re; attic vapor barrier

[daniel nantell]

I recently inspected a home built in 1988 , and has blown in fiberglass insulation, I pulled some insulation back and checked for a vapor barrier and did not see any, I thought all attic spaces should have vapor barriers before blowing in the Insulation. thank for any Info.

[A.D. Miller]

I recently inspected a home built in 1988 , and has blown in fiberglass insulation, I pulled some insulation back and checked for a vapor barrier and did not see any, I thought all attic spaces should have vapor barriers before blowing in the Insulation. thank for any Info.


N1102.5 Moisture control. The building design shall not create
conditions of accelerated deterioration from moisture condensation.
Above-grade frame walls, floors and ceilings not
ventilated to allow moisture to escape shall be provided with an
approved vapor retarder. The vapor retarder shall be installed
on the warm-in-winter side of the thermal insulation.

Exceptions:

1. In construction where moisture or its freezing will not
damage the materials.
2. Frame walls, floors and ceilings in jurisdictions in
Zones 1, 2, 3, 4A, and 4B. (Crawl space floor vapor
retarders are not exempted.)
3. Where other approved means to avoid condensation
are provided.

In cold climates, warm, moist air inside the building can
migrate through the building envelope and condense
on building surfaces or within building component cavities
as the migrating air is cooled. In warm climates,
moist outside air can migrate into a cooled building and

can also condense on building surfaces. Vapor retarders
can help protect insulation and building materials
from moisture damage, degradation and decay by preventing
the water vapor from entering the external
building envelope component cavities. Inmoderate and
cold climates, the vapor retarder is installed on the
warm-in-winter side of the wall (the interior) to prevent
migration of water vapor from the inside to the exterior.
Although not always required by the code, vapor retarders
in warm, moist climates are sometimes installed on
the warm-in-summer side (the exterior) to prevent inward
migration from the exterior.
The warm-in-winter side is the interior side of the
thermal insulation in the Climate Zones where this applies.
Sheet polyethylene is often used as a vapor retarder.
The perm rating for sheet polyethylene is significantly
below the 1 perm (5.7  10-11 kg/Pa  s m2) required by
the code.
The code requires installation of vapor retarders in all
unventilated framed ceilings, walls and floors. Unventilated
areas are framed cavities without vents or other
openings that allow significant amounts of air to move
freely through the cavity and insulation. For example, a
ceiling vapor retarder is not required in a ventilated attic
where the insulation is exposed to the ventilated space.
Per the definition in Chapter 2, the vapor retarder must
have a perm rating of 1.0 or less when tested with the
desiccant method using Procedure A of ASTM E 96.
The code also allows other approved alternatives to
meet the vapor retarder requirements. A variety of materials
act as a vapor retarder, even if not installed specifically
for that purpose. For example, any material with
an unperforated foil facing, some kinds of foam, some
paints specifically formulated as vapor retarders and
most kraft-faced fiberglass batt insulation would meet
the 1 perm (5.7  10-11 kg/Pa  s m2) requirement in the
code. For unfaced materials, the permeability varies
with material thickness. Most product manufacturers list
a materials perm based on a thickness of 1 inch (25
mm). Increasing the thickness of the material will decrease
the perm. For example, 1 inch (25 mm) of
extruded polystyrene, sheet foam product used for insulation,
is about 1.1 perm, not low enough to be a vapor
retarder by itself; however, increasing the thickness
to 2 inches (51 mm) decreases the perm to about 0.55
perm, meeting the code requirement for a vapor retarder.
Note that it is possible to get the required perm
rating based on several materials in series. For example,
most floors are constructed so that the floor
achieves the required perm rating without the addition
of materials specifically designated as a vapor retarder.
The code official or other authority having jurisdiction
must approve this alternative based on reason of accepted
principles or tests by nationally recognized organizations.
The use of the exceptions needs to be evaluated on a
case by case basis. Although the use of exception 3 will
require review and approval of the proposed system by
the code official, exception 2 will be a decision left to the
designer. Although exception 2 does permit the elimination
of the vapor retarder, the designer could still elect to
include one or could elect to locate it at other than the
warm-in-winter side. Such decisions must be made
based on the historical experience in that location and
consider the specific materials of construction. Exception
1 allows exempting exterior envelopes constructed
from materials that are not damaged by moisture or
freezing from the vapor retarder requirement in any
Climate Zone. This exception should probably be reviewed
and determined applicable by both the designer
and the building official before deciding that it is applicable.
Even when using the exceptions, the key principle
which is stated in the first sentence of the section
should always be considered.

[kenny martin]

In our area as long as it is R-25 or greater there is no need for vapor barrier if I remember correctly, (sometimes I don't). I simply look for it, if it's not there look for evidence of any issue with it (usually there is not, if there is going to be it will likely be around penetrations through the drywall, light fixtures etc. where heat escapes) and document what I found and saw and tell them to monitor or if they are overly concerned people recommended infrared by qualified tech.. hope this helps.. Kenny

[John Kogel]

If it's an old house, paint is supposed to help a lot.

Energy Savers: Vapor Barriers or Vapor Diffusion Retarders (http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810)

"Vapor barrier" paints are also an effective option for colder climates. If the perm rating of the paint is not indicated on the label, find the paint formula. The paint formula usually indicates the percent of pigment. To be a good vapor diffusion retarder, it should consist of a relatively high percent of solids and thickness in application. Glossy paints are generally more effective vapor diffusion retarders than flat paints, and acrylic paints are generally better than latex paints. When in doubt, apply more coats of paint. It's best to use paint labeled as a vapor diffusion retarder and follow the directions for applying it."

Where I live, if it's a newer house, no mercy, it's got to have a poly vapour barrier with taped seams. We live and breath mould spores up here.

[Michael Thomas]

In our area as long as it is R-25 or greater there is no need for vapor barrier if I remember correctly...

Does anyone have a cite for this code or standard?

Thanks.

[Robert Hronek]

In most parts of the US poly is not needed. Only extreme climates will need it.

In an attic as long as there is sufficient insulation the building material will not be below the condensing point- the ceiling and rafter will be on the warm side of the insulation. The attic should be able to vent any moisture. If not there is inadequate venting.

Vapor diffusion through the material is very small and is not a problem. Most moisture is the result of air leaks. Generally there are so many penetrations in poly that is does do much.