DESlGN/INSULATlON
Position in the Envelope
The position of the thermal insulation within the envelope needs to based on several considerations
including condensation control, ease of installation, the maintenance of insulation system continuity,
and the relation to elements of the structural frame. In reference to water vapor transport, the
insulation position needs to be based on consideration of the total envelope system, the means of
water vapor transport control and climate. Basically the insulation should be positioned such that
the temperature of low permeance materials are kept above the dewpoint of the interior air in
heating climates and above the dewpoint of the outdoor air in cooling climates. The interactions of
the vapor retarder and the insulation positions are discussed in the section Design/Vapor Retarders
and in the ASHRAE Handbook of Fundamentals.
The position of the insulation within the envelope is also an issue with respect to interaction with
other envelope elements. Positioning the insulation outside of the structural frame makes it easier
to maintain the continuity of the insulation system and protects the structural elements from outside
temperature swings. This protection in turn reduces thermally induced movements of the structural
frame and the need to accommodate these movements in other envelope elements. However,
when the insulation is located in the outer areas of the envelope, it will more often be exposed to
water, and the material selection and attachment must account for this exposure. On the other
hand, locating the insulation within the structural frame makes installation, inspection and repair of
the insulation easier. However, it is difficult to deal with the interruptions of the insulation system
caused by the structural elements while maintaining the continuity of the insulation, i.e., minimizing
thermal bridging of the insulation system.
Design and Installation Requirements
While these guidelines do not address the design issue of how much insulation is required, there
are many other critical design and construction issues relevant to the performance of the thermal
insulation system. One particularly important issue is selecting the insulation material and the
means of attachment based on the environment to which it will be exposed. Issues of drainage,
adjacent materials, and compatibility with adhesives all need to be explicitly considered. The role,
or roles, that the insulation will play also need to considered. While the insulation is being used to
control heat flow, it may also be serving as a vapor retarder or an air barrier. Such a dual role may
be intentional. If it is not, problems can arise. If the wall does not have an adequately designed
and installed air barrier system, the insulation may experience the air pressure difference across
the wall. Unless the insulation and its attachment mechanism is designed for this pressure, the
insulation may be displaced from its intended position, reducing its thermal effectiveness and
perhaps leading to other serious consequences.
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