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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
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    • CommentAuthorKBabs
    • CommentTimeMar 11th 2019
    Hello all helpful people,

    I have a 1930's house with a new, large extension - standard 300mm insulated cavity wall to the ground, timber 1st floor with 115mm celotex (75 between / 40 over internal).

    I am having much of the house rendered, including 90mm EWI graphite eps to the existing part of the house.

    On the extended part, the first floor external build-up is 20mm more than the ground floor. The firm doing the EWI and rendering have suggested using a 20mm EWI on the ground floor to level out the walls before rendering.

    Is this a good option or will it lead to any issues?

    Another option is to add 70mm eps on the ground, with 50mm eps on the first - in theory giving us a much better insulated house. This will cost an additional £2250.

    Again, my question is will it lead to issues, and also, will it actually be worth it from a heating and comfort POV.

    Many thanks and I look forward to your thoughts.

    • CommentAuthorgoodevans
    • CommentTimeMar 11th 2019
    I would suggest the 70mm,50mm option - If the price iyou have been quoted includes labour and adhesive I would guess that option would be the cheaper one - an extra 20mm of EPS thickness should cost around £1.40/m2 extra in EPS but that would be recovered in reduced labour and adhesive costs.
    KBabs, If you have a 20mm over-stand between the first and ground floor now - and you are happy with that, why not leave it?

    Making some assumptions about the ground floor 300mm cavity, namely it is filled with glass fibre this gives you a U value of 0.12

    If you add 20mm of EPS you will improve to 0.11 and adding 70mm EPS will improve to 0.1

    The additional 70mm EPS with the improvement in U value by 0.02 will save you about 0.4W/sqM with a 20 deg difference inside to outside.

    That's quite a long ROI on a spend of £2250

    Putting an additional 20mm on the ground floor will save half the amount of the 70mm fit-up.

    Putting an extra 50mm EPS on the first floor will improve the U value by about about 0.03

    I don't thing there would be any issues with adding the additional EPS to the outside, also I don't think you will notice any comfort difference as the increase is (IMO) marginal.

    Do the sums and make a decision.
    • CommentTimeMar 11th 2019
    Posted By: Peter_in_Hungaryground floor 300mm cavity

    I don't think it's the cavity that is 300 mm, I think it's the wall - i.e. two wythes and a cavity.
    • CommentAuthorKBabs
    • CommentTimeMar 11th 2019
    djh, is correct. Inner (aerated ) and outer blocks with 100mm Knauf 37. Sorry for any confusion on my part.

    Peter_in_Hungary, I'm sure this will make a big difference to the u-value you stated. On paper, the ground floor is .26 and the 1st is .22. As ever, the insulation is not 100% fitted without gaps, however, I have been keeping an eye on it over the course of the build and it is pretty good - albeit with my own intervention at times.
    • CommentAuthorKBabs
    • CommentTimeMar 11th 2019
    Another thing to note, using the second option, thermal bridging will be reduced or eliminated completely, between the existing and new part of the house, as there'd be a continuous wrapping of EWI.

    Another consideration is on the 1st floor. Despite celotex between and over (internal) in the timber frame, there are large parts that will only have 40mm of celotex, with the rest of the wall thickness being the timber stud. Presumably this weakens the u-value?
    With the ground floor at 0.26 an additional 70mm of EPS I would expect to reduce this to 0.18
    The reduction of 0,08 is not going to make a big difference, about 1.6 W/m2 reduction in heat loss (at 20 deg. temp. difference inside to out). If you had 100m2 of wall and a 20 deg. temp difference for a month (unlikely) you would save about 115 kWh for the month and with gas at about 3p / kWh saves about 3.5 quid.

    IMO I don't think the difference will alter the comfort factor and the ROI is over the horizon.
    • CommentTimeMar 12th 2019
    As a cavity wall with (you say) reasonably continuous cavity insulation, you prob will be getting quite close to the nominal 0.26. But the upper floor - does that nominal 0.22 calc take account of the bridging by studs? Even if yes, that construction is liable to a bigger effective degradation - you prob will get a lot or a fair bit worse than the nominal 0.22.

    So, how about saying yes to the 50 EPS on the upper floor but none on the ground floor? It will do wonders for both the upper floor's bridging and its continuity (EWI is much easier to get continuous than any other type of insulation). Or more than 50 while you're at it - will cost little more.

    That would give you a more pronounced overhang of upper over lower, which could architecturally look 'intended' rather than an embarassing mistake.
    • CommentTimeMar 12th 2019
    Chaps, I think the options Ken is considering are:
    - as a given, 90 mm everywhere
    - as option 1, another 20 mm on the ground floor
    - as alternative option 2, another 70 mm on the ground floor plus another 50 mm upstairs

    Note that the upper floor that has 20 mm overhang is an extension, not the whole upper floor, hence perhaps the desirability of losing the overhang in the insulation depth.

    Another possibility would be 90 mm on the original house and 70 mm on the extension. That should be fairly cheap. I would recommend using grey/platinum insulation. Unless the overhang is exactly 20 mm I suspect it will still be visible after rendering, so it might be worth considering a feature in the render - a raised band or somesuch. But the rendering firm will have a more informed view of what's possible.
    • CommentAuthorKBabs
    • CommentTimeMar 12th 2019
    90mm to the existing only.

    The two options are for the new bit only. Again, 300mm insulated cavity wall to ground (100, 100 Knauf 37, 100 aerated), timber first floor, 75mm celotex in-between studs, 40mm over.

    Thanks for all your views so far!
    • CommentAuthorKBabs
    • CommentTimeMar 14th 2019
    Would really appreciate any more thoughts on this.
    You will have to decide what your priorities are and how big the bucket of money is.

    An extra 20mm to even out the step can be achieved without noticing anything after it is done - providing the extra 20mm EPS is put on, then sanded down flush with the protrusion with the difference feathered in, then given a coat of adhesive render with glass mess embedded over the join, then the finish render over the whole lot. done properly this will be invisible and the join shouldn't show a crack in the years to come.

    For the additional insulation as I said above I don't think that you will get any ROI against the heating bills worth much and I don't think any change in the comfort levels will be noticeable. It was once said on here (but I can't find the quote) that changes to the insulation level U value at the 2nd decimal place are not worth the trouble and as far as I can see the additional insulation will only change the insulation value by about 0.08 .

    You will have to do the sums and make the decision.
    • CommentAuthorKBabs
    • CommentTimeMar 15th 2019
    Thanks Peter. The reason for highlighting the build-up again is because I thought there might be some confusion, thinking 90mm was everywhere, then the option 1 and 2 were in addition - this might have caused your u-values to be out.
    • CommentTimeMar 15th 2019 edited
    Posted By: KBabsWould really appreciate any more thoughts on this.
    Given what you've heard so far, can you refine/restate what your question(s) is?
    • CommentAuthorKBabs
    • CommentTimeMar 18th 2019
    OK, so to recap... The new build part (of which is the majority of the whole house), has a 300mm cavity wall (100mm Knauf 37 insulation) .26U. The first floor is timber frame (75mm celotex between studs, 40mm over studs internally) 0.22. There is quite a lot of stud work, particularly at plate level where in some areas, they built it up to meet to existing plate level.

    In order to create more space internally, the first floor stud work is built right to the edge of the ground floor outside wall. With exterior material build up, this will create a 20mm overhang.

    I have two options to create a finished flush wall:

    • Option 1 - 20mm grey eps to the ground floor.

    • Option 2 - 70mm grey eps to the ground floor, 50mm grey eps to the first floor.

    I am motivated by comfort levels and better heat retention, however, not if the pay back to over the horizon.

    My thoughts are this, any additional eps to the ground floor outer leaf should help keep the outer blockwork warm and stop the outside cold from penetrating it - in turn keeping the Innside warmer. So option 1 seems like a good idea in principle. Option 2 seems like a better idea because it should eliminate any cold bridging and hold in the heat way better.

    I hope this help to clarify things.
    Additional insulation to the cavity wall which is already at 0.26 U value will give you approximately

    +20mm EPS new U value about 0.22
    +70mm EPS new U value about 0.17

    The additional 70mm gives an extra 0.09 U value which at a 20 deg difference over 100m2 of wall will save about 4.3kWh per day IF there was a 20 deg difference all day. Look at your heating fuel price and see how long the ROI will take.

    Insulation effectiveness is not linear, it suffers from diminishing returns. Apart from the base question of whether to add insulation or not - I can not see an advantage of using grey EPS over the cheaper white. Whilst the grey gives a better insulation value per mm. IMO you are already at the margins of viable ROI so IMO using the grey makes less sense than using the white.

    And I presume you want to get rid of the 20mm step in the wall anyway - why?
    • CommentTimeMar 18th 2019
    This being the green building forum, is there any mileage in pointing out that reducing energy usage is a positive goal in itself? Saving the planet ranks alongside saving pennies in the ROI calculation.
    • CommentAuthorphiledge
    • CommentTimeMar 18th 2019
    Insulation aside, you mentioned that the extension first floor walls are built off the ground floor outer leaf of masonary?? I'd check that the outer masonary leaf is structurally adequate to carry the first floor wall/roof load. Its normally the inner leaf that carries the loads above.
    Hi Kbabs,
    Depending where you are, the building standards/regulations may require you to insulate to a certain standard whenever you (re)render a significant part of a building. In Scotland the required legal standard is often U=0.22, different elsewhere. Of course you can beat the minimum standard if you choose to.

    If like me you have a mortgage, at the moment I can borrow cheaply so find it easy to pay for thick insulation and still the energy savings cover the extra loan repayment. Obvs you need to make your own choices and don't take investment advice from an internet forum...
    • CommentAuthorKBabs
    • CommentTimeApr 5th 2019
    Thank you for all your comments so far. I wanted to give this some time before I commented back.

    Putting aside the additional cost, I guess the thing I wanted to know is, will there be a tangible difference in heat retention, comfort levels and reduction in heat demand. Peter has done a good job in looking at the u-values and payback, however, I just wonder if by adding EWI to a new cavity wall, with the addition of thermal mass from the EWI, will that have a much greater impact, than if you were just increasing the insulation in a cavity, from say, 100mm to 150mm or 200mm.
    • CommentAuthortony
    • CommentTimeApr 5th 2019
    The mass brings you more comfort and slower temperature swings the additional insulation reduces heat losses (and heat gains in summer ) and reduces your energy demand and heating bills.
    • CommentAuthorwookey
    • CommentTimeMay 6th 2019
    Seems to me that all wall elements should be at least U=0.15 if you can. That my definition of 'reasonably good'. So adding +70/+50 to make a smooth wall that is well-insulated is a good idea. You are already building a whole extension. Another 2K is probably not a big deal. Some PHPP sums might show that it would be more effective to spend that money on better windows or something, but unless you are going to do reasonably detailed modelling my advice is always 'go for 0.15 if possible'.

    Talking about 'payback' on the green building forum should be banned unless we are comparing emboddied carbon to lifetime emissions. Whilst money is not irrelevant, it's a really poor instrument for deciding insulation levels in long-term retrofit IMHO, because it's fundamentally the wrong way to be thinking about the issue.
    • CommentAuthorKBabs
    • CommentTimeMay 19th 2019
    Hi all - thank you for all your comments on this thread.

    In the end, I decided on a "bolt and braces" approach and went ahead with double insulate everything.

    We have an unused chimney breast on an external wall, this was EWI'd from top to bottom, however, I am concerned that wind will still be getting down inside. Is it okay to simply pour down eps balls or vermiculite and cap it off at the pot, or should I be concerned about lack of ventilation?
    The best solution for an unused (and never to be used) chimney is to take it own to ceiling level and then close over and put the loft insulation over to match existing and make good the roof tiling having removed the flashing with the chimney. This gets rid of potential problems with rain ingress around the flashing and gets rid of the cold bridge of the chimney at ceiling level. Unused chimneys that are capped off still need a bit of ventilation.

    A bigger and much dirtier job with a unused chimney is to either take the front or from and side cheeks down to reclaim the room space, but that is serious mess and the amount of bricks always amazes me.
    • CommentAuthorKBabs
    • CommentTimeMay 19th 2019
    I cannot remove the chimney.

    Can I simply 'stuff' some fibre glass insulation down through the pots. Not ideal but at least it will stop the wind.
    Posted By: KBabsI cannot remove the chimney.

    In that case I would open the flue at ceiling level and block it off there and cap off the top but provide ventilation at just above the loft insulation.
    • CommentAuthorKBabs
    • CommentTimeMay 19th 2019
    I cannot get the to chimney via the loft space to do that. Only access I have is through the pots.
    • CommentAuthortony
    • CommentTimeMay 19th 2019
    Wookey a big plus one +1

    I would try to fill the chimney flues with polystyrene beads to above top ceiling level, cap the pots with ventilators and add air bricks just above ceiling level and above the top of the beads.
    • CommentAuthorKBabs
    • CommentTimeMay 19th 2019
    The chimneys have been EWI'd and rendered so cannot add additional airbricks. I'm interested to know why you're suggesting airbricks above the polystyrene beads if there is airflow down the pots?
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