Insulating eaves?

Discussion in 'Builders' Talk' started by soabar, Feb 11, 2014.

  1. soabar

    soabar New Member

    I'm in the process of renovating an upstairs room which is built into the eaves of the house (the outer "wall" of the room is lath & plaster directly onto the rafters). The lower part of this wall is boarded to form a cupboard / storage area running the length of the room.

    One of the jobs is tidying up this cupboard & changing from regular door to sliding doors. As I've got into the job, I've noticed that the area between the outer roof & the plaster in not insulated & trying to figure how to go about insulating it. There seems to be little access from anywhere, so I've been coming to the conclusion that I'll have to strip the plaster, insulate & then re-plaster - does this sound correct?

    I'm looking at Kingspan K7 type board as the easiest way to do this, but really not sure if I'm on the right lines & am concerned about ventilation / moisture;

    The house is a 1930/40's two storey, with a slate/felt/boarding double roof, it's got solid concrete soffit's so I'm assuming it's not ventilated.

    Can I use Kingspan type board here & should there be an air gap between it's top face of the roofing boards?

    Apologies if any of the terminology is wrong & thanks for any advice.
  2. Devil's Advocate

    Devil's Advocate Well-Known Member

    I think - if I've read your post correctly - that the simplest, and least destructive method would be to simply line the insides of the cupboard with Thermal Laminate Board which is 9.5mm plasterboard with a high-value insulation layer bonded to its back.

    Simply cut neatly, mitring the edges where the panels join so's the insulation layer is kept constant, and then screw it in place into the joists behind. Tape and fill the joins afterwards as you would with any wall.

    Or, do it in two stages - insulation and then p'board - might be easier. Mark out where all the joists are first, then neatly cut and fit the rigid insulation sheets and pin them in place, run a bead of expanding foam in between where they join to fill any gaps there, and then over-board with 9.5mm plasterboard, screwing through with linger p'board screws.

    In fact, I think I'd go the second route - easier, and more reliable to get the insulation gaps filled.

    Use whatever thickness insulation board (Celotex/ Kingspan type stuff) you can get away with - the thicker the better, of course, although even 25mm board will make a huge difference.

    Bear in mind, tho', these eaves cupboards will still likely be the coldest parts of the room, and not well ventilated, so you may still find you'll need to keep an eye on it in this respect. If so, it may be worth considering adding a couple of neat vent grills through to the room.
    FatHands likes this.
  3. Sean_ork

    Sean_ork Screwfix Select

    before folks all go shoving lots of insulation into every void and it's worth remembering why there should be a gap
    between the insulation and roof line - if you block it up are very likely to quickly suffer the affects of significant quantities of moisture within the loft space - so don't block the ventilation routes, or fit a one of the many kits designed to enable maximum insulation whilst still allowing adequate ventilation - even if you do have solid soffits there will have been some natural cross ventilation, perhaps not by design - there are many many 1000s of roofs that are slowly being destroyed due to conversions carried out with no regard to ventilation of the timber structure

    there's also a lot of FG top up insulation that's been installed recently by folks who also don't appreciate the need for ventilation

    FatHands likes this.
  4. soabar

    soabar New Member

    Thank for the replies.

    I possibly didn't explain the eave's / room properly first time around, what you say about insulating the cupboard / storage area makes sense, but the cupboard area is only about 1.0m high & there is still quite an area above it that still wouldn't be insulated - the red bit here;

  5. Devil's Advocate

    Devil's Advocate Well-Known Member

    What is in that red bit at the moment? Just exposed roof rafters? In which case use use whichever method I outlined before (either Therm Laminate Board or else Rigid insulation followed by p'board) and screw it straight on to the rafters.

    This will leave the current voids in between the rafters fully ventilated. (Unless there are full-depth noggins going horizontally between the rafters?)
    soabar likes this.
  6. soabar

    soabar New Member


    There is nothing there at the moment, the lath & plaster is fixed directly to the rafters & there is nothing between the plaster & the outer roof backboards, so there is a rafter depth gap (no noggins that I've seen so far).

    I'm still thinking along the lines of the Kingspan type insulation boards followed by plaster - I'm assuming that the insulation boards should be fitted flush with the inner rafter edges, leaving a ventilation gap between it & the actual roof boarding?

  7. Phil the Paver

    Phil the Paver Screwfix Select

    Would it take up to much room in the room if you fit the insulation over the lath & plaster and then board & scim over it, it would save a lot of mess.
  8. soabar

    soabar New Member

    I would if I could plaster........ The room isn't huge either, so I would prefer not to.

    Thanks for the suggestion though.
  9. Sean_ork

    Sean_ork Screwfix Select

    you don't need to know how to plaster - google tape and fill
    Smethy likes this.
  10. Devil's Advocate

    Devil's Advocate Well-Known Member

    Soabar, I am still not 100% sure what that red bit is. It's lath and plaster, yes? With the full depth of rafter behind it?

    In which case your choices are to do as Phil says above (and I said earlier) - simply screw Thermal Laminate Board over the existing lathe and plaster layer. As Phil says, easy and clean.

    Or, rip off the lathe and plaster (cough cough) and do as you yourself suggest - cut the rigid insulation to fit tightly in between the rafters keeping it flush with the room edge of the timbers, and ensuring a 50mm gap remains behind it, between the insulation and the roofing boards.

    This isn't as good, as the rafters edges and the insulation panels with have different thermal properties. But it's fine. You could improve it a significant step further by overlaying the whole lot in a further layer of rigid insulation board, so that there are now no gaps or thermal bridges. You could use the thinnest board for this - around 25mm?

    Or, are you hoping there's some way of getting insulation up behind the lathe and plaster without removing it? I guess that's possible, depending on what the access from the eaves and the loft is like, but I think you'd struggle to get rigid insulation in there neatly, and kept tight against the l & p. If you were to try this, I reckon you'd actually have more success with soft loft insulation, but you'd need to ensure it would stay in place and not slip and block the ventilation route. And you'd only get the thinnest layer in there, so barely effective.
  11. soabar

    soabar New Member

    Yes, it's lath & plaster onto the rafters & then fresh air the depth of the rafters to the outer roofing boards.

    I don't really want to lay anything onto the plaster, so I'm planning to strip it, fit insulation panels between the rafters (leaving an air gap on the "upper" side) & plaster boarding back over.

    I've had a good look at getting insulation into the existing gap & leaving the current plasterboard intact, but there just isn't any realistic method of getting it in there correctly - the roofing boards are peppered with nails that protrude inwards a fair bit taht would tear / snag rigid insulation & snag woolly stuff......

    Thanks again.
  12. Devil's Advocate

    Devil's Advocate Well-Known Member


    Well, as said above, if you can afford to lose a further inch, then it'll be a good move (after you've fitted insulation in between the rafters) to overlay everything with unbroken sheets of even the thinnest insulating board before p'board.

    But, your call :)
    soabar likes this.
  13. DIYDave.

    DIYDave. Well-Known Member

    If you decide to friction fit solid insulation between the rafters then overboard with either insulated board or just plaster board, even leaving a 50mm gap between insulation and roof boards, will this give ventilation between the rafters ?

    If you've got no soffit vents, and no vents at the ridge, how will there be any air flow to keep rafters and roof boards dry and condensation free

    More of a question from me really
  14. Devil's Advocate

    Devil's Advocate Well-Known Member

    'Modern' builds will have proper soffit vents fitted. Older builds - like this one (lathe!) will just be draughty :).

    Basically, if there was enough ventilation before, there still will be with the gap 'closed down' to 50mm.
  15. DIYDave.

    DIYDave. Well-Known Member

    Thanks DA, as I said, a question really

    Was just that I recently asked a similar question about insulating a loft, adding kingspan between the rafters then overboarding
    Was advised to leave a 50 mm gap top and bottom for air flow

    Realise that this set up is different (ain't they always) ! With rafters plastered over now so guess damp isn't a problem

    I guess that with roof tiles and timber boarding the construction will be naturally draughty, as you say, so not an issue as this will provide adequate air flow
  16. Devil's Advocate

    Devil's Advocate Well-Known Member

    "Was advised to leave a 50 mm gap top and bottom for air flow"

    What is 'top and bottom'? I understand that a min 50mm gap should be left between any insulation layer and the whole roof 'felt' or 'sarking board' layer. It isn't so much an issue with using breathable Tyvek membrane, but with old felt you really need a good circulation of air flying around the undersides of the roof.
  17. DIYDave.

    DIYDave. Well-Known Member

    Hi DA

    Top and Bottom ? -
    Well my original question was about insulating between the roof rafters (maybe wrong terminology) from floor level to the ridge
    50mm Kingspan friction fitted in-between 100mm rafters = 50mm gap between Kingspan and rood boards. Cement tiled roof, no membrane

    "Was advised to leave a 50 mm gap top and bottom for air flow"

    What do you reckon then ? And thanks for the input
    apologies for hijacking the thread
  18. Devil's Advocate

    Devil's Advocate Well-Known Member

    Ah, I see :). There's a 50mm gap between the insulation and the roof boards, AND a 50mm gap left at the eaves and ridge for the air flow to get up to that space?

    That all sounds very nifty :D
  19. Aaron Clifford

    Aaron Clifford New Member

    Insulating depends on roof types.
    Sloping roofs are more straightforward - there are more options to choose from.
    Flat and dormer roofs are more of a challenge to insulate.
  20. Ted Leger

    Ted Leger New Member

    A less invasive insulation is blown insulation. There is a type that you can just put a little hole in the wall/ceiling and the wand will go inside the little hole. It will blow in and expand. I think that I am understanding you correctly, but if not I do apologize, and just ignore the post.

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