Part 3: Do super windows make sense

The common perception is that windows are energy black holes. However, some believe that today’s high-solar-gain triple-glazed windows can gather more radiant heat than they lose, and perform better than an insulated wall that can only lose heat.

Such windows achieve this feat by admitting more useful solar heat gains during the heating season than energy lost through conduction, convection and infrared radiation; therefore, acting as a heat generator. The low e coating used allows external solar heat to pass through the window (unlike the regular low e coatings that prevent solar heat passing) while reflecting the inside infrared heat back into the room, this intern reduces the overall heat transfer.

Most triple-glazed windows provide a superior U-factor of between 0.14 and 0.24 and an SHGC of between 0.2 and 0.47; but in doing so reduce VT levels down to below 0.5, and in trying to meet these high standards pay little attention to cost-effectiveness. These investments in very expensive windows are probably a waste of money for most applications and certainly not required in Prescott.

Let’s take a simple example:  We have a basic home with a living area of 2025 square feet and wall heights of 9 feet.  This gives us a wall area of 1620 square feet.  Energy Star recommends that no more than 15% of the wall area be glazing which in this case is 243 square feet

Now if we look at the average pricing for the different glazing designs, double pane configurations cost between $30 and $35 per square foot, triple pane between $40 and $50 per square foot and high performance or super triple pane windows discussed here as high as $90 per square foot.

Now if we look at the cost for the example above, the average double pane solution would cost $7897 and the average triple pane solution $10,935, the high performance solution would cost $21,870.  This shows that the triple pane glazing cost is 38% higher than the double glazing and the high performance cost is 100% higher than triple glazing and 177% higher than double glazing

Now let’s look at the impact on insulation levels; the 2015 building codes require a minimum wall insulation level of R19 (U = 0.0526) and energy star requires a maximum window area of 15%; therefore we have 85% of the envelope at R19 and 15% at the window R value.  The best high performance windows support a U value of 0.16 (R 6). Now R total = 1/ ((0.85*0.0526) + (0.15*0.166)) = R14; down from R19.  Now let’s use the 2015 maximum level for windows U=0.35.  R total now becomes R10. Therefore, the high performance solution provides a 40% increase in R value for a 177% increase in cost

The reason a triple-pain window upgrade fares poorly, from an economic perspective, is that the heating load in an energy efficient home is small compared to standard houses. As an example reducing the U value of windows from U= 0.33 to U=0.2 showed that a heating system reduced its heating cost by 3% per year.

If we look at the energy star estimates, an average homes conditioned space is 1700 square feet and the average cost of home energy in 2009 was $2,200.  If we add the average annual energy increase of 4% through 2015 it will be $2,728 today.  Now the heating and cooling part of the total energy cost is 46%, or $1,254 and 3% of that is $37.  Now looking at the cost difference between double glazed and triple glazed pricing shows the $3,038 payback period would take 82 years and the difference between double glazing and the high performance version 377 years.

An alternative to high-performance windows in cold climates is to install affordable fairly standard, double-glazed windows with low-e coatings and then an energy efficient storm window in winter. Storm Windows are the most economical way to improve existing single or double-glazed windows. The storm window would have to have a durable (hard-coat) low-e coating, since it wouldn’t be protected in a sealed air space, as most low-e coatings are.

Ultimately, the optimum choice of window and glazing systems will depend on many factors. The number of glazing choices can be complex, and you could be easily talked into choosing windows that sound ideal, but that don’t make sense for your climate, your building, or even for the specific wall where you install them, and window replacement isn’t the best place to start energy upgrades.

When planning energy improvements to an existing house, replacing windows should show up toward the bottom of the list as it almost always makes sense to improve an existing home’s air tightness and insulation first, and unfortunately, energy tax credits for window upgrades expired at the end of 2014, so you now have to bear the whole cost,

Modern windows: Energy black holes or not? Part 2

Following on from last months article, window frames are an important part of a window’s overall thermal performance, and improving the thermal resistance of the frame can contribute to a window’s overall energy efficiency, particularly its U-factor. There are advantages and disadvantages to all types of frame materials: however, window manufacturers have generally migrated from traditional wood to more rot-resistant versions, including aluminum, vinyl, fiberglass, and composite. Fiberglass, composite and aluminum are likely to be the most durable choices, but from an energy perspective, fiberglass and composite are preferable to aluminum that is highly conductive. Some manufacturers offer foam-filled fiberglass and composite frames that perform even better.

Wooden frames insulate relatively well, but require a lot of maintenance. Vinyl frames are usually made of polyvinyl chloride (PVC) with ultraviolet light (UV) stabilizers to keep sunlight from breaking down the material. Vinyl windows do not require painting and have good moisture resistance. The hollow cavities of vinyl frames can be filled with insulation, which makes them thermally superior to standard vinyl and wood frames. Fiberglass frames are dimensionally stable and have air cavities that can be filled with insulation, giving them superior thermal performance compared to wood or un-insulated vinyl. Composite frames are very stable, they have the same or better structural and thermal properties as conventional wood, and they have better moisture and decay resistance.

In addition to choosing the frame type, you will need to consider what type of glazing you should use to improve your home’s energy efficiency. Based on various window design factors such as window orientation, climate, building design, etc., you may even want to choose different glazing for different windows throughout your home.

In colder climates, south-facing windows could use high-solar-gain glazing, while east and west-facing windows that are hard to shade would use low-solar-gain units. North facing windows can be either. During the summer, when solar heat gain is less desirable, a properly sized roof overhang will shade south-facing windows during the hottest hours of the day, while allowing the lower winter suns solar heat to enter the home.  In climates with significant air conditioning loads, specify windows with low SHGC values (< 0.40) and in general, high (> 70%) Glass Visible Transmittance.

Single pane glazing has very poor performance, and is generally not used in modern construction.  The more popular insulated window refers to windows with two or more panes of glass. The glass panes are spaced apart and hermetically sealed, leaving an insulating air space, which primarily lowers the U-factor.  Because heat conduction across an air space still contributes some heat loss, performance can be increased by replacing the air with a lower-conductivity gas such as Argon or Krypton. However, most double-glazed, sealed and insulated glazing units have air between the panes. For Prescott’s high altitude conditions, windows should support pressure relief options that prevent seal and glass damage.

Low-emissivity (low-e) coatings control heat transfer through windows. Such coatings are used to reflect the infrared (heat) and the ultra violet portion of the solar spectrum while passing the visible light.  A low-e coating is a microscopically thin, metallic oxide layer deposited directly on the surface of one or more of the panes of glass through a vacuum deposition process. A low-e coating is spectrally selective, filtering out 40% to 70% of the heat normally transmitted through a window.

The diagram shows the green line passing the suns complete spectrum including the Ultra Violet and Infrared heat bands.  The red and blue lines show the effect of low emissive coatings, and the black line shows an ideal, but unachievable spectrum of visual light only.

The higher a windows visible light transmittance (VT), the better. Windows with a low VT look gray and depressing and any window with a VT below 0.40 “would be almost unethical to sell as clear glass.  Another important factor is the interface between a window and wall, which plays a critical role in air tightness. To provide air-barrier continuity a wet sealant between the window frame and the rough opening flashings is recommended.

 Finally, when selecting windows, it’s always wise to compare warranties. The bigger the manufacturer and the better the warranty, the greater the chance that a manufacturer will stand behind its products and resolve disputes. Most big window manufacturers warrant window glass for 10 years and the frame hardware for 20.

Next time we will look at the advantages and disadvantages of high performance triple glazing.



Modern windows: Energy black holes or not? Part 1

Windows are among the most complex building components in a home, and also the most expensive. Look at what we ask windows to do. We want a visual connection to the outdoors that lets in the most daylight. We expect windows to provide fresh air when open, and are completely airtight when closed. We also want them to pass the suns heat in winter and shade the suns heat in summer. We need windows to be durable, resistant to condensation; wind, and driving rain, and we want them to integrate with the rest of the building envelope; and given they are a big investment they should last for several decades.

In addition to the important architectural contribution, windows have far-reaching energy consequences. Their performance, total area, and orientation can make or break the energy efficiency of a high-performance home.

The National Fenestration Rating Council (NFRC) rates windows on three criteria: U-factor, Solar Heat Gain Coefficient (SHGC), and visual transmittance (VT). NFRC ratings are whole window ratings that takes into account the different U-factors of the window’s frame, sash, edge of glass, and center of glass and not glass-only ratings. Glass only ratings can be 10% to 40% better than the whole product value, so look for the NFRC label on rated windows, and be suspicious of those without one.

U-factor measures how much heat is transmitted through the glass due to conduction, convection, and radiation. The lower the U-factor, the more efficiently the window blocks heat transfer. A 2015 Prescott energy code window has a maximum U-factor of 0.35, the equivalent of an R-2.8 insulated wall.

SHGC indicates how much of the sun’s radiant energy striking the window is transmitted through the window as heat. It is the fraction of solar radiation, expressed as a number between 0 and 1. Low SHGC means less heat is transmitted through the glass. A 2015 Prescott energy code window must have a maximum SHGC of 0.4.

Visual transmittance (VT) is the fraction of visible light energy that makes it through the window glass. Visible light is made up of those wavelengths detectable by the human eye. Visible light contains about 47% of the energy in sunlight. The higher the fraction, the more visible light will reach into the room.

Maximizing VT while getting the right combination of U-factor and SHGC can be challenging. All three properties must be considered and balanced to evaluate overall window performance. You may also find that you need different window styles and performance levels throughout your home depending on design preferences, the direction that your windows face and your local climate.

There are many different window designs, with the most popular being:  Fixed or picture windows that cannot be opened; and used for light or visibility alone. Sliding windows that open horizontally, can feature two or more sashes and offer a clear opening for good ventilation.  Single-hung sash windows open vertically (usually the bottom sash) and the other is fixed. Double hung windows open both sashes vertically, and screens can be installed outside the window frame.

Casement windows are hinged at the sides and open outward, with screens on the inside; they may be left-handed, right-handed, or both and are usually opened using a crank. An awning window is a casement window that is hung horizontally, and hinged on top. Casement windows are the dominant type now found in modern buildings.

A bay window is a multi-panel window, with at least three panels set at different angles to create a protrusion from the wall line. Eyebrow windows are a curved top window in a wall or in an eyebrow dormer. Bifold windows have two or more panels folding onto themselves, and louver windows have a series of blades that tilt to open.

Next time we will look at the various window design considerations, materials and performance enhancements.

For more information contact Paul Scrivens

The pros and cons of concrete pavers

Beautifying your outdoor space in an affordable and attractive hardscape definitely requires some careful deliberation. Every aspect of your home is an investment, even your exterior surroundings. Walkways, driveways and backyard patios should all be tied together to accent and enhance the overall personal and architectural style of your home.

Aesthetically, poured concrete is pretty plain and does not really add value to the look of your home; however, there are techniques including stamped and textured effects that can increase character.  On the other hand, concrete pavers come in a vast array of colors, shapes and textures, and are generally more visually appealing than concrete slabs.

One of the most important parts of a concrete driveway installation begins after all of the work is done. Concrete doesn’t dry out; rather, it undergoes a slow chemical curing process that hardens and strengthens the material.  You will need to wait at least a week before driving on a new driveway, and at least a month before parking heavy vehicles on it.

It is a proven fact that concrete and stamped concrete will eventually crack, stain, and fade; therefore, many contractors do not guarantee the installation. Common causes of cracking are when the base moves due to heavy loads or ground settlement, and concrete tends to shrink over time.  As these cracks form year after year, it is often difficult to match the color and composition of the original material, and as such, the concrete surface gets uglier and uglier with each repair.

The alternative to repairing concrete is to replace the entire structure when the cracks become unbearable; unfortunately, repairs can be rather involved requiring heavy machinery and high cost. Concrete slabs can also be slippery when wet. This can cause cars to lose traction and leave tire marks on a newly installed concrete driveway, and a slippery pool deck or patio can be dangerous if someone were to slip.

Concrete slabs are generally lower in cost than pavers. A basic concrete driveway installed over a gravel base will cost $4 to $10 per square foot, and while a concrete slab is more affordable upfront, the long-term costs for repairs and/or replacements will almost always outweigh the initial savings.

On the other hand concrete pavers are considered to be one of the only hardscape products that instantly add value to your home upon installation. Pavers are less susceptible to cracking and breakage because they move with the earth.  They are also molded under extreme pressure and are much stronger and durable than poured concrete. Another benefit is that most manufacturers back up their products with a Lifetime Warranty against breaking and cracking and unlike poured concrete do not require a curing period once installed.

Pavers do require more preparation than poured concrete, and like any good surface material, they perform best when installed over a well-prepared base. In this case, compact sand over a gravel or paver base. Once installed, sweep sand over the surface and compact to fill the joints.  Be careful, because a poor quality installation can leave you with an undulating surface with unaligned stones.

Typical paver maintenance includes resealing the joints every few years, and weeds, moss and grass can grow in between joints, so it is necessary to treat the area with weed killer from time to time. On flat surfaces play sand works well when resealing joints; however, if you have a steep incline the sand will wash away quickly; in this case engineered sand that contains polymers and mortar is a better solution as it hardens with water misting, and rain water runs off, but it is essential to follow the usage instructions precisely. Because pavers are set in sand, utility repairs under driveways and pool decks can be accomplished by easily lifting the pavers and making repairs.

On a greener and safer note, because of the high level of joints in the surface, paver’s aide in the drainage of rain and snow into the soil, recharging groundwater and trapping contaminants.  They also offer greater vehicle and walking traction and reduce surface water glare. Therefore, with a properly graded installation, drainage issues should never occur.  An added advantage is that interlocking pavers are eligible for LEED® credits under the U.S. Green Building Councils Guidelines.

Costs for a paver driveway can vary significantly. A professionally installed job can run between $10 and $15 per square foot, while pavers themselves will run between $4 and $7. Because they are small and easy to handle, and don’t require heavy equipment to install, they are a great material for the DIY enthusiast. However, you might tackle a path or small patio, but for driveways and big projects a professional is recommended.

Finally, homeowners can take pleasure in having a cohesive outdoor environment, and experience an increase in home value without a rise in real estate taxes.

For more information contact Paul Scrivens