Attic Ventilation & How Many Attic Vents Your Roof Needs

Providing proper attic ventilation is one of… or rather the most important issue associated with this area, having huge impact on your house “health” and proper functionality (especially in cold climates).

As long as you have an attic (most likely if your house has a pitched roof and none or partial cathedral ceilings), it should be accessible, and periodically checked for any abnormalities.

Believe it or not – many of the home owners don’t even realize, that there’s an attic, and how important it is to maintain attic ventilation and control amount of moisture migrating into this area. Providing attic ventilation by using a combination of various types of attic / roof vents, plays a key role in preventing such problems as mold growth, and ice damming…

Attic air circulation / How much of that air circulating through your attic you really need…

You can find various attic ventilation calculators online, to give you explanation in numbers, and I’ll try to add a few words to it.

  • Basic attic ventilation principal is to keep that unused section of our house as cool as possible – critical condition during the cold season – dividing amount of the roof upper and lower vents 50/50.

Some of us think absolutely opposite, and seal all of the attic ventilation ports, sometimes insulate roof decking (in unconditioned attics), assuming that such action will lower the utility bill. Unfortunately, by doing that, they are creating ideal attic mold growing environment (moist and warmer than exterior).

You can partially determine how good is your attic ventilation by examining roof surface during the winter (from the ground of course!). After some snow accumulates on a pitched roof surface, observe it over the next few days. If your roof remains snow covered, there’s a good chance that your attic ventilation, and attic floor insulation are adequate.

That might also be a proof that whatever heat is being transferred through the attic floor (house ceiling), and all other penetrations between the house – an attic area, has enough escape routes.

Any roof surface section, where snow starts disappearing, usually corresponds to a heat source in the attic, or pin-points attic part where the warm air has been trapped, instead of being vented to the exterior. With ambient temperature above freezing, such investigation might not be accurate, and you’ll need to open the attic hatch…

The ideal roof / attic ventilation would consists of combination of vents located in:

  • upper section of the attic (ridge vents, turbine, box shaped or dome static vents, electric motor powered vents)
  • vents installed along the bottom parts of the roof overhang, called intake, or soffit vents, hip vents, and / or
  • gable vents

Roofs with no, or very small overhang might be able to utilize “vented drip edge” type vent.

Do not combine attic gable vents with roof soffit and ridge / upper roof vent systems – it might disturb attic ventilation / air circulation process.

How Many Attic Vents Your Roof Needs

Industry standards for proper attic ventilation recommend:

  • For no vapor retarder type of attic insulation – no paper, plastic or aluminum layer between the attic floor and insulation layer – 1 sq. foot of ventilation for every 150 sq. feet of attic space divided 50 / 50 between the inlets and outlets.
  • For vapor retarder equipped attic insulation (for example fiberglass blankets/bats with paper facing), you should have 1 sq. foot for every 300 sq. feet of attic space – assuming that everything else is perfect

Complexity of some roofs might prevent such installations, and compromise proper attic ventilation, but each of those “unique” designs would have to be examined and discussed separately. Lets assume, that we have as many venting ports as we should (based on simple formula above, and house exterior examination), and that everything was installed the way it should be, without cheating…

We can only see roof soffit vent cover, and expect to have an opening behind it. Once installed, it is hard to determine, if the cover has been secured directly over the cut-out in soffit board. The easiest way would be to access attic area, and check for a light shining through the soffit vents, you should be able to see it from access point.

However, in some cases, you might have to walk further from the attic entrance – be extremely careful if you decide to do that – use good source of light and watch your steps, because most of the attics have no floor boards installed, and you have to step on framing members.

If roof soffit vents are visible on exterior, but you can’t see any light shinning through them, there are moisture / condensation stains on the roof framing and / or decking, your attic ventilation could be suffering. Some things to expect:

  • roof soffit vents are clogged with dust or several coats of paint (they should be cleaned if possible or replaced)
  • roof soffit vents are sealed with attic floor insulation, so called – vent chutes / baffles correct this problem, and they should be installed in rafter / truss spaces corresponding to soffit vents on exterior, or in all spaces – they are cheap and more can only do better
  • roof soffit vents might only exist on exterior – there’s no cut-outs underneath (have some new vents installed or correct current installation to improve attic ventilation).

Most common problem with gable and roof top vents is that they get clogged by bird nests or dust / lint, which compromises ventilation. If that happens, just clean it periodically and if wire screen is damaged – replace it.

Some people install screens on the inside (from the attic), covering cut-out in roof decking board…, it is easier and it does prevent rodents penetration, but it is still great spot for a bird nest.

When a Ventilating Cupola Solves Moisture Problems in the Attic

“More gold has been mined from the thoughts of men than has been taken from the earth.” By Napoleon Hill

The American Institute of Architects estimates 90 % of U.S. high levels of moisture in the homes.

Adding a cupola provides passive ventilation to the roof allowing trapped heat to escape with a natural flow in an upward direction through the sides of the cupola.

A roof saver, attic ventilation is all about circulating air to reduce moisture and bring in fresh air.

According to studies conducted by the U.S. Dept. of Energy and the heating and cooling engineers support the benefits of roof ventilation.

Insufficient ventilation can lead to moisture problems during the winter and decreased energy efficiency in the summer.

In an unventilated attic the roof sheathing may reach a temperature of 160 degrees Fahrenheit when the outside temperature is in the 90’s.

The attic heats from direct sunlight on the roof that radiates heat. This is then transmitted through the roofing material to the attic floor or the top surface of the ceilings insulation, causing the attic air to become heated.

The attic floor then acts as a “hot plate,” radiating warm air in the rooms below, causing an increase in your air conditioning requirements.

A ridge vent, such as a functional cupola, helps provide air circulation in the attic and allows for excessive attic air to escape through its sides on the top of the roof.

Suffit vents found along the bottom perimeter of the attic space, work well when used in conjunction with ridge vents to promote the circulation of attic air.

Preventing moisture damage in colder climates is a larger benefit then in warmer climates.

Attic ventilation is required in colder climates to evacuate the warm, moist air that builds from the living space below. This air can condense causing the roof sheathing to rot.

Circulating air from ventilation helps prevent ice, dams, which form when warm air in the attic melts the snow and creates a run off that refreezes on the colder eave.

Suffit vents allow air to enter the attic at the lowest point of the roof. They are more effective when used in conjunction with a continuous ridge vent, such as a cupola.

Adding a cupola to the roof allows a way for trapped heat to escape, by providing a natural flow in an upward direction through the sides of a cupola. Cupolas provide passive ventilation by releasing the warm air while bringing in cool air.

Cupolas were originally designed for functionality, as a ventilating system. Today there has been a rebirth of interest in cupola for decorative architectural accent as well as the functional aspect of ventilation, and are placed on the roof tops of houses, garages, businesses.

Cupolas not only improves the attic ventilation they provide an eye-catching exterior focal point which adds warmth, tradition and a little country charm that will add value to your property for years to come.

Copyright (c) 2009 Elda Titus

Insulate and Ventilate Your Attic For Constant Comfort in Your Home

I’m not sure whether it’s preferable to be stifling hot in summer or freezing cold in winter. But then why should you have to choose? If you make intelligent choices, you’ll be cool in summer and snug in winter. You’ll have the best of both worlds, all year round.

If you can effectively block out excess heat from your home in summer (and you can, very easily) you’ll be a whole lot more comfortable. If you prevent heat loss in winter, you will not only be more comfortable at home when it’s icy outside, but you will also be making a giant step towards conserving energy.

While I have to admit that I’m really more concerned about insulation that will either keep heat in or out of the house, I mention ventilation because the movement of air is also important. It isn’t just that we all need fresh, healthy air to breathe; it’s also vital to have air flowing through our homes. But first let’s focus on insulation.

To get this right, you need to understand a bit about the 3 main ways in which energy (in the form of heat) moves or is transferred:

1. conduction – when heat is transferred quickly by contact through solids (and to a lesser extent liquids and gases);
2. convection – when heat is transferred through liquids and gases (for example in the form of wind); and
3. radiation – which doesn’t rely on any other medium to transfer the heat.

Now the heat from the sun reaches earth as radiant heat, and it is this heat that you are looking to escape from when you look for shade in the garden on an ultra-sunny day. When we insulate our homes, we minimize all this stuff by using double glazing, carpets on floors, cavity wall and timber frame insulation, and attic insulation.

Now, here’s an open secret. Your attic insulation isn’t going to be effective unless you get it right. Right! I say this because people are forever asking me whether to use traditional attic insulation materials – you know, the fiberglass and cellulose types – or whether to go for some type of radiant barrier.

I have two standard answers to these questions that don’t have anything to do with the type or size of house you want to insulate.

1. If you want to be cool in summer and warm in winter, you need to combine good quality fiberglass or cellulose attic insulation with a top quality radiant barrier.
2. “Some type” of radiant barrier isn’t going to cut it. You need the best and it must be double-sided.

Standard attic insulation materials reduce conductive heat flow. A radiant barrier isn’t meant to replace this type of insulation. Rather, it improves the way it works. Why? Well, simply because materials like cellulose and fiberglass also hold heat. That’s really cool – I mean hot – in winter. But in summer it can be hell.

Now, back to attic ventilation. Ideally, you want your attic to be as cool as the outside air. This will keep your insulation and ductwork cooler. When you install a radiant barrier just remember a few things:

o Holes or a gap at the bottom of the installed foil for air to enter the attic space,
o Spaces between the foil and the other insulation material, and
o Holes for the air to escape.

It’s that easy.