Interstitial condensation within aluminium faced PIR insulation in external wall
Posted: Thu Mar 04, 2021 2:05 am -1100
I’m hoping you can help with interpretation of moisture transport in an external wall with aluminium faced PIR insulation
Modelling the equivalent wall using the Glaser method (ISO 13788) results in accumulating condensation between the PIR core and outer aluminium foil facing layer: for London locations between 3 and 14 g/m2 condensate accumulates per year.
Modelling the same construction in WUFI condensation seems to occur but after over time – after 3 years – the moisture content of PIR layers stabilises, apparently to an acceptable level. I’ve a number of questions which I am hoping will help understand the processes and aid interpretation.
Although PIR is given a 0.99 porosity value in the WUFI database, the moisture storage function gives a maximum water content of 2.15 kg/m3 at 100% relative humidity.
1. I’m assuming this is related to the fact that the PIR insulation in the database is a ‘closed cell’ type.
In contrast the European PU association have published data which suggests the maximum moisture absorption of rigid polyurethane foam (PUR/PIR) insulation boards due to diffusion and condensation measured to EN 12088 amounts to about 6 percent by volume, which would be 60kg/m3.
I’ve created a 1mm PIR layer next to the outer aluminium foil layer to assess the condensation risk.
The relative humidity graph of the PIR layer against the outer aluminium foil shows periods of 100% RH which I assume is liquid water i.e. condensate forming:
The water content in the 1mm of PIR insulation next to the outer aluminium foil layer is at a maximum of 22.9 kg/m3 in year 1.
22.9 kg/m3 is in excess of 2.15 kg/m3 value from the moisture storage function but less than the 60kg/m3 European PU maximum moisture absorption.
Main questions
2. What are your thoughts on using the European PU association maximum water absorption value to interpret this condensation risk?
3. If the maximum water absorption value is 2.19 kg/m3 what is the impact of 22.9kg/m3 of water in this location? Will it burst the aluminium foil facing?
4. Is there a way to analysis the WUFI data to understand why and how the water content decreases with time? In other words to answer the question where does the condensate go? Or how does the condensate decrease over time?
5. Or is there another explanation for what is going on here?
Some comments on the modelling:
There are no convergence failures. To get the moisture content balances 1 and 2 to be close I’ve had to run the model with a very dense grid: 1000 number set to ‘Automatic II’ with adaptive time step controls of 10 steps and 20 stages maximum.
Even so if I increase the period of analysis the difference between balance 1 and 2 increases from 0.01 for five years (balance 1: -0.28 to balance 2: -0.27 ) to 0.03 after 10 years (balance 1: -0.29 to balance 2: -0.26 )
6. I’m assuming a 0.01kg/m3 difference between balance 1 and 2 does not invalidate the WUFI analysis.
7. I wonder if there is some inherent divergence or instability with this calculation?
Construction
Outside
Fibrecementboard
Air Layer 60 mm
Metal Deck, unperforated
Air Layer 10 mm
Air Layer 5 mm
Air Layer 10 mm; metallic
Aluminium foil 1000 MNs/g [vapour retarder sd=100 edited]
UK PIR [Polyisocyanurate Board edited]
UK PIR [Polyisocyanurate Board edited]
Aluminium foil 1000 MNs/g [vapour retarder sd=100 edited]
Gypsum Board
Air Layer 25 mm
Gypsum Board
Inside
- wufi-animation.png (59.07 KiB) Viewed 8098 times
Modelling the same construction in WUFI condensation seems to occur but after over time – after 3 years – the moisture content of PIR layers stabilises, apparently to an acceptable level. I’ve a number of questions which I am hoping will help understand the processes and aid interpretation.
Although PIR is given a 0.99 porosity value in the WUFI database, the moisture storage function gives a maximum water content of 2.15 kg/m3 at 100% relative humidity.
1. I’m assuming this is related to the fact that the PIR insulation in the database is a ‘closed cell’ type.
In contrast the European PU association have published data which suggests the maximum moisture absorption of rigid polyurethane foam (PUR/PIR) insulation boards due to diffusion and condensation measured to EN 12088 amounts to about 6 percent by volume, which would be 60kg/m3.
I’ve created a 1mm PIR layer next to the outer aluminium foil layer to assess the condensation risk.
The relative humidity graph of the PIR layer against the outer aluminium foil shows periods of 100% RH which I assume is liquid water i.e. condensate forming:
- PIR-test-05_wufi-v3_wall_london_5y_2021-03-04_05-pir-1mm-against-foil_rh.png (107.43 KiB) Viewed 8098 times
22.9 kg/m3 is in excess of 2.15 kg/m3 value from the moisture storage function but less than the 60kg/m3 European PU maximum moisture absorption.
- PIR-test-05_wufi-v3_wall_london_5y_2021-03-04_03-pir-1mm-against-foil_wc.png (35.99 KiB) Viewed 8098 times
2. What are your thoughts on using the European PU association maximum water absorption value to interpret this condensation risk?
3. If the maximum water absorption value is 2.19 kg/m3 what is the impact of 22.9kg/m3 of water in this location? Will it burst the aluminium foil facing?
4. Is there a way to analysis the WUFI data to understand why and how the water content decreases with time? In other words to answer the question where does the condensate go? Or how does the condensate decrease over time?
5. Or is there another explanation for what is going on here?
Some comments on the modelling:
There are no convergence failures. To get the moisture content balances 1 and 2 to be close I’ve had to run the model with a very dense grid: 1000 number set to ‘Automatic II’ with adaptive time step controls of 10 steps and 20 stages maximum.
Even so if I increase the period of analysis the difference between balance 1 and 2 increases from 0.01 for five years (balance 1: -0.28 to balance 2: -0.27 ) to 0.03 after 10 years (balance 1: -0.29 to balance 2: -0.26 )
6. I’m assuming a 0.01kg/m3 difference between balance 1 and 2 does not invalidate the WUFI analysis.
7. I wonder if there is some inherent divergence or instability with this calculation?
Construction
Outside
Fibrecementboard
Air Layer 60 mm
Metal Deck, unperforated
Air Layer 10 mm
Air Layer 5 mm
Air Layer 10 mm; metallic
Aluminium foil 1000 MNs/g [vapour retarder sd=100 edited]
UK PIR [Polyisocyanurate Board edited]
UK PIR [Polyisocyanurate Board edited]
Aluminium foil 1000 MNs/g [vapour retarder sd=100 edited]
Gypsum Board
Air Layer 25 mm
Gypsum Board
Inside