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

imshapiro@taitem.com

Kapil Varshney, Ph.D.

Taitem Engineering, PC

Ithaca, NY

kvarshney@taitem.com

Javier E. Rosa, P.E.

Taitem Engineering, PC

Ithaca, NY

jerosa@taitem.com

Project funded by NYSERDA

Gregory A. Pedrick, Project Manager Buildings R&D