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Heat Pump - Stone Farmhouse

  • 13-08-2022 3:02pm
    Registered Users Posts: 812 ✭✭✭

    I'm trying to decide on which heating system is best for a refurb. House is a traditional 2 storey farmhouse. 600mm stone wall construction. No insulation or any form of heating at present. Despite that, the house is pretty dry apart from a few damp patches here and there due to cracks in chimneys etc, all of which will be resolved.

    I'm not planning on dry lining the walls internally as I want them to be able to breathe. I'm not going for EWI either. I'm going to upgrade the windows and put in attic insulation.

    I'm on the fence about the heating system. In theory I'd love to put in a heat pump and pv but my fear is that the walls will soak all the heat. The alternative I'm considering is OFCH via an external boiler. Not exactly green but I know it will deliver. I've considered other options but this is where I'm landing at the moment.

    I'd be curious to know if anyone else has installed a heat pump in a similar scenario without dry lining stone walls. I think the thermal performance of dry stone walls is underestimated and leaving them uninsulated isn't exactly the worst thing in the world to do as they act as a thermal mass in themselves.


  • Moderators, Science, Health & Environment Moderators Posts: 18,821 Mod ✭✭✭✭Sam Russell

    The heat pump efficiency depends on a small difference between the 'hot' end and the 'cold' end. The smaller the difference, the greater the efficiency.

    That is if you have a coefficient of performance (COF) of say 4, then 1Kw of electricity will deliver 4 Kw of heat. Now to achieve that in an old farmhouse without insulation is unattainable. The best way is to have a well insulated building, with a sealed building and well controlled ventilation together with underfloor heating, with the heating on low all the time. You really need a BER above B3 but better if it is in the A3 range.

    It does not work to turn it on in the evening until you go to bed. The required insulation will ensure the heat delivered will stay in the building, and not disappear out.

    Think of heat recovery ventilation if that is possible. However, in a farmhouse, I would think it is unlikely.

    Whatever you go for, do go for as much insulation as you can. Look at payback years - how much it costs and how long will the savings take to pay that back.

  • Registered Users Posts: 11,814 ✭✭✭✭Calahonda52

    Step one is to know the exact makeup of the walls, without that, the breathability component is unknown.

    If they are rendered with sand cement on the outside and gypsum skim on the inside then they are not breathable.

    My go to solution here is

    dig out the floor and put in UFH with 250/300 XPS underneath.

    EWI, with rain shield, not the weber acrylic stuff, up to meet the 300=400 in the attic with a warm roof.

    Windows at same time as EWI, with them hung on the outside face of the wall to reduce/ eliminate thermal bridging

    MVHR, with unit in the conditioned attic space.

    Heat pump.

    1 m wide by 1 m deep, or down to top of foundations if less, French drain around the outside, this will also allow the EWI to go down to top of foundations.

    “I can’t pay my staff or mortgage with instagram likes”.

  • Registered Users Posts: 8,093 ✭✭✭Furze99

    One thing to be watchful for if you do leave your old solid walls uninsulated is for surfaces of different temperatures. If the interiors of the solid walls tend to be the coolest, then moisture in the house can condense on them. This is where the breathability of old walls comes into debate and advocates argue that this moisture can pass through or back out into interior etc without mould growth. So you pays yer money and you takes yer choice.

    The only useful advice I can offer is that peoples expectations of heating comfort & ventilation when these old farmhouses were built were different from what many look for now.

  • Registered Users Posts: 8,093 ✭✭✭Furze99

    "EWI, with rain shield, not the weber acrylic stuff" - can you explain what this is? We're in similar boat, though with 1/3rd old building and 2/3rds cavity wall with 60mm insulation. Thought of wrapping the whole but then you read stuff about the delicate nature of some of the external finishes and of course the unavoidable changes to window and door opes etc. Thanks.

  • Registered Users Posts: 11,814 ✭✭✭✭Calahonda52

    build up

    cement based plaster skim

    cement board screwed to timber battens creating a 25 mm ventilated cavity


    breathable insulation


    its on this site somewhere

    re the moisture comment, thats why the MVHR, move the moist air out.

    “I can’t pay my staff or mortgage with instagram likes”.

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  • Registered Users Posts: 812 ✭✭✭bemak

    Thanks for the feedback lads. The existing walls have a total build-up of about 40mm of render on the outside face (layers added over time) so the external walls are not breathable (as you also note @Calahonda52). The walls themselves are about 600mm thick, rubble stone, not pretty. About a year ago I did a trial of hacking off a patch of external render which was clearly wet form a small crack. The render popped easy enough but once I moved away from the damp patch, the render was very hard to take off - and any bit that did come off often pulled a bit of stone with it. Pic below with helper.

    So I quickly decided that I wouldn't be going down the route of hacking the render off and replacing it with lime. The costs associated with same would kill the project anyway. About 85% of the external walls are in good shape and sound solid so I think I might remove the hollow sections and patch them up.

    Then, I've started to think that EWI might be the best solution, particularly as the walls aren't currently breathable anyway. As noted in others comments, this will allow the insulation to go down to foundations when we dig the French drain. I think I would do this in tandem with hacking the internal render off the main damp areas within the house and then let the house dry out to the inside for a good few months. The house is unoccupied for the most part at the moment anyway so I think it makes sense to do this.

    So this initial phase of works (roofs, windows, insulation, drainage etc) essentially focuses on the envelope. After I give the house time to dry out a bit, I should have a better understanding of how the remedial works are performing which will help me to decide on the best approach in terms of heating the house.

    This isn't a time-pressure job - so I might as well use that to my benefit