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Attic ventilation

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The Wet Attic:

Without proper attic ventilation condensation can form on the attic sheathing. Typically a homeowner will first notice frost on the exposed roofing nails. When the sun warms the roof in the morning the frost from theses nails turns to water and falls onto the insulation. If you look at your insulation you may see a series of black dots from the dripping water (see image Below). The insulation then becomes wet and compacted loosing some of it's R-value. When the insulation warms up, the water evaporates and forms new condensation on the attic sheathing and roofing nails starting the whole process all over again.

While this is happening the mold spores that are everywhere and floating around the attic, land on the damp sheathing and start to grow. The water vapor that is added to the attic can cause additional mold growth.

At this point the insulation starts to become contaminated with mold and may have to be removed.

What can a homeowner do?

The first thing to do is determine where all the water vapor is coming from. Here is a list of common moisture producing items.

  • Cooking and showers.
  • Gas appliances.
  • Fish tanks and plants.
  • Excess humidity from improperly vented clothes dryers.
  • Wet basements including dirt basements, foundation and floor cracks and  open sump pump pits.
  • Efflorescence caused by water infiltration into the basement usually due to improper yard grading.
  • Leaking ductwork due to improperly installed HVAC systems
  • Bathroom fans venting into the attic or soffit area.
  • An un-insulated bathroom fan exhaust pipe.
  • Open chasses into the attic from around plumbing pipes, electrical wires, light fixtures or the chimney.
  • A lack of solar drying (trees and shrubs)
  • A missing attic staircase cover.
  • Roofing leaks including missing shingles, chimney flashing, skylight flashing, and sewer pipe flashings.

Usually excess moisture enters the attic through the ceiling, utility chases, electrical fixtures, and missing or improperly installed insulation and the attic entry way. This warm moist air is condensing on the colder roof structures during the fall, winter, and spring months. Without proper ventilation, excess moisture cannot be carried away.

You may require a complete evaluation of your home from top to bottom to determine the sources of the attic moisture problems. We can put together a list for for you of recommended repairs. In some cases the homeowner may be able to eliminate most of the problems themselves.

Below is a photo gallery of some common problems.

1)"An improperly installed ridge vent."

The Problem: In this first image the ridge was only cut on one side. The problem

was then made worse by installing a "roll" vent. This vent was crushed

by the ridge caps providing almost no airflow at all. (Image Two)

Image three is a brand new ridge vent (2013) installed by a roofing contractor. You can clearly see that the vent is completely crushed by the ridge caps providing no attic ventilation. This vent will have to be removed and replaced. Image four is the new ridge vent before the caps are installed.

The Solution: Remove the existing ridge cap, re-cut the ridge and install a collapse resistant ridge vent .

Uncut Ridge VentRoll Vent

Collapsed Ridge VentShingle Vent II Ridge Vent

2) "Improperly installed soffit vents"

The Problem: These perforated soffit panels were installed over solid wood. (Image One)

The Solution: Remove the panels, cut open the soffit and re-install the panels. (Image Two)

Uncut Soffit VentsCut Open Soffit Vents

3) "Inadequate soffit ventilation"

The Problem: The builder did not install the required nine square inches per foot of intake ventilation. (Image One)

The Solution: We removed the existing soffit vents and installed a two inch continuous strip vent instead. (Image Two)

Insufficient Soffit VentilationTwo Inch Strip Vent

4) "Blocked Soffits"

The Problem: The builder pushed the insulation tight against the plywood restricting airflow from the soffit to the ridge. ( Image One)

The Solution: Pull back the insulation and install soffit vent chutes. Image two shows a typical soffit vent with blown in cellulose insulation.

Blocked Soffit VentsSoffit Vent Chutes

5) "Condensation"

The Problem: Excess water vapor from the home forms condensation on the roofing nail and drips back down on to the insulation. Image one is an attic covered in black mold with rusty dripping nails and frost on the sheathing. Image two shows the drip marks on the insulation. Image three is an attic staircase cover.

The Solution: Reduce the amount of water vapor entering the attic. You can reduce some of the water vapor entering your attic by installing an attic staircase cover. This one is shaped like a tent and sets over the pull down staircase. We recommend a cover that has an R-Value of 38 of more. An average attic can lose up to 27 percent of it's insulating properties from a missing staircase cover.

Frost In AtticCondensation Drip Marks In Attic

Attic Staircase Cover

What is the purpose of attic ventilation?

It seems like a simple question, easy enough to answer. Unfortunately, all too often, that's not the case. Most homeowners, and even some experienced builders and contractors, believe the purpose of attic ventilation is to remove heat that builds up in the summer.

That is accurate.  But what that answer leaves out is just as important as what it includes.  If you understand the principles of attic ventilation, you know an effective venting system provides year round benefits.

  • During warmer months, ventilation helps keep attics cool.
  • During colder months, ventilation reduces moisture to help keep attics dry. It also helps prevent ice dams.
  • Not all ridge vents are the same. Some of the rolled style ridge vents can be completely collapsed during installation and may not provide adequate air flow at the ridge. We recommend a collapse resistant hard plastic ridge vent. Do not install a new ridge vent before mold remediation in the attic.
  • The soffit vents must equal the volume of the ridge vent. Typically a properly installed 2" strip style soffit vent will yield 9 sq. inches per linear ft. and a 3/4 " ridge vent will yield the same proving a balanced ventilation system.

Ventilation During Warm Weather 

Dealing with the effects of heat. Why, on a hot day, are the upper rooms of a home always warmer?

Part of the answer, of course, is simple physics: hot (lighter) air rises while cooler (denser) air falls. But in most homes- the vast majority of homes without adequate attic ventilation- a far more important factor comes into play: the downward migration of heat.

Consider what happens in such a home on a typical summer day. Radiant heat from the sun hits the roof. The roof temperature increases, and heat travels into the attic.

As heat builds up in the attic, it radiates to the attic floor, then into adjacent living areas, raising temperatures there.  Eventually this accumulation of heat begins to have more practical-and costly - consequences.

The most obvious are the actions taken by homeowners to cool themselves. To reduce the effect of heat - not only the daytime heat gain but also the excess heat being stored in the attic - they turn on fans, window air conditioners or central air conditioning systems. As the hot weather continues, these appliances run longer and longer - a fact well documented by utility companies across the country. Homeowners pay for all this added energy consumption in higher utility bills.

Exhaust fans may create to much negative air pressure in the attic space and actually draw air conditioning from the living space raising the temperature of your home. Make sure there is sufficient intake ventilation if you are using an exhaust fan.

Over the gutter soffit vents are a bad idea in colder climates. In the summer time water in the gutters can evaporate directly into the soffit vents (commonly called hicks vents) and end up in the attic.

Another problem is exhausting bathroom fans into the soffit area. Soffit vents are intake vents. The moist bathroom air vented into the soffit will be pulled back into the attic.

Insulation: A 5% gap between the fiberglass insulation and ceiling joists can lead to a 20% loss of heat. Do not install insulation with a vapor barrier over insulation that has a vapor barrier already. Do not block the soffit vents and do not install the insulation upside down. Consider blown in insulation (14") to help prevent heat loss and ice dams. Blown in insulation should not be installed without a vapor barrier.

If you are upgrading your insulation to the new current standard of R-38, be sure to add additional ventilation to reduce the moisture in the attic that additional insulation will create.

Insulating a basement can be counter productive. If the basement is wet, insulation can trap moisture leading to a mold problem. Consider insulating the joist end pockets only for draft reduction.

Ventilation During Cold Weather 

Improper ventilation can cause ice dam formation which is the result of continuous freezing and thawing of snow due to escaping heat from the house or from gutters being backed up with frozen slush.  Ice dams occur when the following conditions exist: warm air accumulates near the peak of the attic, lower areas of the room remain cold and/or a heavy snow accumulates on the roof.  When any of these occur, water may be driven under the roof which may cause ceiling, wall, insulation and gutter damage.

Ice dams can be prevented from forming by:

  • Installing a vapor barrier above the home's warm space.
  • Insulating the attic floor.
  • Ventilating the attic properly.

Damage from ice dams, if they do form, can be reduced by:

  • Removing debris from gutters so that it does not build up over time.
  • Making sure that the outer edges of the gutters are lower than the slope line. This will allow for snow and ice to slide clear.
  • Installing eaves flashing, any brand of Ice & Water shield will do.

The amount of ventilation needed is determined by the size and design of the roof.  For roof and attic spaces above an insulated ceiling, the vent ratio is one square foot of net free ventilating area/300 square feet.   For low slope roofs or roofs with cathedral ceilings the vent ratio is one square foot/150 square feet.

  • Typically an attic will have a minimum ridge vent opening of 3/4 per linear ft. on each side and a minimum soffit vent opening of 3/4" per linear ft. on each side. (a net area of 9 sq. in. per linear ft. on each side)
  • You cannot have to much soffit ventilation , but you can have to much ridge ventilation. An undersized soffit vent will force the ridge vent to draw warm moist air from the living space.
  • A portion of this article was reprinted from ventmaster.com)


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