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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    I fitted a new woodburning stove that had the option of fitting an external air pipe. I connected ducting between the stove and a vent on an external wall. The vent works fine and there is no change to the operation of the stove from when it was vented to the room.

    I then had to box off the ducting so I thought I'd add a radiator to that wall (it is a cold and damp corner away from the wood burning stove) and then box-in both the rad and the stove ducting with a bespoke radiator cover.

    I built a frame around the radiator and installed insulation in the frame behind, above, below and at the sides of the radiator to send the heat outwards. I also insulated around the stove ventilation ducting with some spare wool batts I had.

    I left this for a couple of weeks then with the recent cold weather I thought I'd check how cold the insulated ducting was. I removed the insulated panel and pulled out some of the wool and found that the ducting was really cold and had condensation on the flexible ducting pipe and on the wool surrounding it.

    The question I'd like to as is what to do about this condensation? Should I remove the insulation around the ducting or fit some sort of breather membrane over everything to prevent moisture getting thru to the ducting or anything else?

    I've attached photos that show the ducting when it was first fitted, when it was covered with the insulation panels and the last one showing the insulation panel removed and the wool insulation pulled aside.
      woodburner vent.jpg
      insulated rad surround.jpg
      Insulated vent.jpg
    Removing the insulation could make matters worse i.e. more condensation. Putting on a vapour tight covering that is effectively sealed will be difficult and if it isn't fully sealed to prevent the room air getting to the pipe it won't work.
    You could replace the wool with PU foam (squirty foam strikes again !!) which is closed cell so would be its own air tight vapour barrier. Down side - future investigation will be difficult.
    • CommentTimeJan 15th 2021
    The WBS air inlet pipe is similar to an MVHR air inlet (or outlet). They contain air at outside temperature so are cold in winter. Therefore they have to be kept isolated from indoor air. They need insulating with vapour tight insulation. Either inherently vapour tight or some other kind with a vapour-tight wrapping/sleeve over the outside. I believe there are special products made for MVHR pipes, but I don't know the details because I opted for semi-rigid ducts made of XPS foam, and then I've wrapped some more rockwool over the outside of that, since its surface is above the dewpoint.

    I see there's stuff like https://www.ductstore.co.uk/acatalog/DEC-Isodec-Insulated-Flexible-Ducting.html but I've no idea how good it is or whether there are better products.
    I've had a delivery of the insulated flexible ducting. As the insulated ducting is 100mm diameter and the existing ducting is 70mm in diameter, I was thinking of just sliding the insulated ducting over the exiting one.

    It'd make it easier for me as I only need to undo one end of the original ducting, it would also prevent the flexible ducting from getting compressed and reducing arflow. I'm also thinking that the air trapped between the 70mm pipe and the 100mm ducting would give a little more insulation.

    Given that the insulated ducting will be attached to the original ducting with airtight aluminum duct tape and that I will make sure the original ducting is completely free of condensation, is there any down side to this approach?
    I bit the bullet and installed the insulated ducting over the top of the existing pipework. I pulled out the wool insultation and could feel just how much damp there was in there. While it is a pain in the backside to undo and redo the installation of the vent pipe, after feeling how wet the wool insulation was in there, I'm glad I did it. Relieved that a whim of wondering just how cold the ducting was inside the insulation led me to pull off the front of the panel to check, finding the moisture.
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