Viewing 27 posts - 1 through 27 (of 27 total)
  • Alternative to Thermalite building blocks?
  • sparkyspice
    Free Member

    We are building a single storey extension and our plasterer is insisting that we don’t use thermalites on the outside as the render will crack. He’s a good plasterer with lots of experience and I trust what he says.

    Concrete blocks will do, but they have low thermal properties. Is there an alternative that will give better thermal values but won’t draw all the moisture out of the render or crack it due to the expanding properties of the blocks?

    Any help would be gratefully received!

    matt_outandabout
    Full Member

    Use a different render instead.

    Stoner
    Free Member

    you can get a 3.6 N/mm2 block that’s ligthweight, decent thermals and doesnt suck all the render dry. Cant for the life of me remember the brand we used, will need to googleify…one tick…

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    Stoner
    Free Member

    theyve changed their brand, and these look different but this was my supplier

    http://www.bradfords.co.uk/details/100mm-thermal-insulation-block-36n

    it’s not the same though, and TBH these look more like thermalites so prob not your plasterers fave.

    BlindMelon
    Free Member

    Use a proprietary render system like K Rend. A high polymer base coat like HPX with s poly mesh embedded into it followed by your choice of finish coat is perfect. The base coat minimises the high suction of the block and allows the top coat to dry at a suitable rate.

    BlindMelon
    Free Member

    It’s the dry density of the block that’s important if you are going to use a site mix. The block would need to have a minimum dry density of 1400 kg/m3 for a site mix. Thermalite more like 700 kg/m3

    sparkyspice
    Free Member

    Thanks chaps.
    I’ll see if he likes the idea of the Fibolite blocks, or changing his mix.

    pedropete
    Full Member

    Hemelite blocks are definitely the ones to go for. Most house builders won’t use aircrete blocks, such as thermalite/celcon because they always crack.

    tillydog
    Free Member

    We are building a single storey extension and our plasterer is insisting that we don’t use thermalites on the outside as the render will crack. He’s a good plasterer with lots of experience and I trust what he says.

    He’s right 🙁

    Concrete blocks will do, but they have low thermal properties.

    I used thermalites for the outer leaf and they have been nothing but trouble. Currently looking for someone to come and re-render them (for 3rd time in places).

    My £0.02: Use the bog standard concrete blocks for the outer leaf. If you’re worried about thermal properties then beef up the cavity insulation – polyurethane rather than mineral wool, or an extra 10mm, or so of polyurethane if you’re using it already.

    wrightyson
    Free Member

    Using thermalites externally is just poor practice full stop. Hemlites are good but a pain to lay as they cut rubbish with hammer and bolster. What blind melon says is all well and good if you’ve got pounds to spend!
    Increased cavity to get your ú values?

    BlindMelon
    Free Member

    What blind melon says is all well and good if you’ve got pounds to spend!

    Not really wrighty. £35 per sqm supplied and fitted compared to around £25 for a site mix that you have no guarantee from and will require painting regularly. Also means that you won’t need to render 3 times like tillydog above.

    @tillydog what preparation did the plasterer do for the application?

    wrightyson
    Free Member

    A simple site mixed through colour monocouche onto a standard block has a guarantee, requires no painting and is cheaper than that. No need for prep work.

    BlindMelon
    Free Member

    Op was asking about thermalite so prep is needed. How much are you getting a site mix supplied and fitted for? Also who supplies the guarantee for a site mix? The plasterer?

    fionap
    Full Member

    My £0.02: Use the bog standard concrete blocks for the outer leaf. If you’re worried about thermal properties then beef up the cavity insulation – polyurethane rather than mineral wool, or an extra 10mm, or so of polyurethane if you’re using it already.

    This. Standard 100mm med-dense blockwork to both leaves, 50mm Celotex CG5000, 50mm clear cavity will get you 0.28 W/m2K for an extension. Celotex have a calculator here which might be useful: http://www.celotex.co.uk/technical-services/online-u-value-calculator

    wrightyson
    Free Member

    Op hasn’t used thermalites yet and most advice is swinging to using something like a hemlite on the external leaf.
    My plasterers used pre bagged webber.

    matt_outandabout
    Full Member

    Do away with a cavity and the compromises needed to reach decent thermal performance?

    Solid wall, external insulation, render direct.

    Cheaper and simpler than adapting a system?

    BlindMelon
    Free Member

    My plasterers used pre bagged webber.

    So not a site mix, a proprietary render which is what I recommended!

    wrightyson
    Free Member

    When you said site mix I presumed that’s what you meant? It’s mixed on site no? We’re you referring to good old sand and cement in a mixer as a site mix?

    BlindMelon
    Free Member

    Aye sand cement and lime! Old school style. Yes it all need mixed on site however. K Rend Weber etc will recommend a primer and one coat but I prefer the high poly base with scrim in it on thermalite. Less likely to crack.

    tillydog
    Free Member

    @tillydog what preparation did the plasterer do for the application?

    The walls were soaked with a hose (really soaked), coated with PVA and then they put the sand / cement base coat on that. I think they may have had some lime in one of the coats, but can’t recall.

    He did warn me that it would be difficult.

    The rendering on the front wall dropped off after a few months, and they came back to re do it. That’s still sort of OK, but it’s blown on the other walls now (8 years on).

    (Sorry for the hijack, but I would appreciate any advice what is the best way to sort it out – is K-Rend the way to go?)

    honeybadgerx
    Full Member

    coated with PVA

    think they may have had some lime in one of the coats

    I can see where some of your problems may be coming from… In my opinion PVA is used in lots of inappropriate situations where cement based products are used – it really doesn’t work that well. Also, unless you’re doing a fully traditional, i.e. all lime, render mix then I would say an admixture should be used for workability rather than just adding one part lime to the mix.

    sparkyspice
    Free Member

    More input! Thanks.
    My plasterer has tried soaking walls etc but the thermalites still draw out the moisture. Plus the high expand/contraction rates exacerbate the problem.

    FionaP – Thanks for the calculator – very useful!

    sparkyspice
    Free Member

    I think I’ve settled on these, (rather than the Hemelites), because they’re £100 cheaper from a local supplier and have similar properties (although 1350kg/m3 rather than 1450kg/m3 for the Hemelites). Hopefully BlindMelon that’ll still be OK?

    http://www.hansonbuildingproducts.co.uk/thermalite-and-aggregrate-blocks/aggregate-block-detail/fenlite-background-aggregate-block

    Thank you all for your help it’s been an education and in the long run a big time and money saving exercise!

    BlindMelon
    Free Member

    Sparky yes that will be fine. Just make sure the plasterer dampens the blocks.

    @tillydog if you want a solution that will work and be guaranteed for 30 years then a proprietary render like K Rend/Weber/Parex is the best solution.

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