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Thread: Ceiling insulation R value required in Cape Town

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    Ceiling insulation R value required in Cape Town

    Hi

    I've been reading SANS 204 and it prescribes a total roof R value for Cape Town of 3.7. This doesn't make too much sense when comparing with the R value of 3.2 for Pretoria or the R value of 3.7 for Johannesburg.

    Having spent many years in Cape Town I struggle to understand how it can require as much insulation as Johannesburg or more than Pretoria. These last two towns seem to be a lot colder in winter and hotter in summer due to their higher altitude and a lack of ocean to moderate the temperature.

    If you compare a climate graphs of Cape Town to Johannesburg or Pretoria at:
    http://www.climatetemp.info/compare/

    You can clearly see that both are colder than Cape Town during winter (June through August) when it comes to minimum and average temperatures. Which if you are insulating for cold, as per SANS 204, would seem to be the important ones.

    Maybe I'm missing the plot but it seems that Cape Town has a far more moderate climate than the Cities in the interior and thus the R value for insulation is on the high side.

    Basically what this boils down to is that adding insulation with an R value of 2.5 (e.g. 100mm of Think Pink / Aerolite) to whatever the existing roof space provides should be sufficient.

    Please note that the insulation chosen is an example and I couldn't care less if it makes you itch if you don't wear appropriate clothing while working with it. Discussing this would have no bearing on my question so please save it for another thread. :-)

    Reasoned analysis based on facts is welcomed. Personal experiences if well written might also count for something.

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    Diamond Member wynn's Avatar
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    I think the R value is so that you can compare apples with apples?

    I am sure that black garbage bags filled with scrunched up newspaper will give you enough insulation but will also be a fire hazard.

    Macerated newspaper impregnated with a fire retardent gas that is blown into your ceiling is an eco-alternative.

    As I said 'apples for apples' and 'horses for courses'
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    I'm specifically interested in the amount of insulation required for Cape Town, not the type so allow me to state the underlying questions a bit clearer:

    Is the R value specified in SAN 204 too high for Cape Town?

    If so what R value of insulation is enough for Cape Town?

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    Maybe it's not just a function of temperature. Could wind be a factor?

    You certainly have a lot more wind in Cape Town compared to Gauteng, and I'll tell you now - 2 degrees on a still highveld morning doesn't feel nearly as cold as 10 degrees on a windy Cape Town morning.

    My other thought is humidity.
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    Diamond Member wynn's Avatar
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    Go with SAN204 I am sure the price will outweigh the extra R or two in insulation
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    Hi Dave


    Hmn we get a lot less wind in Cape Town when it's not summer but probably still more than up north so you may be onto something. Wind chill certainly has an effect at low temperatures of making it seem colder.

    The humidity factor is a bit of an odd one. Above a certain temperate humidity make it feel hotter. In Cape Town the cloud cover complicates things as it is an effective insulator. The cold nights and mornings are the clear ones!

    But thanks you've given me some things to chew on.

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    Hi Wynn

    I'd definitely not want to have to do it twice so would rather put in the right amount the first time.

    According to SANS 135mm of Aerolite / Think Pink would give an R value of 3.38 which when combined with an R value of 0.32 for an uninsulated roof would get to the recommended R value of 3.7. Similarly 160mm of Isotherm would suffice for Cape Town or PE.

    So does anyone know how to calculate the R value of an uninsulated roof plus ceiling?

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    Diamond Member AndyD's Avatar
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    Maybe the condensation or dew-point in Cape town reduces the r value of the roof covering or possibly there's variations in roofing materials used in different parts of the country. Just a couple of thoughts, I have no data to support either.
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    Diamond Member wynn's Avatar
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    Andy just what I was going to mention, the wet winter means a wet roof with wind blowing over it would be like a giant air conditioner on max cool

    Smitty have you spoken to any insulation installers for their reccomendations?
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