Duct insulation in cold loft

[LinearPancakes]

Our MVHR unit is going in our cold loft. To insulate the ducts I'm planning:

• 25mm armaflex wrap on supply ducts, as these could be cooled to ~14deg in summer due to ComfoPost
• 10mm armaflex wrap on extract ducts

 

If I've modelled this correctly in Ubakus, it looks like there will still be a bit of condensation on the outside of the Armaflex in the worst case scenario (assuming heat wave, loft getting very hot). Applying a foil wrap outside the Armaflex looks like it will prevent this. Though I imagine it would be tricky to apply this well enough to provide a sufficient vapour barrier in practise.

 

I also bought some foil faced fibreglass duct wrap that I'd planned to use outside the Armaflex to reduce heat transfer where not covered by loft insulation. But condensation skyrockets when I apply a similar setup (mineral wool). Better with foil wrap, but again, I doubt in my ability to make it a good enough vapour barrier. I find this curious, as if there's no condensation build up on the Armaflex.

 

So my questions are:

• Would you expect the foil wrap outside the armaflex to help condensation that much?
• Will I get huge condensation issues applying the fibreglass ductwrap around the [possibly foil lined] Armaflex?

 

I guess I could just double up to 50mm armaflex, but it's pretty expensive stuff. I was hoping 25mm Armaflex and then fibreglass ductwrap would be fine.

 

Thanks for any insight you can give. Screenshots of Ubakus attached.

[JohnMo]

Are you sure the ambient and humidity levels are correct?

[LinearPancakes]

Not for certain, but I was trying to pick a worst case. Maybe 50 degrees would be a more realistic peak in a heatwave, but I don't think it would change the calculations significantly.

[JohnMo]

If the roof space is at 50 -55deg, it unlikely to have a humidity level of 60% at that temperature? Also if you are cooling the air the humidity level is likely to increase, the air in the pipe is likely to be at or close to 100% humidity.

[LinearPancakes]

You're right, thanks. I believe the last heatwave was 41deg peak and ~25% RH. So in a loft heated beyond that, the RH would be even lower. Looks like the plan for 10mm armaflex + 50mm ductwrap for extract and 25mm armaflex + 50mm ductwrap for supply will not have any significant condensation, even in worst case.