Not sure which "David" this is addressed.
Moisture is an issue in housebuilding, especially insulation.
Warm air holds more moisture than cold air (water condensing on
outside of a glass of ice water is an example).
In the winter, the air inside the house is warmer, and capable
of holding more moisture, any of that warm moist air that
escapes into the insulation will cool and shed water, leaving
the insulation damp. (same thing in reverse for warm humid
summers and air conditioned homes).
Also winter can include rain and liquid water which also needs
to be prevented from entering the house walls. Liquid water
behaves different from water vapor (water molecules in a 'gas'
state). See goretex and other fabrics that try to block liquid water
while allow water in gas state to pass.
So efforts are made to reduce the amount of moisture that
can collect inside wall, by creating "vapor barriers".
But no (few) wall is totally waterproof, and you don't want
an airtight house, you need the air exchanges inside the house
(ideally without too much loss of heat/cooling).
Most exterior systems of the house are encourage to shed
water, allowing it to flow down and away from the house, rather
than leaking into the structure. Tyvek house wrap or tar impregnated paper before it)
was part of the this barrier. Interior vapor is directed toward fans and other vents
rather than allowing it to flow into insulation inside walls.
However a certain amount of ventilation is essential, most attic have systems
to allow ventilation of the attic space to reduce the accumulation
of moisture.
So, its not an easy/perfect subject. Housed used to be
"drafty" and poorly insulated. They had fewer problems
with stale air inside and moisture trapped in walls.
With the advent of tight, super insulated housed, there has been
a learning curve of how to deal with moisture that gets into the insulation
(and stale air trapped inside). There is more to be learned.
On 4/9/14, 3:10 PM, stratmister@gmail.com wrote:
Moisture is an issue in housebuilding, especially insulation.
Warm air holds more moisture than cold air (water condensing on
outside of a glass of ice water is an example).
In the winter, the air inside the house is warmer, and capable
of holding more moisture, any of that warm moist air that
escapes into the insulation will cool and shed water, leaving
the insulation damp. (same thing in reverse for warm humid
summers and air conditioned homes).
Also winter can include rain and liquid water which also needs
to be prevented from entering the house walls. Liquid water
behaves different from water vapor (water molecules in a 'gas'
state). See goretex and other fabrics that try to block liquid water
while allow water in gas state to pass.
So efforts are made to reduce the amount of moisture that
can collect inside wall, by creating "vapor barriers".
But no (few) wall is totally waterproof, and you don't want
an airtight house, you need the air exchanges inside the house
(ideally without too much loss of heat/cooling).
Most exterior systems of the house are encourage to shed
water, allowing it to flow down and away from the house, rather
than leaking into the structure. Tyvek house wrap or tar impregnated paper before it)
was part of the this barrier. Interior vapor is directed toward fans and other vents
rather than allowing it to flow into insulation inside walls.
However a certain amount of ventilation is essential, most attic have systems
to allow ventilation of the attic space to reduce the accumulation
of moisture.
So, its not an easy/perfect subject. Housed used to be
"drafty" and poorly insulated. They had fewer problems
with stale air inside and moisture trapped in walls.
With the advent of tight, super insulated housed, there has been
a learning curve of how to deal with moisture that gets into the insulation
(and stale air trapped inside). There is more to be learned.
On 4/9/14, 3:10 PM, stratmister@gmail.com wrote:
David,Just when I thought I had it figured out . . . as it concerns this discussion about A barriers such as Tyvek,your statement : "does not block moisture"!is misleading.
here copied straight out of DuPont website and I quote:"Whether it's a skyscraper or a single-family home, the building envelope is an essential line of defense against air, water and wasted energy.DuPont Building Envelope Systems offer solutions that meet or exceed codes, help extend building life, and help reduce fossil fuel consumption. They resist moisture and air, but are highly permeable, to reduce the risk of condensation damage, wood rot or mold growth."
-- Dave Uebele (daveu@sptddog.com)
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