drywall;
  • utility chases; and
  • baseboards.

By using thermal imaging, inspectors can more quickly identify problems with insulation and the sources of air leaks.

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