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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthordnfh
    • CommentTimeDec 31st 2007
     
    We have bought a 1770 2 storey stone walled house. Curiously in 1996 permission was given fand the first floor was dry-lined with standard plasterboard with c. 50cm air gap - and vinyl paint. The ground floor was directly plastered with modern cement plaster (NOT lime). Over the last 3 months neither first nor ground floor has any damp detectable (first floor in the air gap, ground floor directly meassured with probe meter).
    Neither floor has any formal insulation, and we wish to improve the heat loss to allow heating with Air source heat pump and thus reduce our carbon footprint. While generally wishing to follow green principles, these have already been breached with no apparent problems. This poses two questions.

    1. Ground floor: As it has already been impermeable for 10 years without apparent problem, is there any reason not to put eg urethane foam on top of the plaster?

    2. First floor: If (again) standard plasterboard has been fine for 10 years, we can presumably put standard insulation on top of the plasterboard. But would putting urethane foam IN the cavity produce problems despite the apparent lack of current problems?

    (Urethane seems to be much cheaper AND more effective than eg cellulose).
  1.  
    Posted By: dnfh The ground floor was directly plastered with modern cement plaster (NOT lime).

    When you say cement plaster do you mean that the wall is covered with Ordinary Portland cement, which is unusual, or do you really mean that it is gypsum plaster, which is the normal plastering material. Gypsum plaster is pretty vapour permeable - not very different to lime plaster. Maybe your walls are about as water vapour permeable as they have always been.
    • CommentAuthordnfh
    • CommentTimeJan 2nd 2008
     
    That is very interesting. A Lime plasterer who came said 'cement plaster', but I suspect it is standard. How can I tell?

    If standard is permeable, why do English Heritage et al go ape about using it on solid walls? Any way to test?
  2.  
    Hi dnfh. I'm afraid it is a bit more complicated than just saying one is more permeable than another.

    Permeability will depend not just on whether or not cement or lime has been used, but whether they have been used in isolation or together; and in what ratio to each other/sand/other materials; and also whether any waterproofing additives have been used.

    I suppose colour can be an indicator as can the hardness of the render. lime mixes are usualy lighter and softer

    You could try spraying the wall with some water and seeing how absorbent it is. If it is very porus you will be able to see and hear the moisture being taken in.
    • CommentAuthortony
    • CommentTimeJan 2nd 2008
     
    May be you have so many draughts that you wont be able to tell anything about permeability.
    • CommentAuthorTheDoctor
    • CommentTimeJan 3rd 2008
     
    my research suggests, that unless your house is 'extremely' well draft proofed (which is difficult in old houses without a complete gutting) then ground or air source heat pumps are not very efficient, as they have to work very hard to make a difference, due to the relatively low temps involved.

    I ruled it out for this reason.

    the newly refurbished and extended end of my house will be quite suitable for this technology, but the other half will not, without thorough gutting, which is beyond our means at present.
    • CommentAuthortony
    • CommentTimeJan 3rd 2008
     
    Not true a heat pump will always be capable of doing the job. The heat loss of the area in question will determine the size of the heat input that you need.
  3.  
    Posted By: TheDoctormy research suggests, that unless your house is 'extremely' well draft proofed (which is difficult in old houses without a complete gutting) then ground or air source heat pumps are not very efficient, as they have to work very hard to make a difference, due to the relatively low temps involved.


    This is a common misconception. People think that the heat from a heatpump is "weaker" because it is at a lower temperature than that from a boiler. However, all that is important is the amount of heat delivered. It doesn't matter at what temperature it is delivered at - in fact, lower temperature delivery can feel more comfortable than high temperature due to the reduction of convection currents (which give the feeling of drafts) and stratification.

    I live in an old house - 110 years old this year and it is certainly not to current air tightness nor insulation standards. We have a GSHP with forced air delivery and it is quite able to heat the house both comfortably and efficiently. Today it is currently -19C outside but is a comfortable 21.5C inside. Admittedly the GSHP is running continuously, but that's what it's designed to do at such low outdoor temperatures. The heatpump is not working "hard" at all - it is just running at its design conditions. With the volume of air delivered, the temperature rise is about 13C between the supply and the return. Because this is low, there is virtually no stratification in the house so everywhere is at an even temperature. Despite it being forced air delivery, the ducts and distribution system are designed well so there are no noticable drafts - even a candle on the dining room table a mere feet from a vent does not even move.

    Just to get back to the point, all that's important is the heat load. At -23C outside, our heatload is 15kW - and this includes all fabric and air leakage losses - the heat source doesn't care how this load is made up, though a leaky house will feel more uncomfortable than an airtight one (assuming the airtight one is correctly ventilated).

    Paul in Montreal
    • CommentAuthortony
    • CommentTimeJan 3rd 2008
     
    Cripes, did we agree there Paul! HNY
    • CommentAuthorbiffvernon
    • CommentTimeJan 4th 2008 edited
     
    Posted By: dnfhThat is very interesting. A Lime plasterer who came said 'cement plaster', but I suspect it is standard. How can I tell?

    If standard is permeable, why do English Heritage et al go ape about using it on solid walls? Any way to test?


    Dunno what 'standard' is, but you need to distinguish between the three very different materials, lime, gypsum and Ordinary Portland cement. 90+ % of walls these days are plastered with gypsum. Putting OPC on walls is just daft but that doesn't stop some folk. Gypsum and lime have rather similar vapour permeabilities but differ in many other ways. One would not normally use gypsum on the walls of most period properties in which English Heritage have an interest. It's use was mainly confined to decorative mouldings and other places where the quick setting properties were needed. Lime has been the material of choice for wall since time immemorial and is still far superior though requires a certain skill and, most importantly, time, which the modern building industry seems to lack.

    Test? They look different. Lime plaster usually fizzes with vinegar while gypsum plaster usually doesn't. Gypsum is often, but not always, pink while lime is often, but not always, white. Lime often has something hairy embedded within, horse or goat hair or straw. Lime often is diluted with sand, 2:1 or 3:1 sand:lime ratio. Gypsum is often used neat so doesn't have the grainy texture. etc. etc.
    • CommentAuthordnfh
    • CommentTimeJan 7th 2008
     
    Thanks all.
    Definitely not lime. Does look pink, and almost certainly was the cheapest and quickest thing they could put up.

    As the stone wall itself is permeable, normal theory says use lime, let it all breathe etc. But IF it had had a toally impermeable layer on for 10 years without problem, then adding more impermeability wouldn't matter (and even for EH it wouldn't be too bad as the new layer plus the nasty old plsater could all be removed back to pristine Georgian stone if needed!).
    However if the layer is only so-so impermeable, then adding more might indeed be bad.

    Mike George suggested spraying water on and seeing - is tha accurate enough for any decision, or what else can I do?
    • CommentAuthorMike George
    • CommentTimeJan 7th 2008 edited
     
    Hi dnfh, can you tell us how thick the pink stuff is? Is it around 3mm or much more, say 10-20mm? If the latter, it is likely to be some kind of renovating plaster, such as Carlite Browning.
    • CommentAuthordnfh
    • CommentTimeJan 8th 2008
     
    I will look on Sunday when we next go there and measure! (might even be able to photo it)
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