Is Your Home's Siding Waterproof?

If siding contractors are honest, they will tell you that, while many of their products are highly water resistant, there isn't a material today that is truly waterproof. There are some materials that are more moisture resistant than others, but their performance can vary depending upon how it's installed and whether you use additional protection beneath the siding itself.

Vinyl Is Water Resistant But Can Have Problems

While the actual vinyl material is waterproof, the siding isn't entirely waterproof. Because the planks have to overlap each other and lock into place for secure installation, there is always the possibility that some moisture will seep into the space behind the planks in a high wind or at the seams. With proper installation by professional contractors, the overlapping edges of vinyl will naturally channel water away from areas where moisture is prone to pool.

Most brands of vinyl siding also feature weep holes. These are simply very small holes in the bottom edge of each panel projection so that any trapped rain or snow can safely drain through the holes and away from the interior walls. Weep holes also encourage air circulation along the seams so that if some moisture vapor from the interior seeps through, it will quickly dry out.

Fiber Cement Is Highly Water Resistant

While vinyl has been around for decades, fiber cement is a relative newcomer that siding contractors are praising for its superior resistance to the elements, including rain, snow and ice. It is made up primarily of Portland cement with some wood fibers (about 10%) to give it a natural appearance. Because the material is extremely dense and primarily rock and silica, it is practically waterproof, although it is also touted as being water resistant. The individual lengths of these boards are longer than vinyl planks, so there are fewer seams and fewer places for rain or snow to seep under an overlapping edge.

Compared to vinyl, fiber cement is much stronger in general and that strength means it can't be damaged as easily. Many homeowners discover leaks beneath their vinyl after lengths of it have sustained cracks, holes and splits during a storm or by hailstones. Unfortunately, moisture can quickly insinuate itself into the interior walls before you have the chance to do repairs or replacement. On the other hand, fiber cement can withstand winds up to 150 miles an hour, hail and just about anything a storm can dish out without warping, rotting or cracking. There are far fewer opportunities for water to get beneath fiber cement, making it extremely water resistant.

Another advantage of having fiber cement siding is that some fiber cement manufacturers also manufacture their own line of weather barriers and house wraps that balance moisture resistance with breathability. When these barriers are installed beneath the product, they don't allow outside moisture that makes its way beneath the fiber cement to penetrate into the studs of the walls. The breathability of the material, however, allows water vapor formed inside the walls to dry out in the wall cavity. The installation of flashing around windows, doors and edges also prevents moisture damage.

If you have any questions about the relative water resistance of various kinds of siding, talk to some siding contractors in your area. They should be able to help you weigh the pros and cons so that you can find the perfect exterior material for your home.