The "attic area vented" is larger than the foundation

Attic Ventilation; How I do it…

I normally talk for a few minutes with my clients about proper attic ventilation (90% of homes are under vented). In central Texas good ventilation is critically important to proper heat removal.

For example I had a home yesterday that was 1008 Sq foot (tax record) of habitable foundation space, but this is not the correct number to determine proper attic ventilation. I use a rolling tape to measure the drip line of the attic floor around the building’s eave parameter. I measured 172 linear feet. So…

172/4=43. Then you take 43x43=1849sq ft. This means that the “area of the space vented” is 1849 Sq ft. For some reason the local builders just don’t get it… and I even had the city building inspector sheepishly agree he figures the areas wrong too, and then had to go back and correct the builder… Any way, I’ve digressed…

Now on that home the only venting was gable vents and no vapor barrier was installed, so you gotta use the 1/150 method to calculate requirements. 1849/150=12.32 Sqft, or 1774 Sq In. That’s a good bit of required vent area (NFA to boot, mind you) Now this home had 2 gable peak vents triangle in shape (mostly) The rule of thumb I use is to calculate the square of the triangle and divide by 2. These vents were 14 by 85… 14x85/2=595 NFA each, but wait, there are louvers on them thar gables, so they are only 75% efficient… 595x.75=446 per vent of actual NFA. 446x2=892 NFA…so the actual NFA ventilation of this old home was only 882” when the when the general requirement is 1774. 1774-892 gives us a deficit of 882 Sq Inches.

As a matter of practice I shoot various points of the under decking inside the attic with my infer-red, lazer powered, heat seeking thermometer… I routinely see deck temps of 125 Deg F when the actual outside temp is about 70 to 72 on a bright sunny day (on homes with under vented attics).

Do any of you guys do this quick calculation for your clients, and what method do you use to figure the “area vented” My city inspector was seriously lowballing the venting requirements, and my clients are paying higher cooling bills as a result. In Texas most AC evaporators are in the attic, and so I normally make a recommendation to improve attic ventilation (on 90% of homes I inspect). It’s just that I’ve been catching a lot of grief from local contractors for calculating the attic floor area, and not using the foundation habitable foot print. I’m just wondering what some of you folks are seeing and doing about attic venting.

Regards Cort

Cort, I believe you have a serious error in your calculation assumptions. Your process only works for a square home foot print. Take your example of a home with the 172 linear feet of drip line and assume it’s a rectangle. Now, for an extreme example, say one of the walls is actually 10’ long. That results in walls of 10’,76’,10’,76’ again equaling your 172 linear feet but the square footage of that footprint is only 760 s.f. In that case, the stated ventilation is more than adequate. Now, go pull out your old 8th grade Geometry books and prove me wrong. :slight_smile:

You are absolutely correct! :slight_smile:

All that math must be a Texas thing, here in the Low Country we just look for lots of holes to let air out…:mrgreen:

Something like this? Yes, we use that method here in Texas as well:

Despite the calculation errors, I think Cort still has a valid point for construction in this area. Generally I find that even if they have enough soffit vents, which is rare, they simply pounded a couple holes under the vent with a hammer. When I upgraded my ventilation at my house it was about 60% of what it should be.

I generally only make a comment attached to the type of bentilation comment stating that improvement in ventilation is possible and could improve efficiency. I personally feel stronger about it, but am uncomfortable making a big deal about it.

Has anyone found or know of some real study numbers of the differences ventilation efficiency will make to the homeowner’s bottom line in electric bills in the summer? As in improving ventialtion that is 60% of what it should be to 100% will translate to xx% savings in cooling costs? I have read some studies on radiant barriers, which I can tell a HUGE difference in the attic temperature when in the attic and am a big fan of, and because of the insulation the actual energy savings to the cooling bill was smaller than you would think for making a 60 or so degree difference in the attic temp.

If we had some reliable non biased numbers to point customers to, it might help us help them better.

Hay Mike, did you recommend capping those “vent holes”???:mrgreen:

A home inspector make an error? That should never happen, so I wiped the egg of may face and recalculated all my past home inspections where I mentioned attic ventilation improvement may be needed in my report… God must be smiling over me or something, for every one of them was still short of the needed ventilation, its just that some weren’t as short as others… :slight_smile: And I’m not just talking short, but falling short of the 50% minimum vent space in the upper portion of the area vented rule. (where that type of split venting system was in place)

Any way, I goofed, swallow my pride, and beg for mercy. At any rate, I did find my 8th grade geometry book, unopened and still tightly sealed. Damn girls, why was I so interested in them and not math, during 8th grade! Curses on them all… mama was right, they are the devil, and will be my downfall… Yes dear, I’ll get off the computer now, and come to bed… hahaha. Thanks for the corrections and input.


See my pic on how they do venting in new construction here in Abilene


One cannot use mere linear feet either, however. There will be a signficantly different ventilation requirement between an attic for a low-slope roof versus one for steep slope. One needs to evaluate the attic space itself, not the footprint of the home and not necessarily the footprint of the attic.

If an A/C unit or duct is located there it can be a bunch.
Heat transfer rate is Q = U x A x Delta T.

Q = R-19/1 x Area or exposed duct x temperature difference of air in duct (50 degrees) - attic air (200?).

In short, 1 degree F is 1 multiplier in the equation. 200-50=150.
Lower the attic space to the outdoor air 100-50=50.

The transfer rate is 66% less.

No a/c duct in the attic? Plug in the info of the ceiling x the inverse of the ceiling R Factor x The Temp Diff.

A 3 ton A/C = 3 HP. Which = 2238 watts/hr.

That is a savings of 1492 watts/hr.
Cost to operate = 746 watts x your elect. cost./hr.