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The Optimum Cavity Width for Pumping with Beads

  • 18-02-2013 12:19AM
    #1
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭


    Basically I am trying to establish, at what point do you achieve a level of insulation whereby there is no point in having a wider cavity because the wall insulation effect is no greater.

    Obviously there are various parts of the house that require insulation, and work collectively but as someone seriously considering a 300mm pumped cavity, is it above and beyond what I need? Would it provide any greater insulation than say 225mm?


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Comments

  • #2
    TheChizler
    Registered Users, Registered Users 2 Posts: 9,559 ✭✭✭TheChizler


    Thermodynamicly, you won't do better than air as an insulator. As far as I understand the advantage from pumped insulation is that it stops air movement and draughts inside the cavity. So the thickness isn't hugely relevant to that.


  • #3
    caesarthechimp
    Closed Accounts Posts: 204 ✭✭caesarthechimp


    TheChizler wrote: »
    the thickness isn't hugely relevant to that.
    For any given kind of insulation, the thermal resistance (insulation value) increases with thickness.
    The are 2 issues here.
    1. A structural one, in that if you keep increasing the width of a cavity, the wall starts to behave as two separate walls, ie less stability.
    2. Cost effectiveness, ie at some point there is so little heat escaping that it is pointless to keep spending on insulation.

    Its hard to get exact guidance on either of these, and there are a lot of variables.


  • #4
    TheChizler
    Registered Users, Registered Users 2 Posts: 9,559 ✭✭✭TheChizler


    caesarthechimp wrote: »
    For any given kind of insulation, the thermal resistance (insulation value) increases with thickness.

    Oh yeah I'm well aware of the thermodynamics, I'm just saying that the effect of preventing internal draughts far outweighs the change in thermal resistance. As in you won't do better than air for thermal resistivity, but the insulation stops the air moving about.


  • #5
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    Thanks,

    To answer question 1, it will not be a particularly big house. A 1600 square foot bungalow, so therefore there will not as much span of wall away from corners etc. that needs support compared to a larger house.

    Question 2 is the one that nobody has the answer to, how to identify that very point of minimal heat loss.
    caesarthechimp wrote: »
    For any given kind of insulation, the thermal resistance (insulation value) increases with thickness.
    The are 2 issues here.
    1. A structural one, in that if you keep increasing the width of a cavity, the wall starts to behave as two separate walls, ie less stability.
    2. Cost effectiveness, ie at some point there is so little heat escaping that it is pointless to keep spending on insulation.

    Its hard to get exact guidance on either of these, and there are a lot of variables.


  • #6
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    Thanks for the response, though in theory it is easier to prevent draughts than in practice. Room doors will always be opening and closing, external doors will also be opening and closing. In effect, a house will never be a sealed fridge.
    TheChizler wrote: »
    Oh yeah I'm well aware of the thermodynamics, I'm just saying that the effect of preventing internal draughts far outweighs the change in thermal resistance. As in you won't do better than air for thermal resistivity, but the insulation stops the air moving about.


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  • #7
    Outkast_IRE
    Registered Users, Registered Users 2 Posts: 7,511 ✭✭✭Outkast_IRE


    sheff the ref wrote: »
    Question 2 is the one that nobody has the answer to, how to identify that very point of minimal heat loss.

    Lots of people will have the answer to question 2, i could do if i was bothered but thats not my area in particular.

    this should be bread and butter stuff for an architect and QS to sort out.

    Have you hired any sort of professional help or are you trying to figure it all out on your own ?


  • #8
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    Architects and QSs can sort out that based on the entire insulation of a house, e.g Windows, Doors, Roof, Floor Insulation being of a certain standard. but are they able to devise an accurate calculation based solely on the walls? Haven't been talking to the architect in a few weeks, but trying to gather information and be as informed as possible before discussing it further with him.


  • #9
    BryanF
    Moderators, Home & Garden Moderators Posts: 10,147 Mod ✭✭✭✭BryanF


    sheff the ref wrote: »
    Question 2 is the one that nobody has the answer to, how to identify that very point of minimal heat loss.
    you have to quantify what the considerations are? define 'minimal heat loss' -perhaps its the min wall insulation your BER/part L calc will accept?


  • #10
    rockabaloo
    Registered Users, Registered Users 2 Posts: 103 ✭✭rockabaloo


    I was asking the same kind of stuff in this thread - http://www.boards.ie/vbulletin/showthread.php?t=2056811453&pp=15&page=1.

    I think you really have to do your sums to see if there is a cost justification for the wider cavity.


  • #11
    caesarthechimp
    Closed Accounts Posts: 204 ✭✭caesarthechimp


    A lot depends on what the price of energy will be in, say, 10 years time smile.png

    A 200mm cavity filled with graphite bead gives a U-Value of around 0.16 (depending on the type of blocks and wall ties used)
    A 300mm cavity gives a U-Value of around 0.11

    Now, lets say you had 200 sq metres of wall.
    0.16*200*85 =2720
    After one year, 2720 is the number of Kilowatt/hours (units on an ESB bill) that you have lost through the wall.
    The 85 figure is explained in the other thread linked to, but basically it represents an average difference in temperature of 9.7 degrees between inside the house and the outside, averaged through the year.

    Say you are currently paying 10 cent/KW of energy. That is €272 per year that you are losing through the wall.



    Do the same calculation for U- value of 0.11, and you get €187 per year, a saving of €85.

    But if energy is going to cost 20 cent/ Kw, or 30 cent.........

    BTW You also want to make sure that the inner leaf is not losing heat at the bottom, into the foundations, and at the top, bridging across to the outer leaf.


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  • #12
    sydthebeat
    Subscribers, Paid Member Posts: 44,924 ✭✭✭✭sydthebeat


    sheff the ref wrote: »
    .....

    Question 2 is the one that nobody has the answer to, how to identify that very point of minimal heat loss.

    to answer this in my own view...

    full compliance with 2011 regulations, using a MHRV, is a very acceptable break point in the cost / benefit of elemental insulation.

    A house build has to be looked at holistically, and not as separate elements.


  • #13
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    The type of answer I was looking for,

    Granted I would be talking about closer 150 sq metres of wall, but if energy costs go up, the extra expense is probably worth it. In fairness if you are saving €85 a year, you wouldnt be long making that back for the extra cost of the beads.

    I have seen the Quinn lite bricks on a site last week, and will probably opt for them.
    caesarthechimp wrote: »
    A lot depends on what the price of energy will be in, say, 10 years time smile.png

    A 200mm cavity filled with graphite bead gives a U-Value of around 0.16 (depending on the type of blocks and wall ties used)
    A 300mm cavity gives a U-Value of around 0.11

    Now, lets say you had 200 sq metres of wall.
    0.16*200*85 =2720
    After one year, 2720 is the number of Kilowatt/hours (units on an ESB bill) that you have lost through the wall.
    The 85 figure is explained in the other thread linked to, but basically it represents an average difference in temperature of 9.7 degrees between inside the house and the outside, averaged through the year.

    Say you are currently paying 10 cent/KW of energy. That is €272 per year that you are losing through the wall.



    Do the same calculation for U- value of 0.11, and you get €187 per year, a saving of €85.

    But if energy is going to cost 20 cent/ Kw, or 30 cent.........

    BTW You also want to make sure that the inner leaf is not losing heat at the bottom, into the foundations, and at the top, bridging across to the outer leaf.


  • #14
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    I am more concerned with Insulation, Thermal Bridging etc. than MHRV, air tightness etc. To be honest, it is something that. A bit of natural air circulation does no harm.

    How many times per day does at least one external door open in an average house?
    sydthebeat wrote: »
    to answer this in my own view...

    full compliance with 2011 regulations, using a MHRV, is a very acceptable break point in the cost / benefit of elemental insulation.

    A house build has to be looked at holistically, and not as separate elements.


  • #15
    Rabbo
    Registered Users, Registered Users 2 Posts: 637 ✭✭✭Rabbo


    Do you not realise that poor air tightness will effectively allow a bypass of the insulation/thermal bridging.
    What's the point of putting such effort into your insulation if you are going to allow cold air in behind it?
    As syd said, you need to take a holistic approach.


  • #16
    sydthebeat
    Subscribers, Paid Member Posts: 44,924 ✭✭✭✭sydthebeat


    sheff the ref wrote: »
    I am more concerned with Insulation, Thermal Bridging etc. than MHRV, air tightness etc. To be honest, it is something that. A bit of natural air circulation does no harm.

    How many times per day does at least one external door open in an average house?

    do you think your concern on insulation and thermal bridging would be best served by drilling 4" holes in all your room walls?

    like i said, you need to think of the whole build holisitcally and equally.

    If you want to aim for a low energy usage, pick a "kw/hr per m2" figure you are comfortable you can afford and work the specification back from that.

    It would be VERY pointless to end up with a build with a 0 psi value and really low elemental u values, if you dont aim for low air infiltration levels.

    natural air circulation is not just "no harm" but essentially vital. you need to design for this as well.

    Have you anyone engaged to look at this overall specification for you?


  • #17
    sinnerboy
    Closed Accounts Posts: 5,433 ✭✭✭sinnerboy


    sheff the ref wrote: »
    I am more concerned with Insulation, Thermal Bridging etc. than MHRV, air tightness etc. To be honest, it is something that. A bit of natural air circulation does no harm.

    How many times per day does at least one external door open in an average house?

    To comply with building regulations there is no choice - it is not optional. You must concern yourself with air tightness and ventilation.

    Now you can debate the wisdom of the regs all you like - but they are what they are no matter what you may think of them.

    You cannot zone in on one element only - in this case the cavity wall thickness - and get a meaningful answer to your question. You have to know the entire house in all it's measured calculated glory to start to get answers.


  • #18
    rockabaloo
    Registered Users, Registered Users 2 Posts: 103 ✭✭rockabaloo


    @sheff the ref - The extra cost isn't just the extra insulation. It's also extra foundations, extra wall ties, longer wall ties which are more expensive.

    €85 for 25 years is only €2125.

    An extra €1000 of mortgages money at 5% interest rate over a term of 25 years will cost €1933. So if you're extra materials and work for your walls cost more than €1000 (which they will) then you're talking massively long payback times.


  • #19
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    It is something I am trying to broaden my knowledge on, at this current stage and to be honest I am considering doing a one day air tightness course later in the year to get a handle on it.
    sydthebeat wrote: »
    do you think your concern on insulation and thermal bridging would be best served by drilling 4" holes in all your room walls?

    like i said, you need to think of the whole build holisitcally and equally.

    If you want to aim for a low energy usage, pick a "kw/hr per m2" figure you are comfortable you can afford and work the specification back from that.

    It would be VERY pointless to end up with a build with a 0 psi value and really low elemental u values, if you dont aim for low air infiltration levels.

    natural air circulation is not just "no harm" but essentially vital. you need to design for this as well.

    Have you anyone engaged to look at this overall specification for you?


  • #20
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    Ya but once its in, its in. What will heating costs be in 10 years time?
    rockabaloo wrote: »
    @sheff the ref - The extra cost isn't just the extra insulation. It's also extra foundations, extra wall ties, longer wall ties which are more expensive.

    €85 for 25 years is only €2125.

    An extra €1000 of mortgages money at 5% interest rate over a term of 25 years will cost €1933. So if you're extra materials and work for your walls cost more than €1000 (which they will) then you're talking massively long payback times.


  • #21
    rockabaloo
    Registered Users, Registered Users 2 Posts: 103 ✭✭rockabaloo


    sheff the ref wrote: »
    Ya but once its in, its in. What will heating costs be in 10 years time?

    Using the same argument I could ask what will the interest rate on your mortgage be in 10 years time?


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  • #22
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    Essentially my focus is to establish the construction cost differences between say 200mm and 300mm cavities. Reluctant to slab with insulated plasterboards inside if I can avoid it at all for numerous reasons.

    Other associated costs include window surrounds and lintels etc. for a wide cavity

    PS: Does anyone have a 300mm cavity under construction at the moment that I could call to see? Will travel if I have to


  • #23
    Andrew_Doran
    Registered Users, Registered Users 2 Posts: 227 ✭✭Andrew_Doran


    Given a near perfect wall, to halve the rate of heat loss you must double the thickness of the insulation. See the below image (I hope it displays).

    While this is a good rule of thumb, buildings are never perfect and that's where it gets complicated -- thermal bridges etc.

    EDIT: don't take the numbers below literally because they have no context. It's the more the shape of the graph that is helpful!

    graph_relationship_between_thickness_insulation_and_heat_loss.jpg


  • #24
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    If those figures happened to be accurate, that chart would appear to suggest that heat loss levels out significantly after 150mm of insulation. Where was this chart sourced?


  • #25
    rockabaloo
    Registered Users, Registered Users 2 Posts: 103 ✭✭rockabaloo


    @sheff the ref
    Andrew_Doran wrote: »
    EDIT: don't take the numbers below literally because they have no context. It's the more the shape of the graph that is helpful!


  • #26
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    Ya I know that, but there must be a set of figures somewhere that match a graph like that. The point at which the graph levels out, is the point whereby any further expenditure on the thickness is no longer necessary
    rockabaloo wrote: »
    @sheff the ref


  • #27
    beyondpassive
    Registered Users, Registered Users 2 Posts: 558 ✭✭✭beyondpassive


    Heat loss is u-value x area x delta T (change in temp across fabric). U value being the reciprocal of thickness devided by conductivity. Take out surface resitivity and the curve is fairly linear. When the delta T is 20+ degrees, as it is in a low energy design, the cost benefit curve doesn't begin to flatten until nearly 400mm of insulation.

    From my own analysis, 250 cavity with stainless steel wall ties and quinlite thermal breaks is the cost benefit sweetspot. End of.


  • #28
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    Beyondpassive,

    I assume you have obviously priced materials to make this evaluation????

    If so, was there much of a cost involved in wider sills etc?


  • #29
    beyondpassive
    Registered Users, Registered Users 2 Posts: 558 ✭✭✭beyondpassive


    In terms of cost, the compactness of the build is a big factor. The sills are standard sills and sit on an angle. The cavity closing detail at opes and tops of walls is complex but prohibitively so.

    our main criteria for determining insulation thickness is internal surface temperatures. using thermal bridge software to work out how the wal will perform if its -5 outside and +20 inside what is the wall temperature. If its over 19 degrees then it is aiding internal comfort and acting as a thermal buffer. We also analyse the critical points such as window cills and reveals to make sure they are over dew point or 13 degrees.

    So its not purely about heat loss, but more condensation risk, comfort and thermal storage. By keeping all the internal concrete above 18 degrees our buildings become a big storage heater and the underfloor can purr away with 32 degree water fed from the accumulator middle, thereby maximising the performance of the heat pump or autumn/spring solar.

    Does that work for your own situation skeff?


  • #30
    sheff the ref
    Registered Users, Registered Users 2 Posts: 620 ✭✭✭sheff the ref


    The world is my oyster at this juncture.

    The Quinnlite blocks are possible, saw them on another site recently. Basically from what you are saying, I assume the compensation for the wider cavity is filled by the internal reveals, which will basically be insulated plasterboard.


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  • #31
    fclauson
    Registered Users, Registered Users 2 Posts: 1,691 ✭✭✭fclauson


    Do the maths on price and benefit of Quinn vs Bead

    from memory a 4in/100mm quinn adds the equiv of about 5mm of bead - and quinn is nowhere as near as good for thermal mass

    I used a couple of rows in the base of the walls to stop downward thermal bridging


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