What are capillary tubes and why do windows at altitude have them?

Small breather tubes that equalize the pressure between the sealed space in an insulated glass unit and the outside air. A unit assembled near sea level and shipped to high elevation experiences a pressure differential that can bow or stress the glass. Manufacturers either fit capillary tubes or build the unit for the destination altitude.

Is triple glazing worth it in Colorado?

Sometimes. It improves U-factor and reduces the cold surface effect near large glass, which is a comfort gain more than an energy one. It costs more, weighs more, and slightly reduces visible light. On a high-performance home at elevation it is defensible. On a Front Range production home it is usually not the best place for that money.

What does a low-E coating do?

A microscopically thin metallic layer that reflects long-wave infrared while admitting visible light. Different coatings are tuned to admit or reject solar gain, which is how two windows with similar U-factors can have very different SHGC values.

Should window specification match each side of the home?

Yes, and it is one of the higher-return decisions available. South glass specified to admit winter sun with proper overhang, and west glass specified to reject afternoon summer heat, is a meaningful comfort difference at little or no added cost.

Specifying Windows for Colorado Homes at Altitude

In short

Window performance is set by two numbers, U-factor for heat transfer and solar heat gain coefficient for how much sun energy passes through. In Colorado the correct SHGC differs by elevation of the wall, so a single package for the whole home is almost always wrong, and units must be built for altitude.

Windows are the weakest part of any wall. Even a very good window underperforms the insulated wall around it by a wide margin, which is why the specification deserves more thought than it usually gets.

In Colorado there are two additional wrinkles. The altitude affects the units physically, and the sun intensity makes orientation matter more than it does at lower elevations with hazier skies.

The two numbers

U-factor measures how readily heat passes through the assembly. Lower is better. It covers the whole unit, glass and frame together, which matters because a good glass package in a poor frame does not perform the way the glass number suggests.

Solar heat gain coefficient measures the fraction of solar energy that passes through. It runs from 0 to 1, and unlike U-factor there is no universally better direction. High SHGC admits solar heat. Low SHGC rejects it. Which you want depends on the wall it is sitting in.

Both are certified by the National Fenestration Rating Council and appear on a label on the unit. Ask for the numbers rather than accepting a description like “energy efficient,” which means nothing.

One package for the whole home is wrong

This is the most common specification error and it is free to fix.

A builder orders one window package. Same glass, every elevation. It is simpler to order, simpler to install, and it produces a home that is uncomfortable in predictable places.

Consider what each elevation experiences in Colorado.

South. Winter sun is low and comes in deep. High SHGC glass on south walls admits genuinely useful heat in December and January, when you want it. In June the sun is high, and a modest overhang shades that glass almost entirely. The geometry does the switching for you. This is passive solar in its least exotic form and it works.

West. Afternoon sun in July and August arrives low and hot, and no reasonable overhang shades it because it is coming in nearly horizontally. West glass wants low SHGC. This is where most overheating complaints originate.

East. Morning sun, less intense, generally welcome. Moderate SHGC is fine.

North. Almost no direct gain. SHGC barely matters. Optimize U-factor, because north glass is losing heat and gaining little.

Specifying by elevation costs little or nothing on most orders and produces a home that is comfortable in August without working the air conditioning against its own windows.

The altitude issue

An insulated glass unit is sealed with gas between the panes at the pressure where it was assembled. Move it up several thousand feet and the pressure outside drops while the pressure inside does not.

The result is stress on the seal and visible bowing of the glass. Manufacturers handle it either by fitting capillary tubes, which are small breather tubes that let the unit equalize, or by building the unit for the destination elevation.

This is routine for manufacturers and distributors who serve Colorado. It is not routine for someone ordering from a supplier who mostly ships to sea level. Confirm in writing that the units are specified for your elevation.

A side note on capillary tubes: they equalize pressure, which means the unit is not fully sealed, which means it cannot hold an argon fill as reliably over the long term. That is a real tradeoff and a reason to prefer units manufactured for altitude where the option exists.

Ultraviolet exposure

At altitude there is less atmosphere between the sun and the building, and ultraviolet intensity is correspondingly higher.

For windows this shows up in two places. Low-E coatings reduce UV transmission, which protects flooring, furniture, and artwork from fading. And exterior sealants around the units degrade faster here than the manufacturer’s general-purpose rating suggests. Specify sealants rated for high UV exposure, and expect a maintenance cycle rather than a lifetime.

Frames

Frame material affects U-factor substantially and it affects durability at altitude.

Vinyl. Inexpensive, good thermal performance, and it expands and contracts more than other materials. In Colorado’s wide daily temperature swings and high UV, quality varies more than the price difference suggests.

Fiberglass. Expands at close to the same rate as glass, which is good for seal longevity. Strong thermal performance. Costs more.

Wood clad. Wood interior with aluminum or fiberglass exterior. Good performance, attractive, and the cladding takes the UV so the wood does not.

Aluminum. Conducts heat readily. Only appropriate with a proper thermal break, and even then it is the weakest performer of the four thermally.

For custom homes at elevation, fiberglass and clad wood hold up best. For production homes on the Front Range, good vinyl is defensible and the money is often better spent on the envelope.

Installation matters as much as the unit

A high-performance window installed badly performs like a cheap one.

The failure modes are air leakage around the perimeter, water intrusion from an improperly integrated flashing detail, and a rough opening that is out of square enough to stress the frame. All three happen on site, after the window arrives, and no spec sheet prevents them.

Ask who is responsible for flashing integration and how the window opening ties into the water resistive barrier. If the answer is vague, that is the answer.

A practical specification

For a Front Range new home, a defensible starting point:

U-factor as low as the budget supports, prioritizing north and any large glass areas
South elevation: higher SHGC, with overhang sized for the latitude
West elevation: low SHGC, no exceptions
East: moderate
North: optimize U-factor, SHGC largely irrelevant
Units built or fitted for elevation, confirmed in writing
High-UV-rated exterior sealants
Flashing integration explicitly assigned to a named trade

None of that is exotic and most of it costs nothing beyond the attention to specify it.

For how windows interact with overall building performance, see the envelope decides everything, and for equipment sizing implications, do heat pumps work in Colorado winters.

Windows, and what to specify at altitude