Download Controlling Moisture.

Controlling Moisture.
Norbord.com/NA
Controlling Against Moisture Damage For Wall Sheathing . . . . . . . . . . . . . . . . . . 3
Sweaty Vents. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Controlling Condensation In The Home . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Norbord.com/NA
Controlling Moisture
Controlling Against Moisture Damage For Wall Sheathing
Moisture can cause many structural problems and
create a very unhealthy home for your family when
mold and mildew begin to form. Building a home that
deals effectively with moisture requires due diligence
from design to installation and on to maintenance. The
home must be built in such a way as to prevent leaks
which are the biggest cause of damage and it must also
eliminate condensation which occurs naturally in every
home.
According to the APA: “Good construction practices
control condensation within the walls and influence the
rate at which water vapor moves around and through wall system components, and mitigate the effects of
humidity and temperature differentials between the inside and outside of the structure.”
All wall sheathing panels already contain some moisture and their moisture content will fluctuate in
accordance with the moisture content of ambient air. They can also get wet when leaks occur in the
building envelope. If wood panels get wet, their structural integrity is not affected. Prolonged exposure
to water will cause damage over time. To prevent damage, take extra care with flashing around windows,
doors, roofs, chimneys and walls.
Wall sheathing panels will also get exposure to moisture when water vapor accumulates within the
wall structure. This happens when air leaks carry moisture-laden air into the wall systems. You can
prevent excessive water vapor damage by minimizing air leaks and reducing the conditions under which
condensation occurs i.e. preventing surfaces from being colder than the dew point.
There are many ways in which to prevent condensation from occurring in your walls. You can reduce air
leaks by up to 60% when you utilize taller wall sheathing panels like Windstorm and TallWall. You can
also utilize a vapor barrier or vapor retarders. From the APA: “The proper placement of vapor retarders
within a wall system will help manage the diffusion of vapor through the wall system. To determine
proper vapor retarder placement and minimize moisture-related problems, the building designer must
consider the interior and exterior environments, as well as the wall’s thermal and water vapor permeance
characteristics.”
The ability of vapor retarders to limit the amount of water vapor that is able to pass into the wall assembly
is measured in perms. Vapor retarders with higher perm values allow more water vapor through. The vapor
barrier perm value is another consideration to take into account when designing wall assemblies that will
effectively deal with moisture.
In section R318.1 of the 2006 IECC code, a vapor barrier of 1.0 perm or less on the side of the wall that
is warm in the winter is required. This does not apply the Southern US states where hot, humid summer
temperatures take precedence.
Norbord.com/NA
3
Controlling Moisture
If you live in a climate where you need vapor barriers on different sides of the wall to correspond to the
season, use OSB wall sheathing panels. OSB wall sheathing panels are ‘smart’ vapor barriers as their
permeance is low when relative humidity is low, but it increases as relative humidity increases in the
warmer months. This allows for greater air movement which enables the wall to dry faster.
Avoid creating impermeable vapor barriers on both sides of the wall as this will inhibit water that inevitably
gets into the wall system from drying.
From the APA: “Wood structural panels are semi-permeable, and are smart vapor barriers because their
permeability is low when the relative humidity is low and increases as the relative humidity increases. By
increasing permeability when the wall humidity is elevated, wood structural panels help aid water vapor
diffusion through exterior walls, mitigating moisture buildup in concealed spaces. This characteristic
makes wood structural panels an important component of a properly designed durable exterior wall
assembly.”
Sweaty Vents
Question
The walls under some of our air vents sweat in the
warmer months causing water marks and streaking. Is
there anything we can do about this?
Answer
The problem is usually caused by air filtration into your
walls. Moist air from outside is leaking in near the vents
and condensing. You can start by sealing all obvious
gaps and cracks, but if the problem persists, it may be
time to check the wall insulation. This will be costly, but it will improve your energy consumption which will
go a long way to mitigating the costs.
Water marks can be wiped away if they are relatively new. If they are stains, you will have to repaint. Use a
primer made to cover stain to prevent streaks from showing through.
Controlling Condensation In The Home
For condensation to occur, all you need is moist air and
a cold surface. In our homes where temperatures are
controlled these conditions can occur in wall, floor and
roofing systems depending on the season. Condensation
is moisture that can lead to a myriad of problems
from water damage to mold. With the tighter building
envelopes of modern structures, this condensation
problem is exacerbated as reduced air flow can prevent
Norbord.com/NA
4
Controlling Moisture
moist areas from drying out. Luckily there are ways in which you can mitigate the conditions that lead
to condensation and keep moisture out of your home. Dealing with condensation requires a threepronged approach: reducing the amount of water vapor in the air, diverting moist air out of the home with
ventilation or using a vapor barrier to prevent moist air from reaching cold surfaces.
Reduce Indoor Moisture
Prevention is better than cure in this case, so it’s best to keep moisture from the home altogether. This
means that indoor pool you’re thinking of including in your next build may not be the best idea. Vent dryers
to the outside rather than to crawl, attic or garage spaces within the home. The same goes for other
moisture producers like bathrooms and stove hoods where moisture should be carried outside rather than
to other areas of the home like attics.
Seal all light fixtures and plumbing to prevent moist air from entering the home or rooms that are not
adequately vented.
Prevent moisture from coming up through the ground with a vapor-barrier ground cover. Start by sloping
the ground away from the home to move moisture away from the foundations. Homes with crawl spaces
can be adequately protected with a 6mm polyethylene layer. In homes with basements, vapor-retarder
barriers must be installed beneath the basement floor and on the exterior of foundation walls.
Walls can be protected with a vapor barrier and the sealing of any fixtures, wiring or plumbing. Vapour
barriers should be installed near the warm winter side of the wall to prevent water from penetrating the
wall where it will condense and cause problems down the line.
Proper Ventilation
In homes with a tight building envelope, adequate ventilation is key to preventing moisture and ensuring
good indoor air quality. Consult your local building codes for the ventilation requirements in your area,
manufacturer’s guidelines and design professional’s recommendations when determining the best
ventilation systems for your build.
While building codes denote the minimum ventilation requirements for your area, additional ventilation
may be required if you put that indoor pool in after all, if you have a sauna or hot tub, or where buildings
have large kitchens or laundry facilities. In this case, refer to the recommendations of your design
professional and manufacturer.
Ventilation will also differ from one area of the house to the next. In attics and cathedral ceilings, a
combination of ventilation and vapor retarders will help to keep moisture out.
Modern homes with basements will usually come standard with a ventilation system where forced-heating
systems are utilized. When heating systems (like baseboard heaters) are used which don’t circulate air,
separate ventilation systems will have to be considered.
Norbord.com/NA
5