Could an expert please explain the statement made on page 107 of Dec issue regarding heat transfer through thick solid stone walls & slate tiles – surely if heat from the sun was coming into the building via walls & tiles then more importantly in winter any heat generated inside the building would also transfer out making it a very poor performing building. We are also converting a very large church into our home, have installed double glazing & 450mm of loft insulation and would prefer not to internally insulate the existing 900mm thick stone walls which are plastered internally – can someone convince me that these thick stone walls will perform well and how do they compare U value wise to other acceptable wall constructions. We are using 100 kw gas boiler with rads (70,000kw pa usage) plus 2 log burning stoves for a 900 m2 of floor space over 3 floors. – thanks for your help

  • Henry Smate

    Am in a similar position, but our stone walls are clad internally in wood, not thick wood either, the gap between that and the stone is just a few inches. If our experience living here (we’re still pre-restoration) last winter is anything to go by, you’re going to need a lot more than plaster to protect you from the elements – even with that amount of heating. These buildings were never designed to retain heat or get rid of the moisture that living in them generates, I’d be incredibly skeptical of advice suggesting leaving the walls unclad.

  • Tony Taylor


    Not my field of expertise, but basically heat will move from a warm surface or object towards a cooler surface or object, the effect being solar gain in the summer months and heat loss in the cooler weather. The ‘U’ value scale identifies the rate at which heat transfers from one side of the material to another, the lower the ‘U’ value the better resistance to heat transfer.

    I have worked on several church projects in the past and some of the largest heat losses will be through the walls. It sounds as though you have the opportunity to add a layer of insulation to the inner face of the external walls (The building envelope) and you should take it wherever possible.

    Any opportunity to insulate the building (any building) should be grasped with both hands, making up for poor or no insulation in a building by installing larger or oversized heating plant and equipment is false economy, energy prices are never going to go down, only up. Reducing the heat a building requires can only be a good thing.

    A word of caution in your instance, stone buildings and the majority of churches in the UK are built on stone or slate foundations, slate pieces laid into the soil at 45 degrees, with the stone walls built on top. This construction technique can result in moisture being drawn up the foundations and up the wall both internally and externally.

    This does not usually cause a problem as most stone building were never designed to be heated. When you add heating to the internal space you have to be very careful with your design to ensure the moisture movement is not affected or stopped. If you get it wrong the walls and mortar will dry out too much and start to crumble, which will ultimately affect the buildings structural integrity in the long term.

    One of my projects was St Peter’s Centre at Peterchurch (, where we installed a biomass boiler in an energy cabin in the grave yard with under-floor heating throughout the space, both ground and first floor. The biggest design element for the heating system was to install the oak floor with a 4" pebble border between it and the stone walls, allowing the walls to breath and providing sufficient heating to the space.

    Hope this helps


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