Sunday, 9 September 2012

Build Week 5 - Beam & Block Floor

This week the floor structure went in. We chose to have a beam & block floor structure so we could use the existing foundations and allow ventilation and drainage to the basement walls. We used a specialist polystyrene block instead of the conventional concrete building block. Although more expensive it has many benefits over the traditional system. Expanded polystyrene it is a very good insulant, requiring little extra insulation to bring it up to the very high standard we set ourselves. This along with its T profile, which lets it sit in between and on top of the beam, means that the floor construction depth is reduced. The product uses a wider beam spacing meaning less beams, saving time and money. The polystyrene blocks are manufactured in 2.5m lengths and are easy to cut and fit into place, again saving time, money and a lot of work. This floor construction also scores A+ on the BRE green guide contributing to the CfSH materials assessment.

The day that the beams arrived was a very tough one as there was torrential rain all morning. Luckily we had hired a crane for the morning to lift the beams from the lorry directly onto the low level masonry. In hindsight we would have struggled greatly without the crane as the torrential rain made the clay ground very slippery and dangerous. As it was we managed to get the beams in place by lunchtime and could retire for the day absolutely soaked to the skin, but with all the beams insitu. The next day the polystyrene blocks were delivered and were quickly fitted into place. A job well done in difficult circumstances. The rest of the week was spent finishing the low level masonry in preparation for the SIPS installation..........


  1. Just found your blog as a result of reading on the Green Building Forum.

    I have been in my house for 3 years now, timber frame, not designed to a high code level (don't think it was necessary when we started but don't remember) I was very anal about all the detailing during the build, taped every seam, taped and foamed wherever I could. I wanted it air-tight.

    The SAP air test only required 9 (whatever they are) but I managed 1.7 and that was with one of the loft storage doors open! He would not test again with it closed as he told me it was as good as he had ever seen and wanted to rush off to the next job.


  2. Hi, hope you found the blog interesting. Do you have an MVHR system and what SAP score did you achieve? I am interested to see how you have found your house to perform in reality as opposed to the theoretical world of SAP.

    1. Yes, I found the blog very interesting. I wish we had done some of the thing you did, but when push comes to shove, as a non-architect, I have to go with the advice of the 'so called' experts. I'll expand in a moment.

      To answer your question, yes, I do have a MVHR unit. Since there are only two of us in a 240 sq.m house, I only have it running for about 10 hours per day. As for the 'as built' SAP, I was disappointed; figures are TER 23.37, DER 20.47; SAP Rating 77C; SAP Energy cost £512; EPC C.

      The various recommendations were to fit low wattage lighting (we had halogen downlights in living areas and have those terrible low energy fittings in bedrooms. Now all halogens GU10 have been replaced with LED. Also, we don't have any solar stuff whatsoever. If I had, the EPC would only have gone up to B; which I find amazing because when I look in local estate agent windows, there are some refubs with an A rating!

      Why was I disappointed with the figures? I did a lot of research and wanted above average insulation (insulate, insulate, insulate) and air tightness (build tight, ventilate right.) It is a timber frame with 180mm of Warmcell insulation throughout and about 300mm in the roof. All internal walls and the g/f ceiling are Warmcell. The drylining is Fermacell. The windows are 3g with glass fibre frames. The individual u-values look good, but the materials were not on the SAP assessor's list, so he had to a/ take my word and b/ round down. The result was therefore, not as good as I had hoped. However, in the real world, the house is warm, appears to retain its heat well and during the winter, if the heating goes off, only looses about 1-2 degree C in 24 hrs.

      The house is about 250 sq.m. inc a double garage and over the last two years, the gas and elec. costs have been about £1300, which is slightly less than we were paying for our previous, 1960's built, two bedroom maisonette.

      So all in all, not too disappointed.

      The house shape is quite complex compared with yours; think Huf Haus with some more ins and outs. A simpler shape would certainly have made life easier (and cheaper!) I would have liked the beam and poly block as you have, but my ground workers and the t/f design team planned for a one piece, poured, reinforced slab.

      I also wanted a f/f screed using Lewis Plates and included that in the initial 'wants.' Somehow during the design and structural engineering process, that was not included. This oversight only came to light when the frame was up and I was told by the structural engineer that the floor/walls had not been engineered to take a dead load on about 75 kg/m2. So my f/f UFH is set in polystyrene sheets with ali spreader plates. Keeps the house warm but is a REALLY bad idea. I would not recommend it to anyone.


  3. I am glad in reality it has worked out pretty pleasing as that is the most important thing. It is quite a big house as well. I can understand your frustration at the EPC rating. As we were going for the level 6 CfSH becoming zero carbon was essential therefore I ran a SAP calc alongside the design so I could see how my design and specification decisions affected the rating, so it was a bit of a game.

    I learnt a huge amount through this and I am surprised that the airtightness and MVHR did not bring you up to a higher score as these are a couple of the biggest factors. It also sounds like you have a well insulated thermal envelope as well. I did find the shape of the house did have a very big effect on the calculation so this maybe where a lot of the score got lost.

    We have been really pleased with how the house has performed thermally and as we have so little in the way of heating, it is a good job as well - we only have 2 towel radiators downstairs. The only change which I would make and still may is to put a post heater in the MVHR ducting (which is easily and cheaply done) to distribute a gentle heat around the house if necessary. Another change I would make would be as you wanted have a screed first floor. It is a possibility if using a Floscreed which is a very thin screed - this would be mainly for noise benefits as we have found that to be a little frustrating, but we hope that will be better when the floor finishes go down.

    We have found overall that the house actually generates an income rather than cost us, as all the water & energy bills are outweighed by the FIT income of the PV array. I think that our biggest energy cost comes from lighting in the winter. Due to aesthetic reasons we went with low energy halogen bulbs in our living areas of which there are quite a few. They are 'low energy' but absolutely nowhere near that of LED's.

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