When Is A House Watertight?

Raindrops

You might think your house only becomes watertight when the siding, brick or stone is completely installed. Actually, sidings of any kind, including masonry and stucco, are not as watertight as you might imagine. They are not the last line of defense against water.

I refer to siding, brick, stone, stucco, and other sidings as the “weatherproofing” of the house. These materials keep the bulk of the wind and water out, but even perfectly installed siding material will still let in small, but significant, amounts of moisture. There is no effective way to stop this and it does not indicate the house is poorly built.

Do you remember the old black tar paper that used to be installed around a house before the siding was installed? That layer is the actual barrier against water infiltration. It is the true waterproofing of a house. These days, tar paper has been replaced with several high-tech House Wraps, like Tyvek and Typar by DuPont, Weathermate by Dow, and PinkWrap by Owens Corning.

Materials such as these have revolutionized house waterproofing. Much more resistant to moisture infiltration than old fashioned tar paper, these materials substantially improve the energy performance of the house, as well. By reducing the air infiltration in exterior frame walls, particularly on windy days, these high-tech house wraps help the building insulation remain effective. All insulation works on the principle of dead air being the actual insulator. The insulation simply keeps the air still so it can insulate. You can imagine that if air is moving around in the wall cavity, the “R” value (insulating value) is going to be lost. The house wraps keep wind from penetrating the walls and compromising the insulation’s effectiveness. House wraps will save you a lot of money in heating and cooling costs.

When the windows and doors of a house are installed, the house wrap is taped to the window and door frames with special tape to insure a tight seal. When this is done and the roof has been installed on the house, the house is watertight, regardless of whether any of the siding has been installed. The siding material is there to protect the house wrap, deflect the heavy weather, and for looks.

If your house is being built in a cold climate and you are worried about brick, stone, or stucco being installed in freezing conditions, you don’t have to delay the start of construction. You can begin the house, provided weather permits the foundation to be built, and let the builder frame everything. He can install the roofing, install the windows, and wrap the house up with one of the house wraps. The house will be watertight and the interior construction can continue. The masonry on the exterior can wait until warmer weather without delaying the rest of the construction.

Bill Hirsch

Bill Hirsch

2 Comments

  1. Anonymous on September 21, 2009 at 9:33 pm

    I don’t quite understand what you mean in this part:

    The insulation simply keeps the air still so it can insulate. You can imagine that if air is moving around in the wall cavity, the “R” value (insulating value) is going to be lost.

    • Bill on September 22, 2009 at 8:50 am

      Good question. I’ll try to explain it more thoroughly. Materials vary in their ability to resist the transfer of heat. Perfectly still, unmoving air is one of the best “materials” for this. However, if the air moves, the insulating effect is lost. This is the principle of double paned glass. There is a thin layer of air trapped between two sheets of glass that resists the transfer of heat from one pane to the other, greatly improving the insulating value of the glass. Heat does not move well through the air. The air space must be thin to keep the air from moving. But if the width of the air space is too much, a convective loop can occur. A convective loop is a flow of air where warm air rises and cooler air falls. In this glass example, the air that is near the warm sheet of glass will warm and rise. The air that’s near the cool sheet of glass will cool and fall. As the warm air moves up the warm glass, the cool air is pulled up from below to replace it. And the warm air that rose gets pulled back down across the face of the cool glass. Although the air is a good insulator, the very thin film of air lying against the glass will transfer heat. So as the loop flow continues, warm air continually loses its heat when it touches the cool glass. This is a little hard to follow without an illustration. I plan to add one in another post explaining how insulation works.

      Traditional insulation materials, like fiberglass and foam don’t insulate because of the inherent properties of the material itself. They insulate because they trap air and don’t let the air move around. A blanket doesn’t keep you warm because of the fibers in the blanket. It keeps you warm because the fibers of the blanket trap a layer of insulating air within the blanket. When wind blows against a house it can enter the wall cavity and push the air around. The air is no longer still and its insulating benefit is lost. This happens with fiberglass and rock wool insulations. Foam insulations are not vulnerable to this effect because the dead air is trapped in the thousands of bubbles in the foam. House wraps keep the wind out of the wall cavity so it can not stir the air in the fiberglass insualtion. That’s how house wraps improve the energy efficiency of your house.

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