Infrared Cameras, what is the temperature really?

Infrared cameras are a great tool for the home inspector, but like any tool, they can be misused or misinterpreted.

These pictures show frost on a roof with a roof surface temperature of about 12 degrees Fahrenheit.  I say about, because 12 degrees is pushing the lower limits of what this particular thermal camera can see (FLIR C2, 14 degrees Fahrenheit).

   

When measuring the temperature of a roof at night, one must be careful to compensate for the much colder temperature of the night sky that the camera can also see by angles of reflection.  On a cloudy night this might not be so important–and might not even matter at all.  On a clear night the warm roof will give up the heat it has accumulated during the day, to the cold of space.  As it does so, the temperature of the roof surface can depress significantly below air temperature.

But, is the actual roof temperature even close to what the IR camera is telling us? 

On a clear night, likely not.  For a more accurate temperature we would have to shield the roof from the night sky in such a way that the night sky is not influencing the camera’s sensor.  In the case of the pictures above, the actual roof temperature was closer to 25 degrees Fahrenheit—still well below freezing.  This next picture shows, in a simplistic way, how the camera can see more than the small circle on the roof we think we might be measuring.

The camera is viewing the roof from my own house window. It is also seeing some of the night sky.

With air temperature above 40 degrees Fahrenheit, the temperature of the roof surface can drop below freezing and result in frost on the roof—even though air temperature is well above freezing.  Of course there has to be sufficient humidity such that the dew point can allow the condensation to happen and for frost to develop.

This condition is very common in the NW in the Spring and Fall.  See this link for more information about this phenomenon called night sky radiational cooling.

By Charles Buell, Real Estate Inspections in Seattle

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The Range Hood Exhaust–as Air Intake

Modern tight houses can easily become depressurized when exhaust fans are turned on. What this means is there is no place for the air to come into the home to replace the air that is trying to leave. If there are gaps around door weather-stripping, or gaps around window sashes or similar locations, the air will come into the home at these locations.  Sometimes even chimneys might be the path for this air.

If we operate and exhaust fan in a bathroom the house becomes depressurized, or an area of “lower pressure.” Areas of higher pressure will tend to make balance with areas of lower pressure, so the air outside the building literally “pushes” its way into the area of lower pressure.

Most houses are not tight enough for the air to not find its way in somewhere, and general infiltration was once allowed to be the source of this air replacement.

This small condo unit was too tight for general infiltration to be the source of make-up air, as was evidenced by its finding a path through the range hood exhaust.

Most range hoods have a back-draft damper in them, but there should also be one in the cap at the exterior of the building as well. You can see in this picture there is no damper—but there is a screen.

Exhaust fan vent termination with no back-draft damper

With two bathroom exhaust fans and the laundry exhaust fan running, the purple/violet colors of the thermal image of the chase and microwave/hood shows cold air cooling the chase and the area around the microwave.

  

The screen at the exterior cap location did hold a tissue paper to show that indeed air was pushing its way through the microwave/hood.

So, let’s say we “fix” the cap at the exterior with a proper back-draft damper. Where will replacement air come from? General infiltration may still be adequate, it is just easier coming from where it is now. If it is not adequate, the functionality of the exhaust fans will be reduced. In other words, they will make noise but not exhaust enough air from the room. It is like turning a 100-cfm fan into a 50-cfm fan.

For exhaust fans to do their job, replacement air is necessary and is required by modern codes when houses get to a certain point of air-tightness. This one may be at that point, even though it is an older home in that respect.

Some “positive” means of allowing exterior air to enter the home may be indicated if exhaust fans do not function properly after the exterior cap is repaired and its back-draft damper installed.

By Charles Buell, Real Estate Inspections in Seattle

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Is your insulation doing what you think it is doing?

I do not want to tackle the huge ugly topic of whether fiberglass insulation should be used at all–as it frequently is in attics.

For today, I just want to talk about one obvious issue with fiberglass insulation that prevents it from performing as expected.

The short version of what is wrong with fiberglass insulation is that it is not an air barrier, therefor, if it is not encapsulated and air sealed on all six sides its performance suffers. In an attic, at most, only five sides is likely to be sealed leaving the entire top not sealed.

This post is about the sides, which can and must be air sealed. The vented sides are typically not adequately air sealed, especially along the eaves. Insulation baffles, designed to keep insulation out of the lower roof venting and to allow for air flow into the attic, rarely gets adequately sealed.

Soffit vent that allows air flow into the attic

Because the baffle/top plate connection is not air sealed, and because fiberglass insulation represents very little resistance to air flow, air pushes its way into the attic right through the insulation. As it does this, it either cools the ceiling in the area, or warms the ceiling in the area depending on the season and/or side of the house.

In the winter and/or the north side of the home, the air will tend to make the ceiling cooler in the area of the vent. In the summer, especially on the sun side, the air will tend to warm the ceiling in the area of the vent. This will increase both heating and cooling loads of the home.

This next picture shows what that area looks like at the interior ceiling with Infrared camera on the South side of the home. Warm air is moving through the insulation and warming the ceiling.

In the same house on the North side we can see how the ceiling area near the vent, as indicated by infrared camera, is “cooler.” In the actual picture we can see the fungal growth present because this vent happens to be in the area of the bathroom. The moisture in the bathroom condenses on the cooler surface creating a perfect environment for mold growth.

The only real repair for this condition is to pull back the insulation and properly air seal the gaps where the insulation baffle and the house framing meet.

Spray foams are good for sealing the areas where the baffle makes contact with the framing as indicated in the circle in the picture above. Of course, in a perfect world, we would not use fiberglass insulation at all, and instead use types of insulation that are much better at stopping the flow of air. Cellulose fiber insulation can do a much better job at this.

 

Charles Buell, Real Estate Inspections in Seattle