This week we’ll talk in more detail about a rainscreen, which is a moisture control measure that can be used in new construction. I briefly introduced you to rain screens in episode 134 called "Learn to Control One of Your Home’s Biggest Enemies: Moisture". But in this week mini lesson, we’ll have a more in depth discussion of rainscreens and talk about when they are recommended. But before we get to that, let’s go over a Pro Term.
Pro Term: Flashing:
Flashing is a thin layer of waterproof material that keeps water from getting into places it doesn't belong. Flashing material is placed at openings, intersections and crevices in the home’s building envelope—places like roof valleys, pipes openings, and around windows and doors. Flashing is installed to prevent the passage of water into the structure of the house.
Alright, let’s move on to our mini lesson.
A rainscreen is not a tangible screen at all. Instead, it’s a space— an air gap between the exterior siding and the water-resistant barrier, such as housewrap, Zip system or asphalt felt. Remember that housewrap is placed directly on your structural OSB or plywood sheathing to protect the sheathing from water damage. The Zip system is a two-in-one product. It is OSB that already has the water resistant barrier attached to it. The rainscreen works along with the water resistant barrier to protect the home's frame from water damage.
When wind driven rain, and other water or moisture, get behind your brick, stone, stucco or fiber cement siding, the rainscreen provides a drainage and ventilation space. It’s a space that lets any water and moisture that gets behind the exterior cladding, drip down or evaporate. The rain screen is added protection from wall rot.
Creating the rainscreen with vertical furring strips is one of the most common ways to establish a gap between the exterior siding and the water resistant barrier and sheathing. Ok pop quiz...
Do you remember what furring strips are? We talked about furring way back inepisode 15 called “Size Matters… but so does climate. Building a house for a hot or cold climate.”
Well, furring strips are thin strips of wood, metal, plastic or other material that are fastened to a wall or other surface. Exterior siding or wall panels can be attached to furring strips. Furring is used for several reasons 1) to make an uneven surface level, 2) to make space for insulation, 3) to make a wall look thicker, or 4) to form an air space between one surface and another to prevent dampness. In other words, to form a rain screen
Just a couple of decades ago, very few residential builders even knew what a rainscreen was. But it’s not usual for today’s well informed builders to add a rainscreen behind exterior siding (and, to be clear, when I say siding, I don’t just mean fiber cement siding like Hardie board.
I mean most exterior siding, including brick, stucco and stone.). Vinyl siding usually doesn’t need a rainscreen added. More about that in a minute.
A well-designed rainscreen system needs 4 things:
1. water-resistive barrier to keep water away from the OSB or plywood sheathing
2. air gap between the water resistant barrier-covered sheathing and the back of the
3. flashings at penetrations and intersections
4. weep holes at the bottom of the wall so water can drain out.
An optional fifth element can also be added to create a rain screen. That is
5. optional ventilation openings at the top of the wall
Weep holes at the bottom of the wall are essential, but ventilation openings at the top of the wall are not necessary to control moisture. If, however, there are ventilation openings at the top of the rainscreen gap, the rainscreen has additional exposure to moving air, and that will provide even greater ventilation, and theoretically, accelerated evaporation of any unwanted moisture.
So, the reason you might want a rainscreen on your new house is to help manage moisture and protect our house from water damage. A rainscreen helps to keep the sheathing and the back of exterior siding dry when they get soaked with rain. The rain screen gap also allows moisture to evaporate.
By letting liquid water escape by dripping down through the gap and weep holes, and letting moisture vapor escape by evaporating, a rainscreen provides an escape route for water and moisture and limits potential water damage, such as rot and mold.
How do you decide whether your house needs a rainscreen gap?
Does every house need a rainscreen?
There are several factors to consider when deciding whether to have a rainscreen installed. First and foremost is the amount of rain your region typically gets.
In some areas, rainy and coastal areas, for example, rainscreen installations are required by the local building code. The more rain your climate gets, the more helpful a rain screen will be.
Some experts recommend rainscreen installations for areas with an average annual rainfall of more than 60 inches. In areas with rainfall in the range of 20 to 60 inches per year, a rainscreen may not be as essential, but it’s still a good idea. You can use Google to find out how much rainfall your region gets.
Here are some other circumstances that may make a rainscreen worth the investment.
1. Tall walls. If your house will have tall 2 or 3 story walls and a roof with little overhang, your house is more vulnerable to wind-driven rain and a rainscreen is recommended. For short one story walls that are protected by a wide roof overhang or a covered porch, a rainscreen is probably not necessary.
2. OSB sheathing. Since OSB is more susceptible to water damage than plywood sheathing, some experts recommend a rainscreen for houses built with OSB.
3. If your walls are made of structural insulated panel (SIPs), or if your OSB wall sheathing has closed-cell spray foam on the interior, experts say it’s essential to include a rainscreen because the insulation won’t allow the OSB on these walls to dry to the interior.
4. Brick. Brick veneer installations always require a rainscreen gap.
5. Stucco. Stucco siding in a humid or rainy climates would also benefit from a rain screen.
And what about vinyl siding?
Vinyl siding usually does not require a rainscreen. That’s because vinyl siding already has an air space behind it unless it is installed on top of rigid foam insulation. If vinyl siding is installed over rigid foam insulation that is more than 2 inches thick, a rain screen should be added.
Whether specifically recommended or not, a rainscreen gap is always beneficial because of its ability to control moisture. The only drawbacks to a rainscreen are the extra time, effort and money associated with its installation.
And a rainscreen gap doesn’t have to be very big. Researchers have learned that even a 1/16 inch gap helps control moisture, but gaps measuring 1/4 inch to 3/4 inch are better.
Some experts worry that puckers in the housewrap or waviness in the sheathing can narrow a gap that starts off as 1/4-inch. So, to be cautious, a deeper gap, measuring 3/8 inch, 1/2 inch, or 3/4 inch is most often recommended.
A 3/8 inch gap is commonly used since it’s more than wide enough for adequate drainage and ventilation, even if there is some puckering in the house wrap.
Wetter climates tend to benefit from deeper rain screen gaps, up to 3/4 inch deep. Deeper gaps allow greater ventilation and faster drying. But, deeper gaps also make it more difficult for subs to install flashing and exterior trim, so only use those deep rainscreens if you live in a really rainy climate.
There are three ways to create a rainscreen gap:
1. With Furring strips, usually attached vertically over wall studs
2. With Three-dimensional plastic mesh or mat products.
3. With Bumpy or wrinkled housewrap.
Bumpy or wrinkled housewrap may be less effective than the furring strips and plastic mesh and mat options. That’s because those bumps and wrinkles are not very deep, and they don’t provide much of a gap. Some bumpy housewraps have wrinkles that are less than
20 thousandths of an inch deep. Many experts wonder whether such tiny gaps would have much effect.
Three-dimensional plastic mesh and plastic mat products are good options, but they can be more expensive than the other options.
Good, old school furring strips may give you the biggest bang for your buck, but do some price comparisons before you decide what rainscreen options are best for you.
If you think a rainscreen is right for your house, make sure you talk to your contractor about also installing an insect screen at gaps the bottom and top of your walls. This insect screen will keep critters from using your rainscreen space as a home.
Finally, let’s talk about cost.
Of course every house is different, and rainscreen prices will vary depending on the size of the house and the materials used, but many siding contractors say a rainscreen will add an extra 30% to the cost of your siding.
So, that’s rainscreens. Before you go, let’s do a couple of quiz questions
1. True or false. Rainscreens are recommended, and sometimes required by code, in regions that get more than 60 inches of annual rain.
That’s true. If you live an area that gets more than 60 inches of rain per year, you should strongly consider a rain screen, even if it’s not required by code. If your region gets 20-60 inches of rain per year, a rain screen is not a necessity, but it’s still a good idea.
2. True or false. A 3/8 inch rain screen gap is commonly used since it’s wide enough for adequate drainage and ventilation, even if puckering in the house wrap narrows the gap in some places.
That’s true. Wetter regions may require deeper gaps, typically up to 3/4 inch.
Well that’s all I have for this week. I hope you learned as much as I did. Thanks for stopping by.
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