Windows

Energy Savings Opportunities - Investment

Add Storm Windows

Many older homes have single glaze windows. Older windows are notorious for air leaks and drafts around their not-so-well sealed sashes. Cold air flows in as heat transfers out. Although storm windows may be somewhat less desirable, aesthetically, than window replacements, they will provide good energy performance for a much lower cost.

Storm windows are basically a second window that is installed on the outside of your existing windows. They can be raised and lowered to allow air passage and usually have screens that slide into place. By adding a second layer of glass, an insulating air pocket is formed between the storm window and the original window. The seals around the storm window will eliminate heat loss from drafting.

Storm windows are available with tints and low E coatings, which provide an overall thermal performance that is reasonably close to the newest double glazed windows.

The cost of a storm window naturally varies by size and performance level.  Contractors that you contact should be able to provide documentation on the performance of the windows they propose to install, as well as the cost of the installation.

Window performance will probably be specified as a U value. U value is the inverse of the R value used for insulation. The higher the R value the better, so the lower the U value the better.

If you do not have the funds available to purchase and install storm windows, you can always purchase and install plastic sheeting on the inside of each window during the heating season.  Sold as kits in hardware stores and home improvement centers, a thin, clear, plastic sheet is installed around the interior side of the window frame using a hairdryer to shrink the sheet into a tight surface. If installed properly, the created air pocket will slow heat transfer and should all but eliminate air infiltration.

Replace your Existing Windows

In general, replacing your existing windows with newer, higher performance models will not be cost justified on energy savings alone, unless your windows are in poor condition and/or you live in a very cold part of the country. As a homeowner, an investor in your own property, you would like to obtain as quick a payback as possible on the money you spend for this improvement. Replacing all the windows in your house will likely run into thousands of dollars, while your annual savings are not likely to exceed a couple of hundred dollars.

If you have decided to replace your windows for reasons other than future savings potential, you will likely be justified in spending more for higher performance windows. By contacting three contractors for quotes, you can establish a fair price for several performance grades of windows and their installation (See Selecting a Contractor). Each contractor should be able to provide performance data on the windows he proposes to install. Some will be able to perform calculations on the expected energy savings.

Dividing the difference in cost between any two windows grades by their respective difference in annual energy savings will yield the number of years to obtain a payback on your investment.

(Cost of higher performance window - Cost of lower performance window) / (Annual energy savings for higher performance window - Annual energy savings for lower performance window) = No. of years to recover investment dollars (Payback)

The table below lists the equivalent return on investment for each number of payback years:

Payback Years

1

2

3

4

5

6

7

8

Rate of Return*

100%

41.4%

26.0%

18.9%

14.9%

12.2%

10.4%

9.1%

Compare the expected rate of return with your other investment options (savings account, money market, stocks, bonds). If the window payback is four years, the return is 18.9%. Money market is yielding 1.5%, stocks zero, bonds 6%. Therefore, buying the higher performance window is a good investment, provided you are planning to live in your house more than four years. If you are planning to move within four years, you may be able to recoup your money by getting a higher sale price on your house with the window improvement. The National Board of Realtors has a website page that lists the expected impact of various home improvements on home value.

One final consideration: if you borrow money to pay for the windows, subject the annual interest costs from the energy savings costs when you compute the payback. Financing will diminish the rate of return, but at today's rates, not by much.

To get maximum energy savings for your investment, shop for windows with the lowest U value and air leakage. If you live in a part of the US where heating expense exceeds cooling expense, look for lower Solar Heat Gain Coefficients. Elsewhere, look for higher SHGC.

Shopping Checklist:

- U value less than or equal to 0.38 

- SHGC < 0.39 for heating dominant climate, > 0.71 for cooling dominant climate 

- Visible Light Transmittance (VT) greater than 0.5 

- Air Leakage < 0.3 CFM/Ft

Install Thermal Drapes/Blinds

When drawn during summer afternoons and winter nights, insulated thermal drapes shield the inside of the house will minimize both heat transfer and drafts, if any air leakage is present.

The photo below shows the double air pocket configuration of insulating blinds.  Each pocket of air adds resistance to heat flow in either direction. 

 

Enclose the Open Area Above the Drapes

Regardless of the R value of the drapes, if room air can flow freely between drape and the window from the bottom of the drape to the top, then a convection current will be present. In the winter, as the air between the drape and window cools, it "falls". The falling action draws in warm room air from above and the cycle repeats. If the area around the top of the drapes is enclosed to prevent the passage of air, the convection cycle is broken.

Install Awnings

For windows facing south or west, window awnings will act as a screen from direct sunlight, resulting in less solar heat gain inside the house. Window awnings were quite popular before residential air conditioning became commonplace.

Add Landscaping

By planting trees in strategic locations, much of the sun's radiant heat, both direct and reflected, can be reduced significantly. Trees also block and diffuse strong winds that would otherwise cause a greater heat loss off of the windows. As a bonus, trees also shade the walls and roof of the house, providing relief to the air conditioner during the summer.

For new construction:

In heating predominant climates, position more window area on the south side of the house and use windows that have the following characteristics:

U value of 0.35 or less 

SHGC of 0.60 or more 

VT of .70 for glass only or 0.50 for whole window 

Air Leakage of 0.30 or less

Plant trees or install berms on the north and west sides of the house to block the prevailing winds. To counteract the effect of the high SHGC during the cooling season, use overhangs to block the sun's rays on the south facing side of the house and broadleaf shade trees on the east and west sides.

In cooling predominant climates, position more window area on the north and east sides of the house. Use windows that have the following charactistics:

U value of 0.6 or less 

SHGC of 0.40 or less 

VT of .70 for glass only or 0.50 for whole window 

Air Leakage of 0.30 or less

Use overhangs and/or shade trees to minimize the windows exposure to the sun year round.

In split heating/cooling climates, position more window area on the south and east sides of the house. Use windows that have the following charactistics:

U value of 0.4 or less 

SHGC of 0.40 or less if A/C costs are significant, 0.55 or less if A/C costs are low 

VT of .70 for glass only or 0.50 for whole window 

Air Leakage of 0.30 or less

In addition to selecting the high performance windows, further energy savings are possible with the design of the house. Roof overhangs can be designed into new construction to minimize sun exposure during the cooling season. In the summertime, the sun is high in the southern sky in the middle of the day. With a sufficiently designed overhand on the southern side of the house, the sun's ray will be blocked from the south facing windows throughout the day, which vastly reduces the solar heating load.

By planting broadleaf trees on the east and west sides of the house, the sun's radiation during the morning and afternoon can be substantially blocked during the summer months. During the winter months, with the sun low in the southern sky, the direct sunlight on the south facing windows will help to heat the house. With the leaves off of the broadleaf trees during the heating season, the sun has a clear shot at the east and west facing windows in the morning and afternoon.

Next:  Further Reference

Copyright 2003 HEM Technologies, LLC. All rights reserved.

 

 
 
 

 

 

 

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