Storm window weep holes and why needed (C) Daniel Friedman Inspecting or Installing Storm Window Weep Holes

  • STORM WINDOW WEEP HOLES - CONTENTS: Why are weep openings needed on storm windows. Where & how to make a storm window weep hole. Weep holes on site-built fixed glazing windows. Prevent rot at window sills, frames, and building walls by draining water trapped between window and storm. Guide to energy efficient windows and doors. Guide to diagnosing and curing leaks at basement windows. Window condensation causes & remedies
  • POST a QUESTION or READ FAQs about storm window drainage weep holes

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Window frame or storm window drainage & weep hole requirements:

Storm windows, particularly metal-framed storms and screens are secured to a window opening outdoors using screws through a metal flange.

Many installers and homeowners proceed to make their storm windows as air-tight as possible - which makes sense. But if the weep openings at the bottom storm window flange are sealed by caulk, sealant, or just junk and debris you're asking for trouble.

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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Guide to Weep Holes in Windows, Storm Windows, Window Frames

Window sill rot (C) Daniel Friedman[Click to enlarge any image]

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.


Exterior-mounted storm windows must have "weep holes" at the bottom of the frame to allow any moisture that collects between the primary window and the storm window to drain out. Even though these drainage holes subtract from energy savings, not having them will eventually cause the primary window frame to rot, and possibly make them impossible to operate.

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

Storm window sash in wrong position (C) Daniel FriedmanSo where does this must-drain water between storm window and principal window sash come from?

[Click to enlarge any image]

  1. Condensation between the two windows: if the primary window itself is more leaky than the storm window, interior air and moisture can leak into the interstitial space, accumulating considerable moisture. Condensate may form on the outside surface of the primary sash, or on the inside surface of the storm window sash. It depends.
  2. Condensation indoors on the primary window can also be so extensive that spilling condensate runs down the window sash frame and below the sash into the sill space.
  3. Storm window rain leaks: particularly if someone leaves the storm window upper or lower sash open (a typical triple-track storm window has two movable glass sashes and a screen) wind-blown rain easily enters the window space, forming a little lake between the primary window sash and the storm window frame if that frame is not drained at its bottom.
  4. Storm window sashes in wrong position (photo above): on occasion we find that someone has raised the wrong storm window sash to the uppermost position even though both storm window sections are "closed".

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.

  1. Ice dam leaks into the window space from the roof above. For a dramatic example of ice dam leaks into a window sill

Examples of Storm Window Weep Holes

Window sill rot (C) Daniel Friedman

Our storm window weep opening photo (left) is a close-up of the same window shown at the top of this article.

In this design the manufacturer intends that that outwards vertical "U" shaped bulge or stamping in the aluminum storm window frame (just above the "A" in "InspectAPedia") is intended to drain water out of the sill area between the sash and storm.

But you can also see that over the life of the home it's easy for someone enthusiastically caulking along to spread their sealant right over the outlet intended to drain water.

At this home the outside storm window caulk had cracked away, permitting the interior sill area to drain.

Retrofitting Weep Holes into Caulked Storm Window Frames

Window sill rot (C) Daniel Friedman

This is what we do if the storm windows have been totally caulked along the bottom edge of their frame.

Drilling a pair of 1/4" holes through the aluminum storm window frame bottom vertical edge, working from outside the window (or inside if you're on a higher floor), make each hole about 4" in from the sides of the storm window. In our photo the sill needs painting, but the weep hole is working properly despite the owner-applied caulk.

Watch out: do not drill into the wood of the window sill in either direction - aim your drill so that when it penetrates the aluminum frame into the storm-sash interstitial space, the drill bit is just flush with, not cutting into the window sill surface.

That way water trapped in that space can drain out readily, but you haven't compromised the wood. We have seen weep hole attempts that drilled right through the wooden sill - leading to rot.

Window sill rot (C) Daniel Friedman

If the weep hole you drilled is too large and your window-storm space is being invaded by insects you might want to screen the opening as this owner did.

Watch out: do not use a weep hole screening material that won't readily drain water - doing so will defeat the weep hole.

Photos of Rot & Damage from Missing or Clogged Storm Window Weep Holes

Window sill rot (C) Daniel Friedman

Here we see what an astute home inspector often finds: a history of leaks rotted the window sill, no one understood why, and the rot was "patched" with wood filler and paint, but the damage just continued.

This particular window was installed on a silo converted to living space at a country home. Poor window installation permitted water to leak into the space between window sash and storm window.

Window sill rot (C) Daniel Friedman

Our window rot photo (above left) shows severe window sill rot found at a home inspection back in 1993.

Water entered the space between the main window sash and the storm.

Water was trapped between the window sash and the storm window frame where it sat until the sill became so rotted that water leaked into the wall cavity below.

How to Avoid Leaks and Rot Damage at Aluminum Wrapped Window Sills & Trim

Window casing details (C) Daniel Friedman

This photo shows that it's possible to cover up rotted window sills and trim with aluminum wrap-around trim.

But if the window structure is still trapping water between sash and storm, water leaks into the aluminum-wrapped space, and rot or perhaps insect damage continue to increase.

Window casing details (C) Daniel Friedman

Careful detailing in installing the aluminum window trim wrap combined with proper sealing can reduce the outside sources of water leaks and rot, but if the storm window lacks weep openings that drain outside and on to the upper surface (not below) the new aluminum trim wrap, you're going to have trouble.

Our photo shows (above our pen) an extra piece of aluminum stock that was tacked onto the window sill before we completed storm window installations on this home. We added this upper aluminum covering because in his first attempt our contractor wrapped a piece of aluminum just big enough to pass under the storm window frame but not under the bottom edge of the primary window sash.

That mistake meant that any water entering the space between storm and sash would run under rather than over the aluminum stock, rotting our window.

By adding the upper piece of aluminum (we also sealed the nail heads), water in this space will be conducted safely outside when the storm window is added.

Double hung window (C) Daniel Friedman

Why Install Storm Windows?


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


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.


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