Infrared Training Center

Wednesday, June 2, 2010

Vented Soffits or Cover-up?

By Kenneth Brown, MD

A homeowner recently requested our services to help her understand why her attic temperatures could remain so high. For some reason she had placed an electronic thermometer in the attic. In late spring when the outside temperature was pleasant, i.e., in the 70's, in her home in Pennsylvania the attic air temperature, 24” below the ridge vent, were 40-50 degrees warmer.

A new roof with 25 year architectural asphalt shingles, in a moderately dark brown had been applied  within the previous 12 months. At that time a new ridge vent was put in place, and the attic already had 2 gable vents, each approximately 16X24 inches and the pitch of the roof was 8X15.  Finally, there were two 6 inch by 16 inch vents in the horizontal soffits at the front and at the back of the house.

Why was the attic not cooler?

Sometimes the IR data is critical and other times it is merely useful, or on occasion just interesting. In this case it was certainly instructive and required no removal of any materials to get to difficult-to-access construction.

Figure 526 is one of the vents in question. Nothing striking about it at first glance. What’s wrong in this photo?

Fig 526
Figure 523 and 525 are both thermograms of the soffits on the rear of the house taken > 4 hours after the sun has no longer been on them. They are close enough to each other that there is no significant cooling from the breeze, and if so, it would have had equal effects. So, why the temperature difference between the two? Note that Fig. 523 appears to have irregular patterns of cooler temperature areas. On removing the vent from the soffit, we noted that someone had cut a large opening with approximately 30 inches space for air flow in the soffit. Such is not the case for the “vent” in figure 525. Rather  the screen-backed "vent" was put in its place after the builder drilled a few holes.

Fig 523 (good ventilation)                     Fig 525 (poor ventilation)

So we go to Figure 535 to see how effective these so-called soffit vents should have been. Note that the golden dots- holes meant to provide ventilation, in reality only provided us with a little entertainment.

Fig 535 (restricted air flow)

So, what did we learn from this exercise? It reminded us that:
        Even if you use a B300, you still have to think.

To get air flow in a closed or semi-closed space such as an attic, there has to be air intake in order for there to be air exhaust. Seems simple, but there are many sites on the web that do not seem to understand it.

Ideally, the ventilation should be designed to provide cooler incoming air through the soffits. If you have incoming air only from gable vents, then it will be short circuited and not pass along the slope of the attic roof, closest to the shingles heated by the sun.

If there are gable vents AND soffit vents AND ridge vents, it may be appropriate to actually block off the gable vents in order to avoid the short circuit in air flow described above. The calculations by which you determine the needed amount of area for the vents for incoming and exhausting air (in square feet or square inches) are determined by the area of the attic “floor” and the pitch of the roof (

There is more than a small chance that your roofer (much less your builder) may not calculate it herself/himself - the roofer from whom this customer sought help did not and said “we are not engineers- the manufacturer of the vents tells us how many use.” And the proposal was to put one 3 inch diameter vent between each pairs of rafters, front and back; at 7 square inches apiece, and putting 30 in front and 30 in back, this would have provided 420 square inches, far short of the minimum of 576 square inches needed (30ftX40ft)(144 s.i./s.f)/300. Not to mention how they would detract from the appearance of the soffits.

So, without blushing, ask your supplier of your roofing/ventilation services for the basis of the calculations used on your house. You may add years to the life of your roof.

Just as we are now asked to think and be responsible for our health, we have to do so for the homes we live in. Caveat emptor! Get ventilated, not covered up.