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 (http://www.1728.com/gradient.htm).

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.

13 comments:

  1. I think you would find that ventilation in an attic space is secondary to proper insulation. Proper insulation stops radiant heat gain during hot clear days and also stops heat loss in the winter. Many people make the mistake of ventilating during the summer trying to cool the living space. The problem then becomes clear in the winter when all that good ventilation sucks the heat out of the house. John Mowry Quality Home Insulators.

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  2. Then explain this to me...

    Why is air infiltration a greater energy loss than a lack of insulation.

    Your not one of those people that think insulation prevents pipes from freezing in the winter are you?!

    And what type of insulation are you using to stop radiant heat transfer? Insulation is about conductance. A radiant barrier is not insulation. It is a low emissive, highly conductive and reflective barrier to keep the radiant energy from being absorbed by the insulation.

    Ventilation does not suck heat out of the house!
    The space above the attic insulation is an unconditioned space.

    Q = U * A * Delta T

    Lower the air temp in the attic by removing it and the Delta T (a multiplier in the equation) reduces conductance of heat energy through the insulation. Drop the air temp in the attic from 170 to 70F and heat transfer is 100X less!

    Don't ventilate properly and watch the #1 house destroyer (moisture, also known as latent heat) destroy the house!

    Did you know that for the most part, insulation materials are good conductors of heat, but trap air which does the insulation work? So back to ventilation; it is in fact NOT secondary to insulation.

    I recommend you consider ITC Building Science. Or a weatherization class.

    Air movement is the primary reason of enery loss. That also makes it a superior method of heat removal.

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  3. John,
    Proper insulation and proper ventilation are equally important. Without proper ventilation, warm moist air accumulates in the winter and causes condensation and mold growth. The condensation then drips onto the attic insulation reducing its effectiveness against heat flow. Further, moisture in the attic causes premature deterioration of the roof deck and shingles.

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  4. I'm with John on this, but I think his message was somewhat misconstrued.
    You can use fully vented soffit with matching ridge vent and the attic temperature isn't going to drop much. It certainly isn't the difference between 150 and 70F, more like 150 to 140F. The air flow required to cool an attic, heated by a big, hot roof, is thousands of CFM. You're not getting this from passive ventilation.
    Having more R-value in your attic floor will make a heck of a lot more difference in the house than a few soffit vents.

    Also, I don't think John is saying that you shouldn't have any ventilation, just that it's secondary. Sure, you need to flush out moisture. Of course, if you seal up the penetrations between the living space and the attic, and you properly vent your bath fans, the moisture buildup should be minimal. Numerous homes were built with minimal attic ventilation or just good old gable vents, and never had any moisture problems until someone came along and installed recessed lights.

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  5. One look at the picture (it is a good one) and I immediately saw that the board under soffit vent had not been cut out. To have good ventilation,the air must travel up freely from weap holes who will allow air to vent humidity between walls, up to join cool air from soffits and all that cool air should be sucked out by the ridge vent as hot air rises. Soffit vents should be opened up professionnally, gable vents should be blocked and then the attic will have the same temperature as outside, (Delta T 0) the insolation will not be damp and the roof covering will last much longer. This is a real issue and a priority to have corrected.I come across too many attics which are poorly or not sufficiently vented.

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  6. Giving up insulation for ventilation is not the subject matter.

    The effects of ventilation has a lot to do with where you live also.

    I'm from Concord, Ma. living in Nashville, TN.
    150/70F is a daily occurrence here (specifically when ventilation has been omitted).
    Your "normal" 140-150F attic ambient is just an indication of "inadequate" ventilation.

    >190F is common at design OA temps (98F). Many of you never see 98F.

    Using your numbers:
    If we can reduce the attic temp 50 degrees (easily done here).

    150 - 70 = 80 * 1,000 * (R38/1) = 2,105 btu/hr per 1k sf

    100 - 70 = 30 * 1,000 * (R38/1) = 789 btu/hr per 1k sf

    789 vs. 2,105 is significant and can be achieved through proper ventilation.

    I see trucks with H/AC rather than HVAC. I guess "V" is a common omission in building construction these days.

    Way too much selling of insulation without taking care of the air flow goes on. Adding the insulation often results in lost ventilation when 14 in of insulation is forced into the 6" eave.

    There are three forms of heat transfer, we don't need to be tunnel visioned by focusing on just one.

    Granted that we are lucky to see more than 8" (R-22.4) in a ceiling and more is needed in most cases, but proper ventilation also must be incorporated in the process.

    Perceptions that ventilation only removes heat from the house in winter needs to be put to rest.

    Once the heat makes it past the insulation, it needs to be removed.

    It's not just about efficiency either. A 3 degree rise in average roof temp, due to a lack of ventilation reduces the life expectancy of the roof by a year.

    Ventilation is only secondary to John because he sells insulation.


    12" Loose fill fiberglass (12 * 2.8 = R-33.6)
    .02976 * 80 * 1,000 = 2,381 btu/hr per 1k sf.

    Add insulation to recommended R-49.
    .02040 * 80 * 1,000 = 1,632 btu/hr per 1k sf

    Your upgrade reduced heat transfer by 1,749 btu/hr per 1k sf.

    2,105 ventilation vs. 1,749 insulation does not appear secondary (not that these numbers apply in every case).

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  7. Great debate. This is the kind of hard core information consumers are looking for. Nobody minds investing in their homes when they know there will be savings. But without conclusions from postings like these consumers have no idea of what is correct and are left at the mercy of marketing information.

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  8. Just a side note related to an above comment - " a 3 degree rise in average roof temperature ... reduces the life expectancy of a roof by 1 year" is probably a flawed assumption - there is typically a 40+ degree F difference in surface temperature between a black and white roof surface - and both roofs offer the same number of years of material warranty - going on the above assumption there should be roof failures 13 years earlier on a black roof - my 30+ years as a building industry professional have found little difference in roof life expectancy based on color.

    Additionally if we are concerned about attic temperatures shingle color selection should be a significant component of the discussion

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  9. Great conversation guys. In reading it I am reminded of a couple of questions I have never quite had answered. It seems like there was a Florida study that said that excessive heat had very little effect on shingles' expected lifespan. Does anyone know of studies to the contrary or of a study such as this one. Also, does anyone know of studies showing just how much heat a radiant barrier can remove?

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  10. You are all partly right, and partly wrong. Ventilation in an attic is important, but only so much as the warranty on the shingles will be voided if you don't have proper ventilation. If an attic is properly insulated, and sealed, you shouldn't have to worry about moisture issues, or thermal tranfer. Sealed being the key. All attic bypasses should be sealed. You can pour all the insulation into an attic you want, but if you have open bypasses you have problems, and that can lead to heat being sucked out of your house, and moisture into the attic. When the wind blows through an unsealed attic it creates a negative preasure in the attic. This will draw heat and moisture into the attic. If open bypasses are properly sealed, and you are reading near 50pa with a blower door running at 50pa, with referrence to outside; what's going on in the attic shouldn't matter as it will be considered outside of the house.

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  11. To the above comment...

    I believe you are over-looking one common thermodynamic occurrence: condensation.

    First, why does an attic become hot? Well, as most of us know, it's due to both heat transfer through the envelope (in particular, the ceiling) of the home and solar loading on the roof above. Convection, conduction and radiation all play a significant role here.

    In the winter, we still have a problem with solar loading, despite the extremely cold temperatures. Our attics act as a solar panel, absorbing the sun's heat energy through the inconveniently black shingles and transferring that same heat through to the inner surface of the roof, thus allowing convection to play its part in heating the attic space. Because of convection, we have areas that are much warmer than others, and usually those warmer areas are going to be the highest in altitude, leaving the coldest areas of the attic near the ceiling and soffit.

    Once the attic space becomes warmer than outside, the risk of condensation is present. Remember, warm air has the capacity to retain more moisture than cold air because it's less dense. So, what happens if you cool down warm air to the point where it loses its capacity to retain all of its water content? Well, it releases some of that water (known as condensation). This circumstance is referred to as dew point. To prevent temperature differentials from harbouring dew point, we've got to continuously ventilate the attic space to ensure that the difference in temperature between inside the attic space and outside remain minimal (if any difference at all).

    As a side note, if there is any moisture in an attic due to bathroom or kitchen exhaust fans, that is a problem unrelated to attic ventilation.

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  12. One effective way to combat moisture is by the use of desiccants like Silica Gel. These are the same small sachets you find in packaging of various products like electronics, garments, etc. Silica Gel works by absorbing the moisture in its surrounding area. For most moisture challenges, does the job. With small sachets cost less than a dollar, it truly is a cost effective way of protection from moisture. There is a lot more information and an order form on our website at www.SilicaGel.net

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