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Thermal Looping/Bypass Cavity Block Wall

  • 26-02-2013 2:56pm
    #1
    Registered Users Posts: 23 ✭✭✭ cullym


    Hi All,

    First post so go easy :-). I'm looking at getting EWI on my semi-d which consists of 2 hollow block walls and one traditional cavity wall. The recommendation when EWI'ing a traditional cavity wall is to have the cavity pumped which I believe is to avoid thermal looping.
    When it comes to hollow block the consensus seems to be to just put the EWI on. Is thermal looping not considered to be an issue for hollow block walls?

    Regards,

    Martin


Comments

  • Moderators, Home & Garden Moderators Posts: 10,101 Mod ✭✭✭✭ BryanF


    cullym wrote: »
    Hi All,

    First post so go easy :-). I'm looking at getting EWI on my semi-d which consists of 2 hollow block walls and one traditional cavity wall. The recommendation when EWI'ing a traditional cavity wall is to have the cavity pumped which I believe is to avoid thermal looping.
    When it comes to hollow block the consensus seems to be to just put the EWI on. Is thermal looping not considered to be an issue for hollow block walls?

    Regards,

    Martin
    Where is the looping in cavity block? It is assumed that the air cavity is closed Or to look at it another way, what is the the solution, who will guarantee pumping breeze blocks?


  • Closed Accounts Posts: 5,433 ✭✭✭ sinnerboy


    Thermal looping occurs when cold air gets behind an insulation layer and negates the insulation. For example think of a 50mm thick insulation board in a 100mm cavity . The 50mm airspace in front of the insulation board is nice and cold. So if the insulation boards is not installed perfectly flat to the warm side of the cavity and if there are any gaps where the insulation boards meet , well the cold cavity air passes behind insulation board like Spanish strikers against Traps army - and game over.

    The airspace in the hollows of a hollow block wall will be on the warm side of the insulation. And the mortar joints at each horizontal course will restrict air movement.

    In any event it is good practice to remove gutters and facia to ensure the EWI extends fully up the wall to come into line with the ceiling insulation. Whilst this is being done make sure the top of the hollow block wall is inspected and if required mortar bed thin slate "caps" to seal off the hollows of the top course of block. This may not be required as sometimes builders switch to solid blocks for the last course.


  • Registered Users Posts: 23 ✭✭✭ cullym


    @BrianF, @sinnerboy - these were my thoughts but I was led to believe on another forum that thermal looping would be an issue. The thinking was that on a windy day the hollow of the block would loose heat. I thought EWI if done correctly was pretty airtight/windtight.

    @sinnerboy your description is how I thought it would work. Any air movement is on the warm side and bringing the insulation up the roof or over the wallplate will completely wrap the living area in insulation.

    Thanks for the responses.


  • Registered Users Posts: 30 Fingallion


    There really isn't a great way of insulating hollow blocks and EWI is not very suitable although many suppliers will say otherwise, u value calcs do not consider convection, drafts or thermal looping.
    Hollow blocks usually sit on the edge blocks of a foundation with no insulation below forming a series of tubular cavities around the house perimeter. Even with external insulation, you would most likely have quite a temperature gradient decreasing from the inside of the block to the outside of the block (With cavity air being cooled at the base just above the foundation). Looping would occur with air rising up the inside layer and then falling down the cooler outside layer. A lot of unpredictable air movements will be occuring within these cavities, even if there are no breaks or openings at the wall plate. Also, warmer air will tend to stratify at the higher levels of the cavities. If the house is zoned, you may have a heated zone dumping heat into the cavity block and rizing to a cold unheated zone.
    Maybe drylining with a vapour barrier on the room side might be a solution worth pursuing athough mould will still be a risk. What ever you do, don't waste money filling the blocks. An Architech named 'Little' has a few articles published on line on this topic. Now that I think about it, it could be worth injecting closed cell foam into the top and bottom blocks and perhaps on the blocks between floors, this would prevent some of the issues I mentioned, bt even this measure could cause water retention in the wall.... Its a tricky business. What did you decide on in the end?


  • Registered Users Posts: 23 ✭✭✭ cullym


    @fingallion - I don't like the idea of pumping the hollows with foam, I was putting the question out there to get some other opinions. I haven't decided on anything yet. I think EWI is the best bet but has to go down to foundation and preferrably up over the roof. If the outside of the building is wrapped then the air movement is all inboard of the insulation so does it really matter? Although having said that I'm reading conflicting opinions as to whether EWI is completely wind/air tight. Currently the walls are rendered externally (back and side and part of the front) so maybe if rendered completely before applying EWI then that could act as windtight layer. I've read the papers by Little makes for interesting reading.
    It all a bit head wrecking and I find myself going around in circles at times but I then how else to figure out the best way to upgrade the house.


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  • Registered Users Posts: 30 Fingallion


    EWI will help your situation but not as much as it would if you had filled walls. The air temperature in your cavities will not be the the same as the room temp, even with the insulation on the outside, doesn't matter if you insulated right down to the foundation. The cavity blocks sit on top of cold uninsulated foundation edge blocks.
    I am not recommending pumping the blocks as a standalone solution, this will only reduce the thermal resistance of the block by about a third which would still be rubbish. But, if you wish to spend the huge money for EWI, then you should pump the blocks as best as you can to stop air movement. The other bonus is, you wont have to patch up the holes used for injection as your EWI will cover them up. This will stop thermal looping and a variety of other convection currents that will suck the heat out of your rooms regardless of the external insulation. It will also reduce the final u value. A 9" hollow block wall with render and plaster has a U value of 2.09 W/m2 which would be reduced to 1.33 W/m2 if pumped with polystyrene.
    If you don't fill the blocks with something, then internal insulation will deliver more (with proper vapour control etc) benefit than EWI.
    By the way, be wary of the notion of thermal mass with ewi if you have intermittent heat or timer controlled zones. The response time is very slow. If you have a cold room, you will need to generate enough heat not only to heat the air, but to heat a few tonnes of cold concrete also. The benefit is that it will stay warm for a while but it all depends on your lifestyle and heating strategy.


  • Registered Users Posts: 23 ✭✭✭ cullym


    @fingallion - thanks for that info. This is the kind of stuff though that keeps me going round in circles

    For EWI on a standard cavity wall (100mm) the general recommendation is fill with bead (stops thermal looping) and then EWI.
    For a hollow block wall it's EWI all the way but don't pump the blocks. Intially my thoughts were how can the looping effect apply in one case and not the other. I thought pumping the blocks would solve that problem but reading the con's of pumping the hollows put me off that route. As I'm sure you know, no one can guarantee full fill of the hollow block. Some of the foams look like the could do the job in a wall where the hollows line up perfectly and there is no mortar or other rubbish in the wall. However most of the companies I've come across use urea formaldehyde. Not sure I want this in my walls :-)


  • Registered Users Posts: 227 ✭✭ Andrew_Doran


    Fingallion wrote: »
    even with the insulation on the outside, doesn't matter if you insulated right down to the foundation. The cavity blocks sit on top of cold uninsulated foundation edge blocks.

    I don't buy that. A metre of dense concrete blocks on the flat between DPC and founds is fairly insulating. If it were a metre thick external wall it would have a U value of around 0.3. With perimeter insulation going down to the founds I can only see the wall at DPC level tracking the internal temperature more closely than the external.


  • Registered Users Posts: 6 ✭✭✭ carlitos081


    So, what did you do in the end? I'm in the same situation,
    Hollow block semi detached home and I'm going in a circle thinking about EWI or IWI. Any help? Thanks


  • Registered Users Posts: 10,919 ✭✭✭✭ Calahonda52


    So, what did you do in the end? I'm in the same situation,
    Hollow block semi detached home and I'm going in a circle thinking about EWI or IWI. Any help? Thanks
    EWI


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  • Registered Users Posts: 6 ✭✭✭ carlitos081


    EWI

    Thanks, any consideration? Did you pumped the block on the foundation and the one where the roof is? Will be great if you can tell me what you think about it.
    Thanks


  • Registered Users Posts: 23 ✭✭✭ cullym


    @carlitos081, OP here, due to a change in personal circumstances I never went ahead but the plan I settled on was to do the EWI from the foundations up to wallplate to meet the attic insulation. The junction on the gable wall at first floor ceiling level was a potential issue as attic insulation would not meet the wall insulation directly. If it wasn't a semi-d I would have went up and over the rafters with insulation and then house would have been completely wrapped.


    I decided not to worry about thermal looping in the sense that the air in the block on the inside of the EWI would essentially be in the same space as the air in the room.


    Making sure the top of the wall was sealed as sinnerboy mentioned would be important too.


    Hope that helps a bit. Best of luck with the project.


  • Registered Users Posts: 374 ✭✭ zoom_cool




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