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Insulating mass concrete walls

  • 17-05-2022 3:46pm
    Registered Users Posts: 1,036 ✭✭✭ monseiur

    Hi all,

    Just looking for advice/guidance. House built in 1950's, all walls are mass concrete, no cavity. As expected walls are cold and there is sign of dampness in places. I plan to fix an 62.5mm (50mm+12.5 slab) insulated plasterboard to all the 'external' walls - now this is the crux of my query : Should the slabs be fixed directly on to the walls with the special metal/plastic fixings or should I use metal channel or treated 2'' x 1'' battens to allow air/breathing space between slab & walls. EWI is not an option as house has wide overhanging concrete over barges and the soffit is just 170mm. All windows, doors etc. are to be replaced + 400 attic insulation etc. etc.



  • Registered Users Posts: 1,535 ✭✭✭ Dudda

    I’m working on a project currently on site where we’ve just broken off the concrete overhang to allow us to fit the 150mm external insulation. It’s also solid concrete external walls and replacing all windows and doors with triple glazed. The windows rest on an angle to allow them sit inline with the external insulation.

    The only small part we’re drylining inside is because we’ve original limestone cladding (the building is a protected structure) and we’re using a breathable Isover Optima Dry Lining System for this.

    I’d strongly consider the external insulation. It’s a much better system, you can fit thicker insulation and it will reduce thermal cold bridges. We also broke out footpaths to allow the insulation continue down improving it further.

  • Registered Users Posts: 751 ✭✭✭ C. Eastwood


    Do not fit any timber to the wall behind the slabs.

    The only process in a wall is that heat will flow through it when there is a temperature difference inside and outside.

    You stated that the wall is Damp. Walls do not and cannot breathe.

    Where is the damp. Ceiling level or floor level. -photos.

    If the damp is black/ olive green then it’s condensation mould growth. This should stop when you insulated the wall.

    If the damp is in the form of Salts on the inner surface, then it is either rising damp or ingress of rainwater or both. You must determine what is causing this.

    External plastering should solve the ingress of rainwater.

    Electro Osmosis might solve the rising dampness problem.

    I assume that there is no DPC in the solid concrete wall.

    If you cannot prevent the dampness, then you could drill ventilation holes in the wall to vent a cavity, and construct a 100 mm metal stud partition with insulation, 50 mm away from the inner surface of the wall, to provide a 50 mm ventilated cavity, to allow the damp air to escape.

    This is very complicated and you should engage a Construction Professional to confirm what is causing the dampness and advise you on the best options.

    Please note that I need to state that I am not looking for any work. I only provide free advice here.

  • Registered Users Posts: 1,036 ✭✭✭ monseiur

    Thanks C. Eastwood for advice.

    In relation to the dampness there is no green mould or salts on the inner surface it's just that the paint keeps peeling off in small patches in different rooms and I assumed that dampness was the cause. Some rooms are finished in gypsum plaster (pink hardwall) and some in sand & cement nap type finish. 62.50mm is the maximium insulated slab I can fit as the current doors are fixed to the external walls and it's not possible to widen door opes from other side as the wall is a chimney breast which is 800mm thick. The design of the house is totally different to the more modern bungalows, it's long and narrow, just under 6m. from front wall to back wall. The sitting room is the full width of the house, two bedroom are leading off this room, the wall between the sitting room and these bedrooms is basically a huge chimney stack with fireplace in the centre for the sitting room and a fireplace each in the bedrooms. Door to the front bedroom is literally hung on the front external wall and door to back bedroom is on back external wall if that makes any sense to you. In other words you have to walk thru the sitting room to get to the bedrooms. There's another chimney stack for kitchen etc. and bathroom etc. is off the kitchen. All external and internal walls including chimney stacks are of mass concrete, height from floor to ceiling is 2.950m.! I plan to fit a new lower ceiling to within 150mm of top of windows, it will still be 2.750m. high.

    There is a DPC membrane on strip foundation under external walls, an no evidence of raising damp. When painting external walls I apply a coat of 50 : 50 diluted PVA bond/ Polybond for waterproofing. The external plaster is in very good condition, no hairline cracks etc. visible.

  • Registered Users Posts: 751 ✭✭✭ C. Eastwood

    Thanks for kind words

    Thanks for kind words monseiur.

    That pink hardwall is an oldie.

    I have a reservation about the paint pealing off the wall in small patches in some rooms.

    There are 2 main reasons why paint is pealing off a wall. Something / paint on the wall which is preventing the new paint from bonding.

    The main cause is there is moisture evaporating out of the wall which pushes the paint off the wall.

    A Moisture Meter is required to ascertain if there is dampness in the wall - either rising dampness and/or rainwater penetration- is causing the dampness. You need someone experienced in reading the Meter.

    The dampness must be stopped before you apply the insulated slabs, or the dampness will cause damage.

    Do not put a sheet of polythene on the wall, because it will not stop the dampness. If the builder or plasterer tell you to fit polythene on the wall before fitting the slabs - then you are dealing with Delboy Trotter. And the same applies if they tell you that the wall is breathing and /or sweating.

    Stop the dampness.

    it is best not to be applying Polybond on the external face of the walls.

    Apply Weathershield or a similar good quality external paint. This will allow any moisture in the wall to evaporate out through the paint.

    Mass concrete walls will be very cold in cold weather and will have a U value of approx 2.3 watts/m sq/ degreeC/hr.

    62.5 mm slab with 50 mm good quality insulation will give a U value of approx 0.35 watts/Msq/degree C/hr.

    This is reducing the heat loss down to approx 1/7th, which is very good. Bear in mind the house was lived in for the past 70 years.

    I would normally specify metal mushroom fixings for a cavity wall. But if you use metal fixings, because of the cold mass concrete wall, these metal fixings will form a cold bridge and there will be a slight black mould growth appear on the surface of the 3 mm plaster skim over each of the metal fixings.

    To prevent this, I would usually specify 10 mm of Bonding undercoat over the metal fixings, before the 3mm skim of hardwall, but you are limited to 62.5 mm on the walls.

    Therefore you should not use metal fixings. You should only use Plastic Mushroom fixings to fix the slabs and this will prevent the condensation mould over the fixings.

    Also all internal concrete block partitions built in to or connected to the external wall will be Cold Bridges and therefore it will be necessary to fix slabs with min 20 mm insulation with metal fixings to these walls, otherwise the partitions will form a cold bridge adjacent to the external walls and condensation mould growth will grow in those areas.

    Also use slabs with min 20 mm on all windows and doors reveals.

    Only use PVC (plastic) or Stainless Steel corner beads on all the reveals. Do not use galvanised beads.

  • Registered Users Posts: 1,036 ✭✭✭ monseiur

    Thanks C. Eastwood, greatly appreciate your time and professional knowledge. The paint peels off in small patches roughly 150mm x 150mm here and there but every time in to the pink hardwall. I guess the only way to stop this dampness is to apply two coats of exterior plaster with waterproofer. Hacking off the old plaster will be a major job as it's hard as flint, I guess the mix was 2:1 sand & cement !! Impossible for example to drive a steel nail thru' has to be drilled. There is no evidence of rising damp, the lowest peeling paint patches are over 450mm above floor level. Would some vents at DPC level help keep the walls under insulated slab dry / ventilated and stop mould forming? Say one every 2.000m, could put another row at soffit level and barge level on gables if required. I was thinking 150mm round ones which could be drilled with a core drill bit. Plan to do it once and do it right if at all possible.

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