1. CONDENSATION Predicting Interstitial Condensation
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Predicting Interstitial Condensation

2. Interstitial Condensation
This is condensation that occurs within the
fabric of a composite construction such as
walls and roofs.
Interstitial condensation risk can be estimated
from a knowledge of the thermal and vapour
characteristics of the building fabric and the
environment.
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3. How? By plotting the temperature profile and the dew-point profile
as they pass through the
element of the building
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4. How? Dewpoint This is the critical point where the air is
saturated. Dependent on the amount of
water vapour present in the air.
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5. Why?
To determine the likelihood of interstitial condensation occurring within the fabric
To be able to design against this likelihood
and eliminate possible risk
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6. Method By: Calculation Use of psychometric chart
Use of manufacturers’ literature
Graphically
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7. 6 Step process Calculate overall temperature drop
Calculate Thermal Resistance (if not given)
Calculate temperature drop across elements
Plot temperatures
Calculate Moisture conditions of external & internal environment
Find Vapour Resistance using Psychometric Charts
Calculate vapour pressure drop across layers
Determine dewpoint temperature at boundary of each element
Plot dew-point
Analyse & conclude
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8. Example

9. STEP 1 Calculate overall temperature drop External Temperature = 00C
Internal Temperature = 200C
Total Temperature drop = 200C

10. STEP 1 Calculate total resistance Internal Surface = 0.123
Plasterboard = 0.06
EPS = 0.75
Concrete = 0.105
External Surface = 0.055
Total = 1.093

11. STEP 2 Calculate Thermal Resistance 1.81
Element
Thickness
Thermal Conductivity (λ)
Resistance
Internal Sur
0.123
Plaster
0.010
0.06
0.17
EPS
0.025
0.75
0.03
Blockwork
0.150
0.105
1.43
External Su
0.055
1.81
Thermal Resistance = Thickness/Thermal Resistance

12. STEP 3 Calculate temperature drop across layers Layer Inside air 200C
Thermal Resistance
Temp Drop across each element
Boundary Temp
Inside air
200C
Int: surface
0.123
(0.123/1.093)*20 = 2.26
17.740C
Plaster
0.06
(0.06/1.093)*20 = 1.10
16.640C
Blockwork
0.105
(0.105/1.093)*20 = 1.92
14.720C
EPS
0.75
(0.75/1.093)*20 = 13.72
1.000C
Ext: Surface
0.055
(0.055/1.093)*20 = 1.00
0.000C

13. STEP 4 Draw a scaled section of the construction
Plot the boundary temperatures from step 2 on the section
Join the temperatures together to create a profile

14. STEP 5 Find internal vapour pressure. Internal temp = 20oC
Humidity = 59%
External temp = 00C
Humidity = 100%

15. STEP 5
Internal
20 oC
59%
Vapour Pressure = 1400mb

16. STEP 5
External
0 oC
100%
Vapour Pressure = 600mb

17. STEP 5 Vapour Pressures Internal 1400mb External 600mb Difference

18. STEP 4 Vapour Pressures drop by layer Total = 7.15 MNs/g
Thickness
Vapour Resistivity
Vapour Resistance
Int: surface
negligible
Plaster
0.010 50
0.5
Blockwork
0.105 30
3.15
EPS
0.025
100
2.50
Ext: surface
Total = 7.15 MNs/g
Vapour Resistance = Vapour Resistivity x thickness

19. STEP 4 Vapour Pressures drop by layer Layer 1400 Boundary Plaster 0.5
Vapour Resistance
Vapour Pressure drop
Vapour Pressure
Int: surface
1400
Boundary
Plaster
0.5
(0.5/7.15)*800 = 55.94
1344
Blockwork
0.105
(3.15/7.15)*800 =
992
EPS
0.025
(2.50/7.15)*800 =
Ext: surface
712

20. STEP 5
Using Psychometric Chart find the Dewpoint Temperature for each vapour pressure.

21. STEP 4

22. STEP 5 Vapour Pressure Dewpoint Temperature 1400 120C 1344 11.50C

23. STEP 5
Plot the Dew-point temperatures on section and join temperatures to create profile

24. STEP 5
Plot the Dew-point temperatures on section and join temperatures to create profile

25. STEP 6 Analyse & conclude
Is there a risk of interstitial condensation?
If so, what can be done about it?
Is there a risk of surface condensation?

26. Next??????

27. So that’s a snap shot of why you can see that here at NPTC we have all the ingredients to offer you more, than just an education
Thank you for listening!