Category Archives: Ventilation

Attic Ventilation and Sizing

attic ventilation

Fair warning: I am not an engineer. I’m not a scientist, either. I do make informal hypotheses and then use observational results to determine their merits, but using the scientific method does not make an expert. The truth is most contractors, builders, and even architects aren’t either of these things either. In fact, the whole idea of “building science” is a relatively new one, which is interesting, considering that humankind has been creating non-cave places to live for thousands of years.

I use this as a preface because the home building industry makes an awful lot of its decisions based on what we’ve done before. Whether it’s a green behind the ears tradesman learning their craft from a veteran or one contractor picking up tips from another, this industry is based on handed down learning of what works and what doesn’t, with official code books issued periodically to provide continuous guidance on how homes should be built. While I’m not about to get into how codes are changed here, just know this: neither the codes themselves nor the jurisdictions who decide how and what codes get used (adopted, in the parlance) are done from a purely scientific standpoint.

With all of this in mind, it should be little surprise that attic ventilation is a thing we do because we’ve always done it. And, even more maddening, is that the seemingly scientific way of determining how much attic ventilation a building needs was just made up back in the 1940s. So, as much as I’d love to post that using a ratio of 1:300, with 1 square foot of ventilation needed for every 300 square feet of attic area, with the ventilation balanced between the top (ridge) and the bottom (soffits or eaves) is the best solution to attic ventilation, I can’t say that it’s a 100% data driven fact.

Yet, it works. Because of course it does. Funny, no?

Really, it works because;

  1. attic ventilation, while important to do correctly, doesn’t cause catastrophic failure if

       done incorrectly

2. the original ratio, whatever its origins, was most likely based on observational data

     that correlated with homes that worked right.

I will note that one of the most well-known and trustworthy building science professionals, Joe Lstiburek, does recommend using a 60/40 split for the bottom and top attic ventilation areas, with slightly more air coming in from the bottom of the space to slightly pressurize the space. This makes good sense, as it ensures that the attic won’t be “sucking” conditioned air from the living space.

For practical purposes, we put this together 12 years ago. Note that the 1:300 ratio isn’t on here but, instead, there’s a multiplier.attic ventilation

The .24 multiplier follows the 1:300 guideline but we’ve found that a simple multiplication factor is the easiest way to do the math. And, after 12 years, it’s probably time to update this with the 60/40 information as well. Otherwise, it all still works!

Hopefully, this helps explain attic ventilation sizing some. As always, there’s more to the story, especially in how the practical aspects work, but we get to that another time.



Air Leakage

A blower door tests the tightness of the home’s envelope by pressurizing or depressurizing the structure and then measuring the amount of time for the entire air supply in the house be replaced with fresh air. For the NAHB’s Field Demonstration of Alternative Wall Insulation Products study, four builders insulated different houses with the various insulation materials. Their summary noted the following:

  1. Better caulking, window foam and other air barrier improvements generated the most improvement in the building envelope – in fact, a large measured difference between batt insulated houses was found, when one used improved air sealing techniques and the other did not.
  2. Neither the Blow-in-Blanket System (R14 alternative viagra avis.7) nor cellulose (R12.6) measurably reduced air leakage but
  3. While foam in place (R12.6) had the lowest air leakage, the results were variable when uncoupled from air sealing techniques.

So, where does that leave us? The most important thing is that whatever insulation you use, the details are the most important thing. Paying attention to where air can come into the home is the easiest way to create a more comfortable home – or create headaches later if you don’t. The other important thing to remember is that every insulation system can stop uncontrolled air. Just as the house is a system, your insulation is one too.

Attic Ventilation


Attic Ventilation 

The International Residential Code specifies a 1 to 150 ratio of total net free ventilating area to area of space to be ventilated when it comes to ventilating attics. So, what exactly does this mean and why is it important to ventilate attics in the first place? We’ll start with the second part of the question. Ventilation provides the conditions that allow air to move, obviously. Efficient ventilation systems provide a steady, high volume of air movement. At this point, we should note that simply cutting a 6” hole somewhere in the attic is not creating an efficient ventilation system. The air movement we are discussing needs to be controlled and balanced. The amount of air coming in must be equal to the amount going out, otherwise, the attic will start sucking air from the house or stagnate due to insufficient free vents.

The purpose of ventilation is twofold:

1. During warmer months, it keeps attic air moving and, by extension, make the house easier to cool. Basic physics tells us that hot air rises and cold air falls. By circulating the air in the attic, all the really hot air that would build up in there is pushed up and out. Also, We should note that during really hot months, an unventilated attic seldom loses enough heat overnight to compensate for heat gained during the day.

2. During cooler months, it reduces moisture to keep attics dry & helps prevent ice dams. The warm inside air can hold more moisture than the cool air outside (and in the attic) and as this warm, moist air hits the cold, dry air, it can condense. So, now we’re at the stage to get some practical information. Let’s take a house with an attic with 2000 square feet. Now we’ll divide this by 150 to find the total net free ventilating area, 13.3 square feet. To make sure that the system is balanced, we then divide this number in half because half the vents will be lower and half will be higher. This means that 6.6 square feet (or 950 square inches) of ventilation is required at both the bottom and top of the attic.