Solid external wall insulation

Discussion in 'Eco Talk' started by reno-help!, Mar 3, 2021.

  1. reno-help!

    reno-help! New Member

    Hi all, new here so please go easy!

    I have a 1920s solid wall house (lime mortar) with no external insulation/render. I want to insulate the upstairs external walls only so want to go with the internal insulation method (we plan to extend the downstairs, front, back and double on the side). The walls are cold to touch but there is no sign of moisture.

    The wall has original existing plaster on the inside and when i get the rewiring done and change the windows, i want to do the insulation but am concerned about trapping moisture/dew point etc.

    1. Do i go back to brick?
    2. if so, do i tank the walls?

    It sounds like the batten method is the way to go, but

    3. do i leave a gap between the battens or insulate between with polystrene/rockwool etc,
    4. Do i put a breathable membrane or DPM to the battens and trap it with to make it air tight,
    5. what type plasterboard ? Foil backed? ?thickness?

    6. or shall i not bother at all and just skim the walls to make them neat and tidy?

    I have read so much about this my head is starting to hurt! Any advice would be greatly appreciated.

    ( i also plan to convert a small bedroom to a bathroom which has 2 external solid walls, so want to do something similar but want to tiles floor to ceiling - but that might need a whole thread on its own!)

    Thanks.
     
  2. ElecCEng

    ElecCEng Screwfix Select

    1. Yes.
    2. No.
    3. Insulate with PIR (brands such as Kingspan/Celotex etc) or mineral wool batts, depending on budget. PIR is much more effective for a given thickness.
    4. No.
    5. Use foil backed. Insulated plasterboard would be even better if budget allows.
    6. Yes it is worth doing. Use a U value calculator to see just how much difference it would make.

    Also check your roof void.
     
  3. reno-help!

    reno-help! New Member

    Thank you for the reply ElecCEeng.

    1. So i don't need a membrane at all?
    2. And how do i attach the PIR/mineral wool to the wall? can i use a spray adhesive to the brick? or are they attached to the battens and not in direct contact to the wall.
    3. what do i need to look out for in the roof void?

    The devil truly is in the detail and i want to get this job done right the first time.

    many thanks.
     
  4. ElecCEng

    ElecCEng Screwfix Select

    No membrane. You want any moisture generated in the house (breathing, cooking, showers, drying laundry etc) to exit via a vent you have installed for the purpose (extractor, windows, doors, window vents etc). You need to seal the inner lining of the building to do that. The easiest way to do that is foil backed board with taped joints. Otherwise you will need a barrier behind the plasterboard. Any moisture that condenses naturally out of the air into the building fabric should move outwards or be caught in an air gap. You’re limited in where you can add an air gap unless you put in studwork rather than battens or your external wall has a cavity. One issue you may have is the outside of the wall may have been ‘sealed’ either with paint on products or a coating such as a render. If thats the case it could be more complicated, but not worth worrying about until you do the ground floor extension. When you do that you can install vents in the upper floor walls.

    PIR should be cut snug enough to hold itself up between the battens and you should use expanding foam and aluminium tape on any joints. It doesn’t need anything to hold it to the brick. Mineral wool roll is a nightmare for walls unless you hold it in place with a membrane. Use batts (solid slabs) instead and again fit them snug between the battens. The other thing you can do is put up the walls then fill the void behind with insulation beads. Not as ideal as PIR or batts and you’re left with the issue of sealing the holes.

    In the roof void you need insulation too. Mineral wool roll is ideal. Put at least two thicknesses down, one on top of the other running in different directions. You need a vapour barrier above any bathrooms. The void should be vented to prevent condensation. This is normally in the eaves but you can retro fit vents fairly easily. Be careful not to block vents with insulation.

    Another option to consider is external insulation. Effectively attaching PIR type boards to the outside of your house and then cladding or rendering. You’ll have scaffold up for your extension so the cost won’t be too much more. This gives a much better u-value than putting them inside. Also means your walls are acting as a thermal store. Just dot and dab or batten on some foil backed board inside.

    Hopefully that’s clarified your options rather than just confused you!

     
  5. Joshuaojs

    Joshuaojs Active Member

    Great reply @ElecCEng. Sorry to jump in. You say exterior render that seals the wall will complicate things are you able to expand on that, cheers
     
  6. ElecCEng

    ElecCEng Screwfix Select

    No problem. If the external wall is completely sealed you’re effectively trapping air (moisture) in the building fabric. In a modern cavity wall that’s not a problem because there are vents which allow air to circulate within the wall.

    It could cause problems in an older wall, not for certain. Totally depends on the environment, whether the wall is open to to roof void, if it has a DPM underneath etc etc.

    In the case of the OP, their walls currently absorb lots of heat from the interior of the building as it is uninsulated. This helps to push the moisture out and up. Once you insulate the interior of the outside walls you’re lowering the temperature inside the wall and it effects where the moisture goes. Ideally you want an air gap/cavity somewhere in the structure of the wall to deal with the moisture.

    Much debate out there about air gaps. Especially as most people get wound up about what it states in building regs, which in reality is just a rubric for where and how thick the air gap should be for most modern construction techniques in most parts of the UK. The actual purpose of the air gap is to cause the moisture in the air to condense on one of the surfaces either side of the air gap and either drain away or be drawn away by circulating air. You can work out exactly where the air gap needs to be and how wide, by calculating the ‘dew point’ for a set of environmental conditions.

    Apologies for the long winded response. Put simply a building needs to ‘breathe’. Air from the interior goes in and out through the windows and doors (and other ventilation), so you seal the interior. If you completely seal the outside of the building as well, it can’t breathe.

     
    Joshuaojs likes this.
  7. ElecCEng

    ElecCEng Screwfix Select

    Just to add, clearly the render serves a purpose. You need to stop rain etc from damaging the wall. Brick and stone do the job fine in most places, but if you live near the sea or in the North of Scotland, you need something more robust.

    It’s where a product has been applied to a wall without any further consideration of location or moisture management that there’s potentially a problem.
     
    Joshuaojs likes this.
  8. reno-help!

    reno-help! New Member

    Thank you for taking the time to reply. Yes if you are going to the length to insulate, the generated moisture needs to go somewhere.

    I think i'll go with the snug PIR boards with foam/tape and the foil back boards over the top.

    Do i need DPC strips where the batten fix to the wall? i read that somewhere. There is no external render or sealant that i can see. And the wife doesnt like the rendered look and prefers the bricks!

    Also i saw this - https://www.backtoearth.co.uk/product/thin-wood-fibre-insulation/
    It is breathable and goes directly onto the brick or existing plaster using a lime bond and then a lime skim over.

    Has anyone come across this or used it?

    Thank you for taking the time to reply.
     
  9. ElecCEng

    ElecCEng Screwfix Select

    If it’s untreated brick you’ll be fine. Use treated battens and again no issues.

    I am 100% behind more sustainable and natural products but you’d need to do a u-value comparison with your PIR option and weigh up the cost/benefit. The environmental cost of PIR over natural is negligible if you’re not significantly reducing the amount of hydrocarbons being burned.

     
  10. reno-help!

    reno-help! New Member

    Thanks ElecCEng. I will investigate the natural products and feed back to this thread regarding the u-value. I like it though as there is no `gap' for condensation to form.

    Oh, that reminds me. one more thing you said in an earlier post -

    "The actual purpose of the air gap is to cause the moisture in the air to condense on one of the surfaces either side of the air gap and either drain away or be drawn away by circulating air. You can work out exactly where the air gap needs to be and how wide, by calculating the ‘dew point’ for a set of environmental conditions."

    Do i need an air gap? if i wedge the boards between the battens, will i need to leave space behind, or can they hit the wall? and if so, do i need to fill with foam?

    i think this bit confuses me the most, as i don't want to do something and then in a year, get damp/mold because i have done something wrong. I guess that's where adequate ventilation from window vents is needed.

    Thanks.
     
  11. reno-help!

    reno-help! New Member

    Bump. Can anyone advise whether i need an air gap? behind the PIR boards to the brick wall? thanks
     
  12. Bob Rathbone

    Bob Rathbone Screwfix Select

    In my previous home with similar issues I did not remove the existing plaster. I fitted tilers lath 24 x 38 vertically at 600mm centres and in filled with 25mm thick insulation. A vapour barrier was then fitted and 2400 x 1200 x 12mm board and skim. Did the job great.
     
  13. reno-help!

    reno-help! New Member

    Thanks Bob. In your method there is no Gap then?

    May i ask what type of Vapour barrier you used? there are so many types with different breathability aspects (2way, 1way etc) . I suppose you could use plasterboard with pre-fitted vapour barrier?

    I've started reading about `cold` vs `warm' batten methods. more confusion and headache!
     
  14. Jacob23

    Jacob23 New Member

    1. If you want to insulate the upstairs external walls of your solid wall house using the internal insulation method, you do not need to go back to the brick. Instead, you can install the insulation on top of the existing plaster.

    2. It is not necessary to tank the walls in this case, as long as there is no sign of moisture on the walls. Tanking is generally only needed if the walls are prone to water penetration or if there is already moisture present.

    3. The batten method involves attaching wooden battens to the wall and then filling the space between the battens with insulation. You can either leave a gap between the battens and the wall, or you can insulate between the battens with materials such as polystyrene or rockwool.

    4. It is generally recommended to use a breathable membrane or damp proof membrane (DPM) behind the battens, to allow any moisture that does manage to get into the wall to escape. This will help to prevent condensation and reduce the risk of damage to the insulation.

    5. For the plasterboard, you can use standard plasterboard or foil-backed plasterboard. The thickness of the plasterboard will depend on the type of insulation you are using and the width of the battens.

    6. Alternatively, you can simply skim the walls to make them neat and tidy, without adding insulation. This will not improve the energy efficiency of your home, but it will improve the appearance of the walls.

      You can also find out more on my blog about ----> External Wall Insulation
    As for your plans to convert a small bedroom into a bathroom with floor-to-ceiling tiles, you will need to consider the moisture resistance of the walls and the floor, as well as the need for ventilation. It might be helpful to start a separate thread specifically on this topic, to get more specific advice.
     
  15. I have a similar age house to yours and am currently in the process of insulating the external walls.
    I left the original plaster on and battened out using 25x50 treated roofing batten fixed to the wall with a strip of DPC behind it. I then filled the space inbetween the battens with 25mm pir insulation, taping all joins with aluminium foil tape. I then over boarded the whole lot with another sheet of 25mm PIR again taping any joins. After this I installed 12.5mm plasterboard using 60mm drywall screws. I was advised this method on here and on the rooms I have done all I can say is so far so good!
     
  16. Alan22

    Alan22 Screwfix Select

    Regarding the air gap - I'm currently insulating a sandstone building(stripped out to walls) building regs say 50mm air gap and 120mm PIR insulation thickness(current Scottish)

    I'd be worried about cold and moisture bridging without a gap.
     

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