Another Insulation question...

Discussion in 'Builders' Talk' started by Jayme2189, Apr 11, 2020.

  1. Jayme2189

    Jayme2189 Member

    Hi all, as the last response to concrete question/underfloor heating question was helpful thought I'd check my understanding of insulation. I'm renovating a 3 bed end of terrace house to live in. Downstairs has had an RSJ installed and become open plan. Doing this with my brother an electrician and getting others tradesmen where necessary.

    Plan A Cavity wall insulation and fresh plaster.
    I've got a cavity wall insulator coming to do a borescope survey next week. (Keeping appropriate C-19 distance). Which then means I'll ideally get it and then use saved money for a fresh plaster across house with someone comes highly recommended by family.

    Plan B
    I'm thinking of using KIngspan K118 Kooltherm plasterboard on the 3 external walls and skim. I think I can leave the neighbours wall as it's a heated house.

    Currently there is dot and dab plasterboard. But it's all coming down either damage/rewire and replumb access.

    Id rather batten then use dot and dab. My understanding is damp proof membrane, screw in the batten then insulated plasterboard.

    My brother reckons we can apply a damp proof membrane, plywood sheets directly, then insulated plasterboard. The plasterboard will be firmly attached and it will be easier to attach shelves/TVs etc. This will help us save space as not the biggest property.

    Is there a reason this won't work? Other than cost. Would the ply act as a cold bridge more so than a batten? I think there must be a reason just can't work it out.

    I haven't seen this suggested anywhere. I've seen suggested : batten -ply sheets-insulated plaster; batten-insulate gaps-plaster; batten-insulate gaps-insulated plaster.

    Appreciate any advice, suggestions, thoughts, explanations. Thanks
  2. David Hatim

    David Hatim Active Member

    Insulating the inside of the exterior walls, ground floor and loft is a good idea, it will save on your heating bills and cut down on your co2 emissions. If you do it, make sure you take pictures, so if you come to re-sell it will put your house in a higher bracket, I notice that buyers are using this as a price negotiation technique. There are many ways to go about insulating the walls, and I'm sure that lots of people have there own personal preferences. Lots of old terrace houses suffer with dampness on the ground floor, so best to be safe than sorry, as long as you have some sort of physical damp protection at least 1200mm from ground floor that should be ok. I'll let you know our preferred method, we remove all the existing plaster, (this really saves time on first fix electrics) apply 2 coats of tanking slurry up to 1200mm, then stick the insulated plasterboard on using plasterboard foam (like expanding foam but doesn't expand as much) the boards will be firm in ten minutes and can be skimmed within an hour. I take your point about fixing shelves, TVs etc, but there is a huge amount of different fixings out there, we have never had a problem. Good luck!
  3. Hfs

    Hfs Screwfix Select

    I would batten walls, super quilt on top of battens, batten on top then plasterboard.
  4. Mr Rusty

    Mr Rusty Screwfix Select

    DO NOT DO NOT do this. Over the past 3 years I have renovated a 1902 semi, very similar to many end of terraces.

    I have read, researched, and used a fair bit of science in coming to the following conclusion. Just my opinion, of course.

    IMHO the MOST valuable thing in insulating an old house is a ventilated cavity. It does two things - it dries the inside of the outer brick leaf which is inevitably porous because most are built with lime mortar, and 2 it dries any interstitial condensation caused when you lower the temperature of the inner leaf by insulating internally. This is very important. If you insulate internally, that inner brick leaf gets colder. Any house air that gets to it may well condense. The ventilated cavity ensures that this can dry.

    I disagree with this. Personally I think any type of waterproofing, inside or out is folly.

    start reading here.
    Last edited: Apr 11, 2020
  5. Mr Rusty

    Mr Rusty Screwfix Select

    There is no point in battening for insulated PB. You also don't need a DPM because insulated PB has one. Battening also has the disadvantage that if the wall isn't flat the battens won't be and then the wall won't be. Thick insulated PB (I used 60mm insulation on most ext walls, but 25mm in areas where I couldn't spare the space) is not very flexible but fixes perfectly using either PB adhesive (D&D) where you want the ability to adjust the boards by tapping them in to place, or foam where the base wall is very flat.

    Source insulated PB carefully - it varies in price enormously.

    IMHO the construction you are looking for is to reproduce what is now used for timber framed houses - that is, working from the inside outwards:-

    plasterboard, Vapour Control Barrier, insulation, internal structural element (i.e. timber frame or inner brick leaf) then most importantly a ventilated cavity, then the external facing leaf (the outer brick). I repeat what I said earlier - IMHO the most important element is the ventilated cavity. If you think about it, we know the outer brick is designed to be porous because we install weep holes, cavity trays, and cavity wall ties have drip arresters in the middle, so why anyone ever thinks it is a good idea to fill the cavity with insulation which once it gets wet will never dry!!

    As I say, this isn't just theory - I've done all this to my house and so far all is good.

    One other thing - if you have chimneys, make sure they have ventilated cowls/caps on. We don't have enough open fires these days to keep chimneys dry, and rain coming down combined with old combustion salts is a recipe for problems on chimney breasts. Mine is slowly drying out now I have capped the chimneys, but 100 years of coal fires and very heavily salt laden bricks was/still is a bit of a challenge.

    A little more information - when we inherited the house, an old guy had lived in the house with a coal fire every day, dried his clothes inside, the back room was just flags on sand covered with lino and carpet. The lime plaster was sodden half way up the walls, and I mean wringing. All the symptoms of "rising damp", except when we dropped the rotten plaster and ventilated, the bare brick all dried out - completely!. Proof if ever there was that "rising damp" is really condensation on the colder bottom of walls.
    Last edited: Apr 11, 2020
  6. David Hatim

    David Hatim Active Member

    And what really went on at area 51?
  7. Hfs

    Hfs Screwfix Select

    Just to clear a few things up from above....not all insulated plasterboard has a vapour barrier. If a wall isn’t totally flat when battening then just needs packing out with shims to get it flat...doesn’t take long and is easy for anyone that has basic DIY skills.
  8. Mr Rusty

    Mr Rusty Screwfix Select

    True, but all the PIR I found did, and EPS and XPS doesn't need it because it is impervious anyway - I used on this last project.

    True, but why bother when the manufacturers recommended installation method for cavity walls is plasterboard adhesive?.
  9. glob@l

    glob@l Active Member

    Presumably this would be different with solid sandstone walls. For example forming a cavity to ventilate the internal stone face?
  10. Mr Rusty

    Mr Rusty Screwfix Select

    I guess so - if you look at the Siniat info it suggests metal studding to form a cavity on solid walls. If you read heritage house it pretty much says never internally insulate a solid wall. I discussed interstitial condensation with my BC during this last project and his opinions was as mine - the ventilated cavity on my house should avoid problems, as it appears to be doing.
  11. Hfs

    Hfs Screwfix Select

    You shouldn’t stick foil backed insulation board with plasterboard adhesive:
  12. Mr Rusty

    Mr Rusty Screwfix Select

    True, but insulated PB isn't foil backed. The foil layer is between the PB layer and insulation layer.
  13. Hfs

    Hfs Screwfix Select

  14. Mr Rusty

    Mr Rusty Screwfix Select

    Celotex PL4000 like the Siniat equivalent is not "foil backed" the same way that foil backed PB is. You are correct that you shouldn't use PB adhesive for foil backed PB (because its actually metallised plastic-polyester coated), but all the manufacturers of insulation backed PB confirm using dot&dab PB adhesive for their boards is correct.

    For PL4000 you linked to :- pp34
  15. choppy111

    choppy111 Member

    Are you saying dot and dab is correct and battening is not?
  16. Abrickie

    Abrickie Screwfix Select

    For that product both methods are acceptable by the manufacturer;)
    Mr Rusty likes this.

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