Insulating eaves of existing loft conversion

Discussion in 'Builders' Talk' started by Thresher, Dec 5, 2015.

  1. Thresher

    Thresher New Member

    Firstly I apologise for the length of the query. I have been reading various forums and have found myself increasingly confused and thought I’d get all the information down in one place. As with all DIY queries my case is subtly different to all the others I have already read about on line!

    I’m trying to improve the insulation of an existing loft conversion on a house I have just bought as cost effectively as possible. The loft conversion was built approx. 15 years ago and as such the insulation is not up to current standards and I have found the main room (used as a bedroom) to get cold. I believe most of the cold enters the room through the doors entering into the eaves storage areas which are furthest from the single radiator. The issue does not appear to be just a draught but also general creep of the cold from the eaves into the room (I have already tried to reduce the draught around the doors as much as I can using brushes and foam strip).

    I have a contractor booked in early next year to replace the pitched portion of the roof due to other issues. As such I have asked them to also re-insulate the pitched portion which is inaccessible to me and they are planning to do this using Rock Wall. To save costs I have asked them to not insulate the eaves pitch as I will do this myself. The flat roof is not being replaced and consequently there is no access to these rafters. In the interest of keeping costs (and hassle) down I would preferably not want to remove the internal plasterboard and insulate from inside.

    The current insulation properties of the room and planned improvements are described in the image attached.

    Finally to the questions:

    - If using breathable felt is it okay to use unvented PVC soffits and fascias when I replace the existing ones? (I will also be replacing the soffits and fascias myself)

    - Should I insulate the eaves roof rafters, the eaves stud wall or both?

    - If insulating the rafters shall I insulate between, under or both? If I don’t have enough money for both, which is better? Under, to avoid a thermal bridge across the rafters?

    - How much gap needs to be left between insulation and breathable felt to ensure good ventilation? (I have read both 25mm and 50mm)

    - How do I ensure continuous insulation where the rafters meet the ceiling joists without compromising ventilation? Is ventilation still an issue for breathable felt?

    - How should I insulate the eaves stud wall? Between studs, in front or both?

    - If insulating the eaves stud wall is there a decent way of also insulting behind the door to provide some continuous insulation? (I was planning in overlapping insulation board placed in front of the studwork with the gap between the door and frame and placing some insulation board on the back of the door as well)

    How do I fit insulation board where I am unable to use friction fit (to the front of the studwork, under rafters or to the back of the eaves door)? Screws and large washers or specialist fittings?

    Thanks for any help.

    [​IMG]
     
  2. BMC2000

    BMC2000 Screwfix Select

    I used Kingspan Nilvent membrane for my pitched section, draped it approx 10mm between rafters, filled between rafters with Kingspan TP10, left 25mm gap above insulation and am about to over board with K18 insulated plasterboard. You don't need through ventilation with this product, raadread the BBA cert/install instructions, I did add a continuous fascia vent though as I am a bit sceptical!

    You need a 50mm air gap for impervious bitumen type, non breathable felts, the space is to allow the condensate to collect and be blown out by the eaves and high level through flow ventilation. Check the product requirements of the felt/membrane you are planning to use and design accordingly.

    If you insulate your full pitch roof down to the eaves there's no real need to insulate the stud wall as the pitch is doing the insulation task.

    As for your flat roof, is there through ventilation here? And does it rely on the pitch section for ventilation? You'll need to check this before starting as what I said above relies on the pitch section being effectively sealed.
     
  3. Thresher

    Thresher New Member

    Thanks for the response BMC.

    I have been assured by the roofing contractor that they will be using breathable felt when replacing the pitched roof and the flat roof which is not being replaces is fibreglass. As such I suspect it relies on the pitch roof for ventilation.
    For this configuration what would you advise to ensure adequate ventilation?
    Would a 25mm drape space be sufficient if using kingspan/celotex in the pitch?
    Even with the breathable felt would you still advise vented soffits then?

    If I insulate the full pitch down to the bottom of the eaves, how do I ensure continuous insulation where the rafters meet the ceiling joists?

    I suspect it will be a lot easier to insulate the stud wall due to access not being as much of an issue. Will this be as god as insulting the pitch?

    Thanks again.
     
  4. BMC2000

    BMC2000 Screwfix Select

    My advice, either tell them what brand of breathable membrane you want and design to the recommendations of the membrane.

    I'd review how the flat roof is ventilated, and insulated and see what knock on effects your proposal will have on it. Get someone to help you look at this. What direction are the flat roofs going? Is there soffit vents? Is it a cold roof or warm roof?

    As for insulation of the pitch or stud wall, it's up to you, you still need to get your ventilation right and insulation right.

    If you are doing a loft conversion you will no doubt be putting 100mm acoustic rated mineral wool between the floor joists, this will help with your thermal bridge. I put this is on chicken wire supports for fire rating the floor.

    These are tricky details and require investigation, detailed product specification and a good standard of installation to get the best results.
     

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