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Window frame or storm window drainage & weep hole requirements: this article series explains why weep holes or drainage are needed in storm windows and in some site-built fixed glazed windows.
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Also our home page about windows: WINDOWS & DOORS and see AIR LEAK MINIMIZATION and AIR SEALING STRATEGIES as well as ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE. We also discuss window weep holes for site built windows at SITE BUILT DOUBLE GLAZED WINDOWS.
Question: Why do we need those open air weep holes in storm windows?
One thing that seems bizarre to me is status/need for open air weep holes. Would think they’d reduce air sealing quite a bit. Am surprised no one uses a semi-permeable membrane covering larger weep holes. - G.K., New York
Reply: To avoid rotted window sills, walls, insects, mold
Storm windows are additional windows, fixed or operable, that are hung or installed over the main window sash to reduce heat loss through the window. A "triple-track" storm window incorporates a movable screen and upper and lower widow sashes. Each layer of glazing added to a window cuts heat loss through the window glass by about one third, but if the window is drafty any energy savings will be lost until the drafts are found and sealed.
All factory-built storm window frames will include some sort of weep opening to make certain that any water entering the space between window sash and storm sash can drain safely to the building exterior. But unfortunately folks who don't recognize what these openings are, or even that they are present, often seal them with caulk. The ultimate result is window sill rot and in severe cases wall rot, insect damage, and mold contamination of the wall cavity below.
At WINDOW / DOOR ENERGY EFFICIENT, DOE we read:
Our OPINION is that the energy lost through two tiny weep holes in a storm window bottom frame is trivial compared with the energy savings from adding this additional layer of glazing and stopping outdoor air from blowing across the primary window sash glass. And we're afraid that the permeable membrane you suggest won't adequately pass the large volume of water that is often found in the space between main sash and storm window bottom frame.
How Water Gets Between the Window Sash and Storm Window
The outermost storm window is the one that should be in the fully "up" position, and the innermost storm window (innermost means towards the building interior) should be the one left in the "down" position when the storm window is closed. If you do this backwards rain will run down the sash in the upper position and pass onto the inside surface of the lower storm sash, making another sash lake. We mean "lake" too, not just "leak". Our photo (above) shows how water will pass down the upper sash and behind the lower sash into the window interior space.
Examples of Storm Window Weep Holes
Retrofitting Weep Holes into Caulked Storm Window Frames
Photos of Rot & Damage from Missing or Clogged Storm Window Weep Holes
How to Avoid Leaks and Rot Damage at Aluminum Wrapped Window Sills & Trim
Why Install Storm Windows?
Quoting from WINDOW / DOOR ENERGY EFFICIENT, DOE
Add Storm Windows
If you have old windows in your home, the best way to improve your home's energy efficiency is to replace them with new, energy-efficient windows. However, if you're on a tight budget, a less expensive option is to use storm windows. Some types of storm windows are also a good option for those living in apartments.
Even though storm windows add little to the insulating performance of single-glazed windows (that are in good condition,) field studies have found that they can help to reduce air movement into and out of existing windows. Therefore, they help reduce heating and cooling costs.
OPINION: the DOE comments above may underestimate the benefit of storm windows in areas of high winter winds. Even without significantly increasing the insulating value of the primary sash, by stopping cold winds from blowing directly across the primary sash glass, a storm window should cut heat loss through the window opening by 1/3.
Watch out: if you don't find and stop air leaks in the building first, the benefit of adding storm windows (as well as other energy saving steps such as adding insulation) may not be realized. See ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE in the article series listed below.
ENERGY SAVINGS in buildings
Types of Storm Windows
Storm windows are available for most types of windows. They can be installed on the interior or exterior of the primary window. They range from the inexpensive plastic sheets or films designed for one heating season, to triple-track glass units with low-emissivity coatings that offer many years of use. Mid-priced storm windows may use glass, plastic panels, or special plastic sheets that have specific optical qualities. Those made of polycarbonate plastic or laminated glass also offer a high degree of resistance to breaking during storms and/or from intruders.
For the most part, interior storm windows offer greater convenience than exterior storm windows. They're easier to install and remove; they require less maintenance because they're not exposed to the elements; and, because they seal tightly to the primary window, they're more effective at reducing air infiltration. Interior storm windows also are often the best choice for apartments and houses with more than one floor. If you can afford exterior storm windows, you can probably afford some newer, more energy-efficient windows, which will be a better investment.
Glass pane types offer better visibility and longer life than plastic pane types, but glass is heavy and fragile. In general, plastics are most economical for people with small budgets or who live in apartments. However, while inexpensive and relatively easy to install, they are easy to damage. Plastic panels, such as Plexiglas and acrylics are tougher and lighter than glass, but may scratch easily. Some may turn yellow over time as well. Some plastic films may significantly reduce visibility and degrade over time when exposed to sunlight.
Wood, aluminum, and vinyl are the most common storm window frame materials. There are advantages and disadvantages to all types of frame materials. Although very strong, light, and almost maintenance free, aluminum frames conduct heat very rapidly. Because of this, aluminum makes a very poor insulating material.
Wood frames insulate well, but they weather with age. They also expand and contract according to weather conditions. Wood-frame storm windows installed during the winter may not close easily during the summer, and those installed during the summer may fit loosely in the winter. They can also be quite heavy and thicker than metal frames. This can make storage difficult, reduce the view out the window, and reduce the amount of natural light in the room. Wood frames also require the most maintenance. There are, however, aluminum- or vinyl-clad wood frames that reduce maintenance requirements.
Vinyl frames are usually made of polyvinyl chloride (PVC) with ultraviolet light (UV) stabilizers to keep sunlight from breaking down the material. They, however, may expand and warp at high temperatures, and crack in extremely low temperatures. Also, if sunlight hits the material for many hours a day, colors other than white will tend to fade over time.
How to inspect, diagnose, and repair skylights, sloped glazing, vertical glazing, energy-losing windows, and leaky basement windows
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