GraemeHM

We’re close to finishing our extension now and one of the last jobs is converting the old 1950s built kitchen into a cosy office come spare bedroom. The original floor 3x5M is concrete (with some form of bitumen coating) has always been cold. I am looking for advice and suggestions. My thought was to level the floor with sand, lay down 50mm of PIR insulation then overboard that with chipboard. Then underlay and either carpet or LVT. Not sure about fixing or not the floor ie the celotex or the chipboard. Would it be wise to have in essence a very thick floating floor. any thoughts or guidance much appreciated.

Bit of background.... we are just completing the structure of our single story extension to our two bedroom, two bathroom (one shower, one bath/shower) house. This will add an extra bedroom and extra shower. We are on LPG and currently have a Glow-worm 30cxi combi boiler, which is in good working order and meets our needs more or less. We will be relocating the boiler into the ground floor extension as part of this project. One of the things we dislike about our current setup is that every time you turn on a hot tap, even for a short period, the boiler fires up. My impression (and I'm sure I have read this somewhere) is that firing up the boiler often and for short periods is inefficient and costly in fuel. We are on LPG so fuel is expensive. One thing we do like about our current combi is that we don't run out of water in the tank. Although the existing combi has been well maintained I have been considering our options. One option is replacing the boiler with a system boiler and tank. Tight for space, but possible. Another option I have also been looking at is to replace the combi with a storage combi eg Glow-worm Energy 35 Store combi. My hope is that the HW tank in the storage in the combi will stop the constant start/stop of our existing setup, is that true? Any thoughts and advice welcome.

We’re just getting to the final stages of our extension. I had a similar question when we first started. The issue is condensation forming on the underside of the metal. I spoke to lots of people including a very knowledgable guy from VM Zinc who said. “You can do either (cold vented, warm unvented) especially if it’s a new build and you do everything properly and use the correct materials. But, given the choice he would say that a cold roof I.e vented is more forgiving in the long run”. There is lots of detailed discussions on here about the pros and cons. In the end I went for an aluminium standing seam roof on a cold vented roof. It looks the same although there subtle differences for the installer. They do the same job, but the aluminium is cheaper and doesn’t suffer with the “possibility” of corrosion. Noise is hard to compare like for like. I had heard that cold vented could be slightly noisier, but to be honest our experience so far (and my god we’ve had some rain down here in Devon during May) is that the roof buildup needed for BC soaks up the noise really well.

@dnb @TonyTGreat info in this thread guys, thank you. One follow up question I need a bit of advice on…I plan on putting external lights at the same height as the internal ceiling safety zone. The cables would be run in that zone (batten service void), pass through the timber framed/rendered wall (260mm) and terminate directly into the light fitting Do I need to joint and change to swa before the cable passes through the wall or can I continue in t&e but put in a short length of metal/plastic conduit? Do I need to put a box and blanking plate on the inside wall surface to show presence of cables even though it’s in the safety zone? Cheers

So if I understand correctly, you say that insulation on the outside of the walls? Originally I had wanted to insulate on the outside of the walls too i.e. on the outside of the OSB then the batten, render board etc. I was told by the render technical guy that this was problematic. Any thoughts? Should I recheck this info?

@tonyshouse Thanks for that, I appreciate input. I have read a bit about condensation risk in timber frame at the sole plate, but hadn't followed up on it. So if I can use an aerated concrete block then this should reduce that risk, right.... so what about this... please stay with me.... Earlier @Mr Punterpointed out that I should have a 150mm upstand to ground level, so following his input and yours, I could use two blocks. The base on its side, the send on its edge. So that gives me an extra 100mm to get my upstand. If both blocks are aerated and I face the top block with PIR behind the sts construction board would that reduce my risk of cold bridging from the outside and hence condensation. I appreciate that the concrete slab could (possibly would) cold bridge from the underside through two aerated concrete blocks, but hasn't that got to reduce the risk? Please see attached...What do you think? Cheers On a side note my BC design guy has specified 7.3n blocks which for a single story extension is possibly a bit of overkill? Detailed structure x-section 31-3-21 two block +.tif

@tonyshouse Thanks for the input. Thermal bridging I understand, but what analysis of thermals are you suggesting so I can find out more. Others have suggested Marmox but it's hellish expensive are there any alternatives that I should look at?

@Mr Punter Do you think the attached would cut the mustard. Following your suggestion I have increase the difference between outside and inside and dropped the render carrier board down. Also added detail about perforated pipe and French drain. I am trying (I suppose) to keep to a single block on the perimeter. Many thanks Detailed structure x-section 31-3-21 2nd.tif

@Mr Punter Thanks. I did think about marmox as it came up on another post, but I looked at the cost and whilst I’ve tried to do a good job on the insulation we are tight on budget. So on the DPC is that 150mm up from the outside ground level and under the sole plate? How important is that?

I hope the attached drawing makes some kind of sense. We are just about to start digging down for our extension. There is fairly straight forward trench foundation and concrete pad. We are planning doing a single pour for both trench and pad. Then a single course ring of 140mm wide blocks (excluding the doors) to build off. 150mm (400mm c/c) stud work filled with Rockwool with the wall and floor build-up shown. My question is about the DPM and upstand on the outside. There isn't much detail on our architect BC drawings and want to check with the collective knowledge on how good (or otherwise) you think this construction is. Thank you! Detailed structure x-section 31-3-21.tif

Thank you @Thorfun & @craig for your help, it’s such a subjective decision. Be bold or not ?

@ThorfunWe are wrestling with the same decision right now. Originally we were going anthracite outside and inside but we've had a bit of a wobble just as we're about to confirm the order details. Can I ask, do you really notice the inside colour or has it blended in? How easy has it been to match in other interior colours? What colour paint have you gone for on the walls and reveals? Excuse the cheek but could you DM a discrete picture or two?

@Tom's BarnHi Tom, Thanks for the feedback. I spoke to one of the sales managers who couldn't be more helpful. I was planning on using zinc, but the cost of this product makes it much more attractive. The guy we are using said he thought it was an excellent product, but great to have a customers view. BTW I didn't get your personal note.

@Dunny1234I should say that are also considering using an aluminium roof PREFA / Preflaz and it seems to have a good reputation.

@Dunny1234Yes the condensation is a big concern and as @Russell griffiths says all that insulation on top of our rafters makes our roof angle more problematic. Our architect also preferred warm, but soon changed his mind ? I've read things about there being more noise on a cold roof, but not sure about that. I've also read that a vented roof keeps heat (we are south facing) away from the roof structure. Other considerations (as I understand) if you have rooflights, care must be taken to keep the ventilation open and if you are joining another building like us, then the abutment is slightly more complicated.