First, stucco is a cementatious material. In other words its basic component is cement. If you think of a concrete driveway or sidewalk, which are also made from cement, you see that they have very good qualities. Strong, resistant to damage, long lasting.

As with any cement product however, there is one challenging characteristic. That is it absorbs water like a sponge. This is because of its makeup. As concrete cures, the water in it rises and evaporates leaving small pours or channels. Stucco does much the same.

So you have a material, which absorbs moisture that comes into contact with it. Meaning it is not waterproof and must be used in conjunction with a system that allows drainage from behind as well as drying.

I believe that I mentioned the 4-D’s of protecting a house from moisture damage in an earlier article. Deflection, Drainage, Drying or use of Durable (non-rotting) materials. Stucco is a product that relies on all these components since it allows water behind it. As an example, stucco will Deflect about 92-96 per cent of rainwater that hits the surface. That means between 4-8 per cent will get through the surface. This must be Drained.

However, since you attach the materials using nails and staples, some of this moisture will end up inside your wall, meaning that you need some Drying to occur.

So that’s the stucco portion. Now you can see how important the role of the paper behind the stucco becomes. First, it must be properly installed or lapped, to allow moisture to be transported down. If the paper is lapped incorrectly or not enough, moisture can be directed into the building.

What about the kind of paper used? Should it be single or double layered? Well, you will determine that by what you are willing to pay for. You see a builder will try to use the best material for the least cost, since it’s the buyers that may or may not be willing to pay for these items.

Here’s the difference. Ordinary tarpapers are what we call water resistant. They will absorb moisture if saturated long enough. Therefore you have papers that have a 20, 30 and even 60 minute ratings.  They have very good ability to dry since they “breath” or allow water vapour to pass through them. Incidentally we want anything we use on the outside to breath. And since Alberta has fairly low humidity, the more the exterior papers breath, the better.

House wraps such as Weathermate, Typar, Tyvek, GreenGuard and so on, are a little different. These are non-absorbing, meaning they are more waterproof. And since they come in such large rolls, usually
nine feet tall, they have very few joints. That means they are more airtight which also means better energy efficiency, since they don’t let the air blow through into your walls making them cold.

Tarpapers come in a 36 inch rolls, so they can’t stop the air as well. This is where you can get into a real debate if you start to examine the physics that take place with pressure differentials and the like. Suffice it to say, each material performs differently and has its own unique benefits.

For paper, here are some things to consider:
• House wraps stop air better and are more energy efficient.
• Two layers are better than one.
• First, this allows one layer to act as an air barrier, the other can act as a drainage material.
• Second, if delayed, the outer layer protects the inner layer from breakdown from weather and sunlight.

There must be assurance that there is no bonding of the stucco to the paper used, since that limits the drainage.

Finally, stucco must be allowed to drain any and all moisture that can accumulate behind it, down and out. That means the use of the appropriate materials That means wherever you have batten work, trims, decks, parging, anything that could stop the stucco from draining, that there is a proper detail and flashing allowing that drainage to take place.

Stucco can be a beautiful finish and its long lasting. It also adds value to your home and increases your property value. We’ve used it successfully for years, we just need to use it properly and that’s the trick.

Here are a few resources for stucco
www.stuccoguru.com
www.imascominerals.com
www.nwcb.org/stucco.php

Darrell Paul, Building Consultant, has dedicated his entire career to the residential building industry. He is president of Building Science Resources, is a director with the Professional Home Builders Institute of Alberta (PHBIA) and is an accomplished speaker/presenter in addition to instructing home building and building science courses with SAIT.