Go to Forum Home AECB Member’s Discussion Forum IWI and original box sashes

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    • #48106
      Adrian Fleet

      Hi all,

      I am currently in the planning stages of a retrofit in my 1901 terrace. The area is relatively sheltered. The property is constructed from a soft red facing brick – solid wall construction (215mm). The original sliding sash window frames are in very good condition but the sashes themselves could do with some TLC. Therefore, I intend on upgrading these windows as part of my planned retrofit. The property is located within a Conservation Area and is covered by an Article 4 direction. Therefore, replacement windows must replicate the exact profile from the outside; narrow glazing bars and meeting stile. For information the windows face South.

      It is my intention to use the NBT Pavadentro IWI system at a thickness of 80mm.

      As always, money and time are big considerations with the project. The ideal solution would be to remove the existing windows and replace with spring sash windows. The remaining void from the box sash would then be insulated with NBT Pavadentro – see attached detail. However, given the conservation constraints these windows are expensive to construct and the time involved is a consideration. My alternative option is to just replace the sashes with new that are glazed with DG 14mm (4-6-4) slimlite units. The IWI insulation would then overlap with the existing box frame as shown on the attached detail. However, I am concerned that I may face a thermal bridge/condensation issue if I pursue this route.

      I’m not looking for a yes or no answer but purely an enquiry to see if anyone has faced this situation before and if they did simply replace the sashes have there been any issues?

      Details here

      It would be great to discuss this with anyone even if you don’t have experience of the situation – i’m getting pressed by the family to get the project moving!

      Thanks, AJ

    • #48399
      Tim Gilbert

      Hi AJ,

      I have been waiting to see whether you get a reply from someone who knows what they are writing about. Ten days later you have not had a reply so I will discuss my thoughts as a retrofit student.

      I believe that leaving the box frame in place, or replacing with new, is storing up trouble and will seriously compromise your walls’ overall U-values.

      Voids of any sort should be avoided, even if fully sealed, which it won’t be. Air circulating within the void will very effectively transfer warmth from inside to out in the winter and vice versa in the summer, adding to any overheating. Also the walls will be cooler in winter once you retain more heat in the house with your IWI, thus increasing the risk of condensation.

      If it was my house I would grit my teeth and take the alternate option on replacing the box with insulation.

      I hope this helps and it may even trigger more responses.


    • #48418
      Adrian Fleet

      Hi Tim,

      Thank you for coming back to me. You echo my concerns entirely. I am currently a student on the CLRF course and whilst I haven’t been able to commit too much time to it I am very aware of the potential to store issues.

      Since I posted the above I have been in contact with a couple of consultants in pursuit of a THERM calculation to determine the heat transfer at the junction. I was subsequently advised that I actually needed a WUFI calculation and that NBT may be able to advise. I emailed NBT and they called me last week. Mike from NBT was of the opinion that if anything the detail was better than they would usually expect and given the location of the project did not foresee any issues. He didn’t think a WUFI calc was appropriate in this situation either.

      However, your reply has piqued my suspicions again so I am just going to email him to make sure he realises its a box sash.

      I will report back.

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