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Method to Diagnose Window
Failures and Measure U-Values
on Site
Kapil Varshney, Javier E. Rosa, and Ian Shapiro
Taitem Engineering, PC.
Ithaca, NY
February 28, 2011
What is a U-value?
Overview
U-value (or U-factor) = is the coefficient of heat
transmission
National Fenestration Rating Council
(NFRC)
Tests & Rates windows/door assemblies
Overview
U = 1/R
Overview
R – value: Higher is better
U – value: Lower is better
Overview
Examples
Overview
Window Type Typical
U-values Single Pane, Clear 0.90 – 1.10
Double Pane, Air Filled, Clear 0.45 – 0.50
Double Pane, Argon Filled, Clear 0.40 – 0.42
Double Pane, Air Filled, Low-E 0.35 – 0.38
Double Pane, Argon Filled, Low-E 0.30 – 0.33
Triple Pane 0.23 – 0.27
R-5 0.20
New Energy
Code Compliance
Heat Loss = U x A x (Tinterior – Texterior)
Overview
Tinterior Texterior
U-value of windows on site
are often not known
No NFRC sticker on the window
Storm windows
Leaked insulating gases in high-
performance windows
Overview
Absent or failed weatherstripping
Poor-fitting windows
Low-e coating on wrong surface
Unusual windows (i.e. glass block, stain glass,
frosted glass, etc.)
Overview
R
h c L
1U =
1 1 1+ +
h h U
NFRC Procedure
0.25 4 4
2 3 1 3 1
h
3 1
T -T (T +459.67) -(T +459.67)h (Btu·in/hr·ft ·ºF)= 0.30× +σe
L T -T
U-factor of test specimen
without air-film coefficients
Exterior film coefficient
Interior film coefficient
Overview
= Stefan–Boltzmann constant
e = Emissivity
L = Height of the window
Overview
NFRC Procedure
0 0F for exterior and
70 0F for interior temperatures
Interior Exterior
T1 T2
hh T3 h 3 1 L 1 2h A(T -T ) = U A(T -T )
Proposed Solution
Overview
hc
0.25 4 4
2 3 1 3 1 3 1
L
3 1 1 2
T -T (T +459.67) -(T +459.67) T -TU (Btu·in/hr·ft ·ºF) = 0.30× +σe ×
L T -T T -T
Interior Exterior
T1 T2
hh T3 h 3 1 L 1 2h A(T -T ) = U A(T -T )
hh
Proposed Solution
Overview
hc
Overview
Overall Field U-value
Proposed method does not measure infiltration
U-value interaction with infiltration
Overview
Why IR Thermometer?
Advantages
Rapid temperature measurements
Easy to use
Non-invasive
Compact
Affordable
Accurate
Already available in tool kits of energy auditors
Overview
Disadvantages
Cannot be used on reflective materials, (i.e. glass)
Some models give inaccurate readings in cold weather
conditions
Overview
Project Outline
Testing was done in a custom built chamber
Tested different types & assemblies of windows
Majority of windows tested were new and NFRC rated
In addition non-rated single pane windows and
windows/storm assemblies were also tested
Procedure
Test Chamber
Procedure
Procedure
RTD Temperature
Probes
Point of Measurement
Results
Y
X
Quality Check for IR
Thermometers
-10
0
10
20
30
40
50
60
70
1 5 9 13 17 21 25
Measurements
Tem
pera
ture
(F
)
RTD
IR-1
IR-2
IR-3
Results
Conclusion:
Not all IR thermometers are
sufficiently accurate
Results
Air Temperature Measurement
Results
Can IR thermometers be used to measure air temperature?
Cannot measure a wall, could give a wrong reading.
Need standard air temperature location.
Decided on non-reflective paper, 12” from window, 5’ above floor.
Air Temperature Measurement
Results
Transient Air Temperature Measurement on
Piece of Paper
Lab Test Results
Types of windows Frame Glazing Rated Storm Argon Low-E Type
1 Vinyl Double pane 0.47 No No No Double hung
2 Vinyl Double pane N.A. Yes No No Double hung
3 Vinyl Double pane 0.34 No No Yes Double hung
4 Vinyl Double pane N.A. Yes No Yes Double hung
5 Vinyl Double pane 0.31 No Yes Yes Double hung
6 Vinyl Double pane N.A. Yes Yes Yes Double hung
7 Vinyl Triple pane 0.25 No Yes Yes Double hung
8 Wood Double pane 0.33 No Yes Yes Double hung
9 Wood Double pane N.A. Yes No No Double hung
10 Vinyl Double pane 0.40 No No No Casement
11 Wood Double pane 0.48 No No No Casement
12 Vinyl Double pane 0.30 No No Yes Casement
13 Vinyl Double pane 0.27 No Yes Yes Casement
14 Wood Double pane 0.32 No Yes Yes Casement
15 Vinyl Single pane N.A. No No No Double hung
16 Vinyl Single pane N.A. Yes No No Double hung
Results
Results
Argon Leak Detection
Results
Effect of Outdoor Air
Temperature on U-value
Results
Field-measured U-value were found to decrease slightly
as outdoor air temperature increases
Based on lab test results, an adjustment is being
developed to correct for this
Field Test Procedure
Procedure
Test Equipment
Procedure
Procedure
Procedure
Procedure
How long does it take to perform
a U-value measurement?
Procedure
Results
Field Measurements
Results
Field Measurements
Results
Results
How well will the method
work for unrated windows?
Results
(2005)
Limitations
Measurements should be avoided when windows are in direct
sunlight
Fixed/non-operable windows in upper stories
Measurements should be avoided if a baseboard/radiator/register
is located underneath a window and is ON
Radiator
Conclusions
A method has been developed to rapidly measure window U-
values in the field
Results are within range of NFRC ratings
The method is not intended to exactly reproduce NFRC
ratings – rather to serve as a “field-measured U-value”
The method allows U-value measurement of non-rated
windows and window/storm assemblies
The method permits measurement of U-values for a wide
range of outdoor air temperatures
Conclusions
IR thermometers allow for rapid measurement
The method has the potential to allow diagnosis of argon
leakage
The method has the potential to allow quality control for
failure of new windows
The method is being refined to more fully assess limitations,
and evaluate possible application to measure wall R-values
Conclusions
Other Observations
Conclusions
Single pane window itself: R-value is almost zero, and U-
value depends almost entirely on air film coefficients
Storm windows perform well
Low-e coatings really do work
Leakage of gas fill (eg. Argon) is a vulnerability of high-
performance windows
Even the very best windows (e.g. “R5”) are poor: Always ask
if every window is really needed
Conclusions
Contact Information
Ian Shapiro, P.E.
Taitem Engineering, PC
Ithaca, NY
Kapil Varshney, Ph.D.
Taitem Engineering, PC
Ithaca, NY
Javier E. Rosa, P.E.
Taitem Engineering, PC
Ithaca, NY
Project funded by NYSERDA
Gregory A. Pedrick, Project Manager Buildings R&D