Some
Basic Dynamics of Over-Pressurization |
And
De-Pressurization In A Home |
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Over-Pressurization |
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| A house can become
over-pressurized in many different ways. For instance, localized over-pressurization
can occur when a portion of a house is finished off, but the return air vents are omitted.
Supply air continues to enter the room, but since no means of air escape was
provided, the room "over-pressurizes". This is a very common occurrence I
see on a regular basis, especially when a "do-it-yourselfer" finishes his own
basement. One of the dangers of an over-pressurized
house, is that moisture-laden air will be forced into the exterior wall or ceiling
cavities, where it will eventually reach the dew point and begin to condense. Once
the interior wall & ceiling components reach a certain moisture level, they are
susceptible to mold growth. Many components, such as insulation, will cease to
provide their intended function if they become saturated, and must be replaced with new. |
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Click on the thumbnail image above
to see an expanded view of a wall section in an over-pressurized house in the winter. |
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As you can see from the image on the left,
you will typically find the highest moisture levels and the most interior wall damage
towards the outside 2/3 of the wall, in an over-pressurized house in the winter. |
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Click on the
thumbnail image above to see an expanded view of how over-pressurization has forced
moisture up into an attic, where it has frozen on the underside of the roof decking. |
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Moisture-laden air is forced up into a poorly
ventilated attic, freezing onto the underside of the roof decking. During the first
warm day, the ice will melt and drip onto the insulation and drywall ceilings below. In
severe cases, the insulation and drywall is so damaged, it requires replacement. |
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Click on the
thumbnail image above to see an expanded view of how moisture forced up into a poorly
vented attic space, has lead to mold growth on the structural members. |
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This photo shows the attic of a house
described in "Upgrading An Existing
Home". The damage here could be directly attributed to not properly
upgrading attic ventilation in conjunction with upgrading the exterior "shell".
Notice the extensive mold growth on the underside of the roof decking. |
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De-Pressurization |
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| A house can become de-pressurized
in many different ways. For instance, a wood burning fire-place that has not been
installed with a proper source of make-up air can easily consume over 300 - 500 cfm (cubic
feet per minute) of indoor air. Not only is this very inefficient as the air being
consumed by the fireplace is expensive air (has already been warmed to indoor
temperatures), but also can very easily draw more air out of a house than is being
returned. Localized and house-wide
de-pressurization can also take place through the use of any out-exhausting mechanical
ventilation (i.e. bathroom fan, kitchen range hood fan, etc.). While these
appliances are certainly useful and necessary, their operation without corresponding fresh
air return into the house will create a de-pressurized situation. |
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Click on the thumbnail image above
to see an expanded view of a wall section in a de-pressurized house in the summer months. |
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As you can see from the image
on the left, you will typically find the highest moisture levels and the most interior
wall damage on the inside 1/3 of the wall, in a de-pressurized house in the summer. |
| The charts below help to illustrate some key
principles associated with air-flow and air-pressure, and the potential for hidden
moisture problems in a structure. |
Winter |
Over-Pressurized house in winter
Warm Air  Cold Air |
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There is the potential for hidden moisture problems. |
De-Pressurized house in winter
Cold Air Warm Air |
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There is little risk of hidden moisture problems. |
Summer |
Over-Pressurized house in summer
Cold Air Warm Air |
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There is little risk of hidden moisture problems. |
De-Pressurized house in summer
Warm Air Cold Air |
= |
There is the potential for hidden moisture problems. |
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