Loft boarding and Insulation questions/advice - HELP!

Discussion in 'Floors, Stairs and Lofts' started by Blaggins, 29 Jan 2021.

  1. Blaggins

    Blaggins

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    Hey guys. Apologies for the long post but wanted to be thorough, plus I am somewhat verbose at the best of times! Hoping someone can help me.

    Looking to board and potentially insulate my loft, but I have a bunch of questions and concerns! I'm a competent-ish DIYer, but have a tendency to over-think every task!

    When we first moved in, I slung whatever boards I had laying around up there just to get some storage, and I walk between them stepping on the joists (if that's the right term?). I now want to board an area properly to make better use of the space, as well as be safer.

    Initially I was just planning on boarding onto the existing joists, against the existing between-joist insulation but not really squashing it down (perhaps slightly in a couple of places where it sits a touch higher). I toyed with the idea of adding more insulation, but on the assumption I had 100mm between the boards like all the web results were quoting as standard, I was happy enough with that for the boarded area at least. There's barely enough head-height as it is (highest point in the roof peak is around 2m, sloping down sharply, and at around 180cm tall I can just about walk through the middle comfortably without ducking too much), so I had quickly ruled out loft legs or similar because I didn't want to lose the head height. I figured I could always add insulation to the rest of the loft and see some benefits, but this way I could just get on with boarding at least (we still have our xmas decs piled up in the way!).

    Then I started to read about potential interstitial condensation, and how you should raise the floor to allow breathing space. "But I don't have the head height!", I cried. After much deliberation and with a sudden panic when the nearby DIY chain finally got some more stock of loft boards, I went and bought some packs of chipboard loft panels and some 2400 x 63 x 38 CLS timber, thinking I'd raise the floor by 63mm by laying these at 90 degrees to the existing joists, giving the insulation some breathing space.

    I then started to think about it all some more and decided that I needed some advice from the likes of you helpful bunch, and quickly realised I'd need actual measurements of the existing joists. It was at this point that I discovered I didn't have 100mm joists as I had been assuming, but rather 70 x 35! These seem incredibly small to me, and so I then started to worry about how much load they can carry, and also whether or not I should have been so carelessly walking across them (I'm a big chap!). Also, that means I only have 70mm of insulation, rather than the recommended 270mm. Suddenly, I don't know what to do again.

    I know that ideally I'd add 200mm of insulation on top of the existing, and raise the loft floor up to that height, but I really don't want to lose over 20cm head height and make it difficult for me to get around up there. It would also mean I'd probably need the XL loft legs, which would make the whole process even more expensive, lose even more head height (as they're 300mm tall), and I'd also worry about stability of a platform raised this high?

    So I'm at a bit of a loss now, unsure what to do from the number of different options I can think up. Thus, the questions/concerns:

    1. Is condensation always a concern, or are there specific things I can check to rule it in/out?

    2. How is boarding on loft legs (or similar) with a raised insulation height any less prone to condensation? In the typical examples given (100mm between joists + 170mm above, using 175mm loft legs) there would still not be an air gap between the insulation and the boards, so condensation presumably is still as likely? I can't see how this solves the problem.

    3. I've seen claimed (simpleenergyadvice.org.uk) that a £225 investment in insulation will save me £10-15 a year. Is that accurate? Because honestly, even at the the higher saving of £15, that's still a 15 year turnaround before I break even, and honestly I doubt we'll still be in this house by then! I wonder if it's even worth worrying about? Honestly I think the insulation cost might be closer to £150, but that's still 10 years to break even (based on £15 heating saving a year).

    4. I've seen suggested on here that laying additional joists/studs (sorry, unsure of terminology) at 90 degrees to the existing joists would strengthen the structure, as it spreads the load. Is that right? What about the extra weight of the wood? Is this preferable to using Loft Legs to raise the platform?

    5. How concerned do I need to be about weight? Despite my not insignificant personal bulk (which I AM concerned about!), I have been merrily walking around up there on the joists just assuming it would be fine. Now I've seen how small they are I'm concerned that the permanent weight of stored items, plus whatever flooring solution I employ, coupled with the temporary weight of me accessing said stored items, is too great?

    6a. Is there a benefit in bulking up the insulation in the rest of the roof that isn't to be boarded, and leaving the boarded area with the existing 70mm? Either boarded directly onto, or with a small breathing gap from the studs I bought. Or is it a waste of time unless you insulate the whole roof? Will all the heat just escape through the thinner part? Or will the rising warm air be suitably trapped under the thicker insulation, with a cooler spot under the thinner part?

    6b. I also wondered about adding 200mm insulation to the unboarded sections, and just 100mm to the boarded section on regular loft legs, leaving 70mm breathing room (seems a lot?). Thoughts on that? Presumably this is preferable to 6a, from an insulation perspective?

    6c. I even wondered whether I could leave a board's width along the centre (parallel to the roof ridge) without any extra insulation for me to walk along, preserving the full head height, with a storage platform either side raised up (either 100mm or 200mm)? Thoughts on that?

    7. Any other way you'd recommend approaching this? I'm sure I'm missing something.

    I'll attach a crude diagram I've drawn (inexplicably in Excel, I have no idea why) to show the loft layout, and where I intend to board (the beige area). Joists are 60cm centres. I've drawn in one of the trusses(?) to show the supports. House was built around 1981 I think. 3 Bed detached, timber framed. South-East England. Let me know if you need any other information.
    Loft Plan.jpg

    Really appreciate any help and advice people can give, as honestly I'm at a bit of a stalemate here. It's times like this I wish I wasn't so inclined to overthink, as I'd have just boarded the joists and got on with my life!

    TLDR: Boarding loft area with 70mm x 35mm joists at 600mm centres, 70mm existing insulation between joists. Advice please! Thanks!
     
    Last edited: 29 Jan 2021
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  3. JP_

    JP_

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    Well, I can't say for certain as just a DIYer, but one option might be to use solid board insulation under the area you want to board - 70mm kingspan (or similar) is equivalent of about 140mm of standard loft insulation, so while still not as much as recommended, it will make better use of the space. You can then pile more insulation on the edges that you'll never use.

    One consideration though is access to light fittings or any other electrics that might run through the loft - you might need to work around this?

    I have a flimsy truss loft in my extension, the builders boarded the centre of it and used an internal partition wall to provide some additional support, this works well for storage.

    How tall are your ceilings? A more drastic option would be to insulate ceilings by over boarding with insulated plasterboard, this combined with 70mm PIR / kingspan in the loft would bring it up to current standards, and you'll have the same headroom in the loft still. But a big cost for a bit of loft storage! Room in the garden for another shed?
     
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  4. Blaggins

    Blaggins

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    Funnily enough I was just reading another post on here about board insulation and wondering the same. That's a really good thought, thanks. Do you know how I'd use it? The post I read said to just lay the loft boards on top but that doesn't seem right to me - wouldn't the insulation board compress? presumably I could use a sufficiently sized bit of timber to raise the floor above the board insulation and lay the boards in between, but I'm not sure if i'd still need a breathing gap, and if so how much?!

    I love the thinking outside the box! But our ceilings aren't high enough, and as you say that would be over the top for a bit of extra storage!

    This might not be a bad idea tbh! Our shed needs replacing, so I could build a bigger one I guess. Still feel like we'll need the loft space as we have SO much clutter, and some things we'd want to definitely stay dry.

    Thanks for the reply.
     
  5. JP_

    JP_

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    A shed should be as dry as a loft, and I think a well insulated loft, as in ceiling insulated, is not much different from a shed - you wouldn't want to store paper, photos etc there.
    Sheds are also easier to access as we get older! My wife wants me to board out a large part of our old loft, which as no felt in the roof to gets very dusty - I am pushing for bigger shed instead!
     
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  6. Blaggins

    Blaggins

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    Hmmm, that's an interesting point. I guess i just assumed a loft would be more suitable, but perhaps not. I guess i should move the photos and paperwork!
     
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  8. Blaggins

    Blaggins

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    Bump to see if anyone else can offer any input on my long rambly post?

    Also, if anyone has advice on how best to install the board insulation per jonbey's suggestion above?

    Thanks
     
  9. Jonathan1111

    Jonathan1111

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    As a reundant truss designer, your roof trusses will have been designed to support your weight wandering around in the roof space on an occasional basis for maintenance etc, and also for storage of bits and bobs up to 250 newtons per square meter, (approx 25Kg). The size of the timber used will have been designed for those loads, and your truss span is only 4.5m, so 72mm timber is fine, it is only unsupported for 1.5m.
    As for boarding, using bearers across the joists won't make the trusses stronger, but it will help with load sharing across multiple trusses. In practice this means that the overall load applied cannot be any more than the 25kg/m2, but you will get away with the load being concentrated in one point.
     
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  10. Blaggins

    Blaggins

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    Thanks so much for digging this old post out and replying, I really appreciate that!

    So cross-battening will help to spread the load, without needing to worry too much about the additional weight of the battens and flooring?

    Storage-wise it will be suitcases, some storage boxes of old clothes, and a few other small bits and bobs.
     
  11. Jonathan1111

    Jonathan1111

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    Yes, that's right. The weight of the battens and boards should be knocked off the 25Kg/m2 but in reality, the trusses will carry considerably more than that without any problem. I'm just learning about the condensation issue, which seems to be of more concern than overloading, and i am beginning to think it is probably not wise to board above bathrooms, and especially shower rooms.
     
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  12. Yannick Wawawawa

    Yannick Wawawawa

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    Hi @Blaggins - I have many of the same questions. What did you end up doing?
    Yannick
     
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