Designs That Work
Cold Climate
Introduction - Building Science for the Cold Climate House
An understanding of the regional climate is the starting point for the
design of affordable, high-performance homes. Applied building science is
the next step to create houses that are safe, healthy, durable,
comfortable, and economical to operate. For the Cold Climate Case Study
House, this means understanding and managing the way that four things move
on or through homes:
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Water,
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Vapor,
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Air, and
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Heat
Section Two of this package, The Basic Cold Climate House, focuses on
these four phenomena. The greatest risks for moisture-related problems are
discussed and where possible, the reasoning behind the selection of
enclosure assemblies is given. The house design is based on extensive
experience with what works and what does not work, from forensic
investigations of building failures, and from the results of test houses
and thousands of houses constructed by builder partners of the Building
America program.
To bolster your own professional judgment and building common sense,
the following ten building science principles are offered. It should not
be a surprise that all of these principles are at least indirectly related
to moisture. Even in hot-dry climates, moisture events related to occupant
activities, leaks, and singular climate events can bedevil the performance
and durability of today’s homes.
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Our efforts to save energy and reduce the flow of heat
through building assemblies have reduced drying potentials and,
therefore, increased the importance of controlling moisture flow through
building assemblies.
- Ideally, building assemblies should be designed to dry to both the
interior and exterior. In heating climates, the primary drying potential
is to the exterior (but not necessarily exclusively so); in cooling
climates, the primary drying potential is to the interior (but not
necessarily exclusively so); and in climates with both heating and
cooling, some drying potential in both directions is typically a good
idea (but not necessarily exclusively so).
- Building materials last longer when their faces are exposed to
similar or equal temperature and humidity. This is why the ventilation
of claddings, particularly those that store moisture (reservoir
claddings), can be important.
- Drainage planes, air barriers, and thermal barriers must be
continuous to be truly effective. Being able to trace each of these on a
full elevation drawing without lifting your finger (or pencil or
pointer) from the elevation is a good test of continuity.
- In moisture control, the priority is liquid water first,
particularly when it comes in the forms of rain and groundwater. In
these forms it is referred to as “bulk” water. Following are
air-transported vapor and then diffusive vapor, all other things being
equal. It’s always a question of quantities and rates, of wetting and
drying, and the tolerance of materials (individually and in combination)
for each and all of the above.
- Three things destroy materials in general and wood in particular:
water, heat, and ultraviolet radiation. Of these three, water is the
most important by an order of magnitude.
- When the rate of wetting exceeds the rate of drying, accumulation
occurs.
- When the quantity of accumulated moisture exceeds the storage
capacity of the material or assembly, problems occur.
- The storage capacity of a material or assembly depends on time,
temperature, and the material itself.
- The drying potential of an assembly decreases with the level of
insulation and increases with the rate of air flow (except in the case
of air flow in severe cold climates during cold periods where interior
moisture levels are high).
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