More and more people have domestic power washers, allowing them to quickly clean all kinds of things around the house. Of course one of the first things we want to use it for is to clean off the siding on the house. CAUTION -- you all know that you need to be careful not to damage wood or spray hard enough to remove paint, but did you know that if you get soap inside the wall, it could destroy the wall?
All siding has some kind of provisions for venting and draining to the outside, like the little holes in the underside of the profile of aluminium and vinyl siding. If you spray into these holes, or shingle overlaps, you can get water into the wall. A little water is actually not a problem because the building paper or house wrap under the siding should protect the wood and the insulation, causing the water to simply drain out the bottom of the siding.
Building papers and house wraps have the very special qualities of being 'waterproof' but 'breathable'. Waterproof means that liquid water will not go through, breathable means that vapour can get through. This makes them like one way valves -- moisture can escape from the wall to the outdoors but rainwater cannot get in -- helping to dry out walls and keep them dry. The photo shows one way to illustrate this. Two bottles of water are filled to the same height. One is covered tightly with a house wrap and the other with a polyethylene plastic, like we use for the vapour barrier inside the house. Put both under a heat lamp for a day or so, and you will see that one will get wet and drip inside but no water will evaporate, the other will slowly dry up because the moisture is filtering through the building paper.
So our building paper is waterproof -- UNLESS YOU PUT SOAP ON IT. This photo shows two containers covered with house wrap. Water is put in one and soapy water in the other. The clear water beads up around the edge and does not drip through, in fact it will dry up before dripping into the container. The soapy water removes the surface tension from the water, that is what soap is supposed to do and that is how it cleans things. It does not bead up but flows out on the building paper, in fact, it drips right through. Soap on building paper and house wraps removes their quality of being waterproof -- their reason for being.
When you use a power washer, if you spray soap behind the siding by shooting into those vent holes, you can destroy the waterproof nature of the building paper, perhaps forever because you never get enough water back there to rinse the soap off. The soap residue will be there to help water through whenever you have any rain driven leak in the siding. The second line of defence against rot problems is gone -- because you cleaned the wall with the wrong technique.
It is best to not use soap at all on a wall just to avoid potential problems, but if you must use soap, spray it downhill or at least horizontally. Totally avoid spraying soapy water up into those little vent holes. It is much more difficult to clean a wall this way, but if you don't, you may be setting your house up for water damage inside the walls later.
In the same vane, anyone who adds soap to stucco to make it work better is destroying the waterproof nature of the building paper behind the stucco. There exist special chemicals to make stucco work better. The often used home remedy of adding soap can prove to be an expensive short cut.
Click here for more information on How to clean aluminium siding.