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This article was originally published in the September/October 1997 issue of Home Energy Magazine. Some formatting inconsistencies may be evident in older archive content.

 

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Home Energy Magazine Online September/October 1997


Creating Windows of Energy-Saving Opportunity


by Andrew M. Shapiro and Brad James

Andrew M. Shapiro is an energy engineer with the Vermont Energy Investment Corporation in Burlington, Vermont. Brad James is a master's degree candidate at the University of Vermont. S. Flanders of the U.S. Army Cold Regions Research and Engineering Laboratory assisted with project oversight.


Windows are where we often look to improve the energy performance in old homes. But don't rip out those old sashes yet. A field study in Vermont suggests that remove and replace is not necessarily the way to go when it comes to old windows.
Brad James sets up a test rig to measure air leakage around a window.
Renovating historic homes is a tricky and sometimes onerous task. The desire to retain the historic character of the building, and in some cases the actual historic material, competes with the desire to improve energy performance. One particularly difficult question that renovators of historic buildings often confront is what to do about windows.

From their handmade glazings to their crafted muntins, old windows add much to the character and charm of historic homes. But just looking at their loose jambs and leaky sashes can make an energy auditor shudder. Although the tendency among some contractors has been to replace the windows in older homes, until recently, there has been very little data available to guide renovators in choosing the most energy-efficient window rehab option (See Energy-Efficient Window Retrofits, HE Jan/Feb '97, p. 35).

To help fill this data gap and supply additional guidance to renovators, we evaluated the thermal efficiency of more than 150 windows in 29 old New England homes and one municipal building. We determined the energy savings and costs associated with different renovation strategies, from simply weatherstripping to replacing the entire sash. The study was funded by the National Center for Preservation Technology and Training and the Vermont Division of Historic Preservation.

Comparing Original and Renovated Windows We were not able to test most of the windows before and after renovation. Instead, we tested 64 original windows and 87 windows that had been renovated by contractors, and compared the results.

The windows varied widely in age and condition; a few were at least 100 years old. Many of the original windows had storm windows installed. Some renovated windows still had the original sash, while others had been retrofitted with a new sash. For the retained sashes, the contractors used a variety of weatherization and renovation methods; these are described below. The retrofitted windows had received either a new sash in the old jamb or a new vinyl or wood window insert (also known as a secondary frame) with a new sash. On some homes, the contractors had installed new storm windows.

Although the windows we tested varied in size and shape, we were able to make comparisons across sizes by normalizing the data to a typical window 36 inches wide by 60 inches high.

Figure 1. On the left, typical air leakage sites with Sash Leakage, S, and Extraneous Leakage, E. On the right, a cutaway showing the parts of a typical single-hung window.
Measuring Infiltration and Thermal Losses Window heat loss may be considered as a combination of thermal and infiltration (or leakage) losses. Thermal loss occurs when energy passes directly through the materials of the window. It includes radiation and convection to the interior surfaces of the window from the room; conduction through the materials of the window; and convection and radiation from the exterior surfaces of the windows to the outdoors. Infiltration losses are driven by wind and by differences between indoor and outdoor temperatures. They occur primarily through cracks in the sash, gaps between the sash and jamb, and gaps between the frame and rough opening (see Figure 1).

We calculated thermal losses using WINDOW 4.1, a computer model developed by Lawrence Berkeley National Laboratory's Building Technologies Program. We based our infiltration test method on ASTM E783-93, performing two air leakage tests on each window. We constructed a simple measurement device around the windows by taping a plastic sheet onto the interior trim and attaching an air hose, blower, and pressure tap. First, to test total leakage, we drew air through the window using the blower and measured the flow rate, in ft3 per minute (CFM), at various pressure differentials across the plastic sheet. Then, to test extraneous leakage, we attached a second plastic sheet to the exterior trim of the window and repeated the test. By subtracting the value obtained in the second test from that obtained in the first, we were able to estimate sash leakage.

Some building designers think of window infiltration only in terms of sash leakage. But significant leakage can also occur between the window frame and the rough opening. (Note that window manufacturers report only sash leakage in product data.) To estimate how much rough opening leakage contributes to total window infiltration, we measured the temperature of the indoor air, the outdoor air, and the air being drawn through the window during 33 of the extraneous leakage tests.

We found that, on average, the air drawn through the windows in the study was approximately 30% cooler than the indoor air. Based on this difference, we assumed that approximately 30% of the extraneous leakage was outdoor air coming through the rough opening. We thus estimated total infiltration as sash leakage plus 30% of extraneous leakage. While this method was not very precise, it did allow us to estimate the relative contribution of rough opening leakage to heating load.

Table 1. ELA (in In2) for Baseline and Selected Renovated Windows
  Baseline Tight Baseline Average Baseline Loose Replacement Sash Vinyl Window Insert Original Sash with Vinyl Jamb Liners
Number of windows tested 35 35 47 11 14 37
ELA sash 0.27 0.89 2.19 0.45 0.13 1.46
ELA rough opening* 0.59 0.59 0.59 0.30 0.16 0.39
ELA total 0.86 1.48 2.78 0.75 0.29 1.85
*Air leakage from the outside is assumed to be 30% of extraneous leakage. This is a simplifying assumption.
Surveying the Size of the Hole Using the results of the leakage tests, we calculated equivalent leakage areas (ELA) for both the original and the renovated windows. ELA is the area of a single round hole with a leakage rate equal to that of the aggregate of the leakage sites for a given window.

Air leakage rates of the original windows varied widely, reflecting the wide variation in the condition of the windows. We derived three baseline ELA values--typical, tight, and loose--for use in the comparison with the renovated and retrofitted windows. The loose baseline value was the mean leakage value of all the original windows without storm windows. The typical baseline value was the mean leakage value of all original windows that had storm windows in place. The tight baseline value was set at one standard deviation lower than the typical baseline value.

Table 1 is a summary of the ELAs for the original and renovated windows. The vinyl window insert was clearly the tightest option, having one-fifth the leakage of the baseline average.

Tightening Up with the Original Sash

For those houses where the sash was retained rather than replaced, we found that contractors used a variety of methods and materials to tighten the sashes. These methods and materials are summarized in Table 2.

Figure 2. First year heating cost per window, pre- and post-treatment.
Comparing Costs and Savings Having determined the ELAs, we then analyzed the impacts of the renovations and retrofits on energy savings. Figure 2 compares the infiltration and thermal loss heating costs of the baseline, renovated sash, and retrofitted windows. As expected, the results confirm that the largest energy savings came from tightening the loosest windows. However, the chart also shows that, except in the case of low-E glass window inserts, the difference in heating costs for the renovated sash and retrofitted windows was not great. Compared to the loose baseline, savings for the renovated windows ranged from $14 to $20 annually per window (3%-20% rate of return), while compared to the typical baseline, savings ranged from $1 to $7 annually (less than 1%-4% rate of return). Note that these annual returns actually reflect the first-year savings.
 

Table 2. Weatherization Methods and Materials at Sites Retaining Original Sash
Site Number of windows Method and Materials
A 7 Vinyl jamb liners
B 8 Vinyl jamb liners and silicone bulb weatherstripping (sill and head)
C 19 Vinyl jamb liners and silicone bulb weatherstripping (sill, head, and meeting rail)
D 3 Vinyl jamb liners; silicone bulb weatherstripping (sill, head, and meeting rail); replace single glass in original sash with double-pane insulating glass; new latch at meeting rail (Bi-Glass System)
E 3 Upper sash painted in place; zinc-ribbed weatherstripping on lower sash; V-strip weatherstripping at meeting rail; pulley seals; new storm windows
F 2 Upper sash painted in place; bronze V-strip weatherstripping on lower sash, meeting rail, and sill junction; existing storm windows; no locking mechanism
G 1 Weatherstripping between sash face and parting bead; V-strip weatherstripping at sill, head, and meeting rail

Lead Abatement

The problem of lead paint often arises when dealing with old windows. As shown in Table 3, lead abatement can add significantly to renovation costs (see Getting the Lead Out). For example, weatherizing a loose baseline window with weatherstripping, sealing the top sash, and rehabbing an existing storm costs $75 if lead abatement is not needed. At the annual savings rate of $15, this represents an annual rate of return of 20%. But when lead abatement is needed, the cost jumps to $200, which approaches the cost of replacing the sash.

Table 3. Renovated Windows Annual Heating Savings and Renovation Costs per Window
Renovation Cost Cost with Lead Abatement Annual Savings (Tight) Annual Savings (Typical) Annual Savings (Loose)
Retain sash: Vinyl jamb liner $175  $300 none $0.80 $14
Weatherstripping 75 200 $0.20 1.70 15
Replace sash: Single-glass sash 200 200 0.30 1.80 15
Window inserts 250-500 200-500 1.90 3.40 16
Low-e, double-glaze inserts 250-550 250-550 5.30 6.80 20
Storm Windows New exterior 100 225 1.00 2.50 16
New interior 115 240 1.30 2.80 16
Interior low-e 155 280 4.70 6.20 19
Note: Costs for inserts varied with the material, which ranged from medium-cost vinyl to high-quality wood. Full-sash lead abatement adds $125 to other rehab costs. Savings were based on 7,744 degree-days and oil heat at 90¢/gallon with 75% overall heating season efficiency. Note that the samples of most windows tested were very small. Cost estimates for window upgrades were based on interviews with housing developers and/or builders. Estimates were normalized to a $20-per-hour labor rate, and included contractor markup.
Figure 3. First year heating costs, infiltration only, renovations retaining sash.
Storm Windows

Table 3 also shows the costs and savings associated with adding storm windows. A new exterior storm window added to a loose baseline window has a first-year savings of $16 at a cost of $100 (excluding lead abatement), or a 16% annual rate of return. Adding a low-e interior storm to a loose window saves $19 at a cost of $155, a 12% annual rate of return.

Figure 3 shows first-year heating costs due to air leakage only. The variation in the cost of air leakage in the first four columns suggests variability in workmanship in installing the jamb liners. Also, windows where the jamb was out of square had the poorest seal.

Sites C and D both incorporated weatherstripping at the meeting rail but the much lower leakage rates should not be attributed to that difference alone. Jamb liners require that the sash be fitted precisely to the liner and to the jamb to prevent leakage between the jamb and the jamb liner and between the liner and the sash.

The strategy used at site E resulted in quite low sash leakage. However, the total leakage was approximately the same as that at sites C and D, due to high leakage through the rough opening. Similarly, even though site G had a very low sash leakage, performance was undermined by the rough-opening leakage.

Is a New Sash Worth It?

Figure 4 compares the associated heating costs for infiltration and thermal losses for windows that retained the original sash. The chart shows that, compared to the thermal losses, infiltration accounted for a small part of the total heating cost, regardless of the strategy used.

Table 4 lists the heating costs for windows with new sashes or window inserts. With the exception of the low-e sash and the poorly fitted sash, the total heating costs were very similar, ranging from $12 to $14.

Figure 4. Total first year heating cost, renovations retaining sash.
Table 4. Annual Heating Cost for Upgrades that Include Replacing the Original Sash
Upgrade Description Non-Infiltration Heating Cost Infiltration Heating Cost Total Heating Cost
Vinyl window insert $12 $0.37 $12
Wood window insert 12 0.70 13
Sash & storm 12 1.68 14
Sash & storm (poor fit) 12 4.83 17
Insulated glass sash 12 0.60 13
Insulated low-e sash 8 0.60 9
Note: Savings were based on 7,744 degree-days and oil heat at 90¢/gal with 75% overall heating season efficiency. We did not observe the use of low-e glass in the field and calculated the thermal loss for the low-e retrofit. We estimated the added savings of low-e glass over other sash replacement strategies at $3.40 per year.

Brad James, who worked on much of the testing and analytic work for this study, covers a window with plastic in preparation for the extraneous leakage test.
Preservationists Take Heart Our study of old windows showed that the energy savings are similar for a variety of retrofit and replacement strategies. Rates of return on investment for energy improvements are quite low when starting with typical or tight windows with storms in place, but are significantly higher when renovating loose windows with no storm.

The difference in annual energy savings between renovating an old sash and replacing it with a new one was very small--retrofits saved only a few dollars.

For preservations, the good news is that with a proper choice of renovation strategy and good workmanship, historic sashes can be almost as energy-efficient as replacements. Window renovators and homeowners can give more weight to comfort, maintenance, lead abatement, egress requirements, durability, ease of operation--and historical value--without sacrificing energy savings. For those of us who work with old windows, this is very good news indeed.

Sources American Society for Testing and Materials. ASTM E783-93, Test Method for Field Measurement of Air Leakage through Installed Exterior Windows, Curtain Walls, and Doors under Specified Pressure Differences across the Specimen, in Annual Book of ASTM Standards, Vol. 04.07, 1994.

Grimsrud, D.T., M.H. Sherman, R.C. Sonderegger: Calculating Infiltration: Implications for a Construction Quality Standard, Proceedings, ASHRAE/DOE Conference on Thermal Performance of Exterior Envelopes of Buildings II. 1982.

James, B., et al. Testing the Energy Performance of Wood Windows in Cold Climates, National Center for Preservation Technology and Training, 1996.

Lawrence Berkeley Laboratory, Windows and Daylighting Group, Berkeley, California. WINDOW 4.1: A PC Program for Analyzing the Thermal Performance of Fenestration Products (1994).
 
 

 


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