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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

PLEASE NOTE: A download link for Volume 1 will be sent to you by email and Volume 2 will be sent to you by post as a book.

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    • CommentAuthordavvac
    • CommentTimeJul 9th 2008
     
    Can anyone let me know what factors would help me choose the worst case unit in a block of flats for SAP.

    I'm looking at CfSH level 4 on two current projects for a Housing Association.The unit by unit nature of The Code means that I will have to get the 44% improvement of the DER over TER on the worst flat with the "better" units over performing.

    I work for a medium sized builder by the way, hence perhaps the "noob" nature of the question.
    • CommentAuthorjon
    • CommentTimeJul 9th 2008
     
    Probably the largest external area
    • CommentAuthordavvac
    • CommentTimeJul 9th 2008
     
    I sort of guessed that was one of the criteria, but I think ground, mid or top floor makes a difference too.

    Logic would suggest that top floor would be worst, but with a well insulated roof would it?

    Would the fenestration and orientation of the flat come into play as well?
    • CommentAuthorSimonH
    • CommentTimeJul 9th 2008
     
    Top "corner" flat is the worst (assuming an back to back arrangement) as it has 3 heat loss surfaces (2 walls and a roof). If you have a flat that runs the whole width of the building then it's the top "end" flat - whic might have 4 heat loss surfaces (3 walls and a roof).

    An alternative is if you have an unheated space below the lowest level flat - like when you have a car park below. In this case depending on the level of floor insulation it could be the reverse of the above. Ie bottom corner, or bottom end flat. But it's more likely that you'll be looking at the top floor.

    SAP can also take orientation into account - in which case it would also be the flat on the north side corner or the one with most north facing walls - especially if they have windows in them. :-( - Make sure you don't buy that one!

    This will of course depend on whether the flats are all the same size and have the same heating system.

    Eg. A mid flooor flat with only 1 wall - might be 10m long along the north face and 5 m deep. With only electric on peak heating. So it has 10x2.5 m of heat loss perimeter. 25m2.

    You could also have a top floor flat 5x5 m with a heat loss area of 5x2.5x2(walls)=25m2 + 5x5(roof) = 25m2. But if it has a large south facing window and a gas consdensor boiler + UFH it will have massively higher SAP score than the previous flat. Probably 80ish VS 50ish.

    The only real way to find out is to get some SAP assessments done.

    Simon
    • CommentAuthordavvac
    • CommentTimeJul 9th 2008
     
    Great

    Thanks Simon.

    I am expecting the initial SAPs soon, so all will be revealed.

    As an aside, I know from discussions with our assessor that the worst flat meets the 44% with our enhanced u values, AP rates of 6 and exhaust air heat pumps as the space and water heating system.

    Regards

    Dave
    •  
      CommentAuthorOlly
    • CommentTimeJul 9th 2008
     
    Remember you're looking at a percentage improvement not an absolute emissions figure. Therefore even though a top storey corner flat may perform worse than a central mid floor single aspect flat in terms of CO2 emissions per square metre, its TER will be correspondingly higher so it's often actually easier to improve upon the former as you will benefit more from lower U-values. This also affects the ratio of space heat to DHW which in turn impacts on the % improvement resulting from any renewable energy systems, e,g, Percentage wise SHW will improve the DER of the single aspect flat more than the top corner flat.

    So it could be argued that electrically heated 2 bed detached houses are the way to go as you get a whopping great big TER due to the surface area and fuel source, simply whack in an ASHP and job done. I am over simplifying, but it illustrates the point that it's not actually about creating a dwelling with the lowest emissions possible.

    I'd certainly be interested to see the SAP results for the ASHP flats and how close they are to the 44% requirement.
    • CommentAuthordavvac
    • CommentTimeJul 10th 2008
     
    Thanks Olly

    Your comments are right on the money.

    I have just received a summary of the SAPs for two sites totalling 46 apartments. All have hit the CfSH 44% DER/TER figure for Level 4.

    I have attached the summary of the results and you can see that the best % increase is generally on the units with a higher TER.

    Regards

    Dave
    •  
      CommentAuthorOlly
    • CommentTimeJul 10th 2008
     
    I see they've basically assumed worst case air tightness for all units (10m3/m2/hr) and then improved it on the worst performing units to squeeze them through. The best is 6.5 m3/m2/hr, which should be easily achievable, although some builders still seem to struggle in this country.
    • CommentAuthorsimple153
    • CommentTimeAug 20th 2008
     
    Dave

    Would it be possible to list your other "U-Values" used for the SAP calcs ie walls/windows/roof etc. I am trying to find the most practical solution.

    Regards

    Simon
    • CommentAuthordelboy
    • CommentTimeSep 18th 2008
     
    Worth remembering that if the flats are all in the same building envelope with the same heating systems, then the improvement of DER over TER is AVERAGED to identify which Code level it can meet. Therefore the weakest flat can have an improvement of less than 44% provided the overall improvement is averaged at 44% or more.
    Page 44 on the technical guide: http://www.communities.gov.uk/planningandbuilding/buildingregulations/legislation/englandwales/codesustainable/
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