Vinyl windows, also known as PVC (polyvinyl chloride) windows are a ubiquitous part of the residential window landscape, and not only that, there are a lot of them out there.

There are many satisfied vinyl window customers and I have seen many good installations, however there can also be problems with vinyl windows. Some problems are common to all windows, others stem from the unique properties of PVC. Some problems occur in the window itself, some come from the way windows are joined to one another or to adjacent materials.

What is PVC / Vinyl?
PVC is a thermoplastic polymer constructed of repeating vinyl groups (ethenyls) having one of their hydrogens replaced with a chloride group. That’s it! Grab a tweezers and make some yourself. Seriously, folks, PVC is the third most widely produced plastic and is expected to exceed 40 million tons by 2016.

PCV is made up of 57% chlorine by weight. The remainder is the petrochemical ethylene. Windows and plumbing pipes are made from rigid, or unplasticized PVC, also called “rigid pvc” or “uPVC”. Shower curtains, IV bags and sex toys are made from plasticized PVC due to their need for flexibility.

Properties of PVC
The good news about vinyl as a window material

Section through vinyl double hung sill. (1)frame jamb (2) insulated glass (3) operable sash (4) lift rail (5) sill insert (6) frame sill (7) insect screen (8) screen frame (9) screen track.

• Generally, PVC has better resistance to thermal conductivity than thermally broken aluminum.
• The color of the product is inherent in the material, so if the window frame is scratched, you still see the same color. This is great if you like white windows because the vast majority of vinyl windows are white.
• PVC is a thermoplastic and can be fused at frame corners. If the frame extrusions are designed so that sill, jamb and head are similar to one another, they can be mitered and fused. First the parts are cut at a miter (45 degrees), then the cut ends are pressed against a hot metal plate to melt the ends. Then the ends are pushed together to form a (usually) watertight corner. The aluminum and wood window guys haven’t figured out how to melt and fuse corners with their materials yet.

Properties of PVC
The bad news about vinyl as a window material

• PVC is not as rigid as wood or aluminum. Generally, PVC is used only for relatively small windows as a result. It can be fitted with internal metal stiffeners.
• PVC is a thermoplastic with a relatively low melting point, about 212°F (100 °C). It begins to soften 149°F (65°C). If it heats up too much, it can bend and then “take a set”.
• PVC has a greater coefficient of thermal expansion than does wood, aluminum or glass. This means it shrinks and grows more with changes in temperature. One of the worst areas where this can have an effect is at the glazing area, where the glass engages into the sash or frame. Glazing sealants can shear because the PVC is moving against the relatively stable glass. Leaks around the glass can result.
• PVC is a little tricky for sealant adhesion – sometimes caulk doesn’t stick to it. When working with any substrates, and especially PVC, you must do adhesion tests two weeks before doing the work.

Problems of Vinyl Windows
Here are some problems I have seen with vinyl windows.The numbering does not reflect frequency of the problem, but rather an attempt at creating the illusion of organization.

1. Water leaks through frame joinery
Back in the day before vinyl window manufacturers found out how well the fused corners worked, they simply screwed and sealed the frame corners together. To make things worse, the corner joinery was in some cases not conducive to effective sealing. Add in the problems with sealant adhesion, and you have a leak at the bottom of the window, allowing water into the building.

Water damage beneath leaking vinyl window window. Leaks rusted out the steel opening liner (left), went on to attack the wood (right), ran down inside the wall and destroyed a wood floor.

Top left: Two older style vinyl windows being pulled apart at the stacking mullion. The joinery is "straight cut and screwed" and is prone to leakage. Top right: With the jamb removed, the end of the sill is visible. This is where a critical, too-difficult seal is needed. Bottom: Much better detail where similar frame members are fused together.

Because vinyl changes in dimension with changes in temperature much more than glass, long term movement can shear glazing sealants and bring about glazing leaks

Left: Vinyl window mullion is split open revealing an aluminum stiffener and an absence of sealant between the H-section and windows. This assembly leaked. Right: Looking down on the top of a pair of windows mulled together. New assembly has sealant present between H-section mullion and adjacent windows at exterior plane (inset-sealant location indicated by arrows)

Gaps between brick mould and window must be sealed continuously

When vinyl windows are retrofitted, thin aluminum sheet metal known as capping is formed on site and applied to the perimeter of the window. Left: Capping peeled away to reveal wood far behind so that capping is not supported. This capping was held in place with a face-nail. Right: New capping being installed so that its contour is visible. Outward bend in capping makes a good caulk joint design very difficult.

Too often, installers do not understand that water must drain from the top of the steel lintel - the angle that holds up the brick above. So, they clad the lintel with capping so that water goes into the window system, where it is uncontrolled and eventually becomes a rusty leak. At left, the installer is removing the flimsy capping. At right, steel lintel is revealed. Caulk joint above the lintel on the brick confirms that drainage area of lintel was buried beneath the capping.

The meeting rail of a vinyl double hung window bows in excess of 1/16" along a distance of only 18 inches. This amount of bowing is visible even without a straight edge.

A bowed meeting rail is not just a visual issue. It results in a misalignment between the lower and upper sash. In this picture, the misalignment is fully 5/32" (4mm). The weatherstrip on the lower sash no longer weathers against the upper sash, allowing excessive air infiltration and condensation on the glass

5. Poor component tolerances
In vinyl windows, just like any other manufactured product, the parts need to fit. Sadly, this does not always happen. Window manufacturers use “cutting formulas” to tell them the cut length of each part of a window, given the window’s overall size. In a simple double hung window there can easily be almost 30 parts that all must be cut to the proper length in order to work well. Sometimes there can be an error in the cutting formula. If assemblers on the plant floor are not vigilant, poorly fitting components can be released to the public, resulting in degraded quality and reduced performance.

The horizontal parts on this lower sash are too short, resulting in a large gap between the side of the sash and the adjacent frame. The weatherstrip is supposed to be compressed 30%. Instead it is uncompressed and has a nearly 1/8" gap besides. This is an obvious air leaker.

6. Poor serviceability
Double hung windows utilize a pair of spring-loaded balances that assist in reducing the amount of force need to raise the sash, and to hold the sash in its raised position. It is not uncommon for this balance to fail and require replacement. Sash balances should be easily replaceable, but on one of the jobs pictured here, one would need to cut the frame in order to replace the balance.

7. Insulated glass seal failure
Seal failures occur in vinyl windows, just as they occur in wood and aluminum windows. Often, this is due to the long term presence of water in the area beneath the glass.

Insulated glass seal failure in vinyl window

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