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    I have a little project for the summer that I could use some advice on. My Mum has a conservatory on the back of her bungalow. It is south facing so like oven in summer and freezing in winter, bar the odd sunny day. She wants it replacing with a proper sun room extension that can be used all year round.

    The plan is to take the conservatory down to floor slab level and re-use the existing slab, extending it to one side so that the new extension runs across the whole of the rear gable end of the bungalow (stepped in about 500mm at each side). The overall width will be 5.4m and it will extend 2.4m from the house, so fairly small and cost is an issue as the client is now a pensioner!

    I would like to build it out of a single skin of concrete blocks, externally insulated and finished with a timber rain screen to give some thermal mass to the room and take advantage of passive solar gains. A number of things are currently vexing me:-

    1 Will the building inspector accept the reuse of the conservatory slab? How do I handle this i.e. should I just ask or dig a hole next to the footings before he comes on site?
    2 Am I more likely to be able to re-use the slab if I propose a light weight construction, i.e. timber frame, rather than blockwork walls?
    3 The extension will be next to the two bedrooms of the house which have no windows except those that will be covered by the extension (the exisiting conservatory is currently accessed through the main bedroom). The plan is to put an internal floor to ceiling window between the extension and the main bedroom with an opening casement for ventilation and a combination frame with door and opening casement from the spare bedroom through which the sun room will be accessed. Will this infringe the building regulations as the ventilation for the bedroom with be through the sun room?
    4 Existing rain water off the conservatory roof flows into a water butt with over-flow into a flower bed. We would like to retain a similar arrangement linking the overflow of the water butt to a perforated irrigation pipe wich I will lay in the flower bed under the mulch. Will building control accept this as a reasonable solution or will I be digging a soak away?
    5 The roof will essentially be a lean-to type affair to keep the overall high under 4m to avoid the need for planning permission, but there will have to be a hip at each side because of the level of the roof line of the bungalow. I had planned to use timber shingles to finish the roof. Can these be fitted around a hip and remain water-tight?
    6 Can I build a single storey extension in 100mm block work or will it have to be 140mm?
    7 There will be no insulation in the existing slab so little point in putting any in the extra bit to the side. I don't want to isolate the thermal mass in the slab so can't insulate over the top of it. Can I get over this and comply with the building regs by beefing up the roof and wall insulation?
    8 Wall space for a radiator in the extension will be a premium (bungalow is gas centrally heated with new condensing combi). I had thought about a small amount of electric undertile heating (floor will be tiled) on a separate thermostat to the rest of the house so the heating would be more responsive to passive solar gains. Would this be a mistake given that there will be no insulation in the slab?

    Any advice would be appreciated.

    Thanks, Chris.
    • CommentAuthortony
    • CommentTimeApr 30th 2007 edited
    1) Very likely no. This means a complete new proper extension with all new regs etc new costings will likely preclude this. Could it be done without?
    • CommentTimeApr 30th 2007
    My understanding is that if less than 50% of the wall and 75% of the roof is glazing you will need building regulations approval for the work.

    If thats the case I suspect you will have to comply with the current regulations.

    1.I suspect you will have to excavate to prove the footings are adequate.
    2. Yes but BI will probably want an engineer to confirm that what is proposed is acceptable.
    3. Not sure but it sounds dodgy.
    4. I doubt it unless you can demonstrate that it has the capacity to deal with a storm even when the tank is full.
    7. Dont think so - each element must now achieve a minimum standard. 0.25W/K/m2
    8. Yes a big mistake - this is the green building forum :bigsmile:
    It is the high amount of glazing in the roof that is causing the summer overheating problems and the cold in winter, so I can't see any point in re-building to slip under with those percentages.

    Guess I need to dig a hole and see what these footings are like. I really don't want to have to scrap the existing slab because of lack of insulation, particularly as it seems to me far from conclusive that it does much good. I thought you could do some sort of overall SAP calculation for the building?

    As for the water butt drainage, maybe I could put a higher overflow into a second water butt and prove that the capacity would be sufficient for a storm event?

    Heating wise, I will have to try and fit a radiator in somewhere. I guess if it has a TRV it will switch off if passive solar is providing enough heat?
    • CommentAuthorken davis
    • CommentTimeMay 7th 2007
    knock it down and do it properly.
    • CommentTimeMay 7th 2007
    If you are extending your existing heating system I think you will need planning permission (or is it just Building regs - I can't remember now)
    • CommentAuthorcaliwag
    • CommentTimeMay 8th 2007
    Might be worth posting on www.ebuild.co.uk
    there are some helpful building inspectors pass through that forum and, as I recall, people that have carried just such works.

    Best of luck Caliwag
    • CommentAuthorGuest
    • CommentTimeMay 8th 2007

    A conservatory is usally classed as temporary, so if to the rear and not excessive in size wouldn't have required PP. A sun lounge would be considered an extension so would probably require PP and Building Regs approval. The criteria can be on all sorts of aspects. For example, consevatoroes generally have a an exterior lockable door between them and the house and dont interconnect with heating systems. Bets to bite the bullet and do it with approval to full house standard or better.
    Already established that the extension doesn't need planning permission as it meets all the criteria set out in the permitted development rights and I have checked that these have not been revoked. Building Notice is in so it will be done with full building regs approval. If anyone can comment further on the points raised in my first post I would be grateful (will check out the ebuild site, thanks caliwag).

    My first concern is existing slab and footings which I do not what to have to dig up, cart away and then replace. Is is possible to build an extension with no insulation the floor by compensating with extra insulation elsewhere in the extension or in the property? I thought it was but Nigel's comment above seems to suggest otherwise i.e. there is a minimum for each element.
    • CommentTimeMay 8th 2007

    There are some exemptions for substantially glazed extensions you need to refer to paragraphs 22-24 of part L1b of the building regulation which you can download.

    Apart from that all new buildings and extensions have to meet the minimum standards for each element there is no set off.

    I've just been looking at those paragraphs Nigel. It seems to keep refering back to compliance with paragraph 22 which requires that no element is worse than column b) of Table 4 which states minimum U-value of 0.25 for the floor.

    Given that I am extending the slab to one side, adding about a third to the area, perhaps I could super insulate that section meaning the average for the floor as a whole exceeded the minimum required value for the element?

    Failing that, I'm stumpted. It seems perverse to me that I'm probably going to have to spent time, money and energy digging up what would be a perfectly adequate base for the extension and replace it with the same plus a layer of insulation underneath. Sure the energy pay back on that would be really impressive...
    That is an option, and probably what we'll have to do, but then we loose all the thermal mass in the slab which, given that it is a due south-facing sun-room with significant glazing, is not ideal. Will also provide a nice little tripping hazard for my mother as she enters the room from the rest of the house (perhaps I'll mention that to the building inspector).
    Good luck - to me, it sounds as if you are trying to do things right but on a budget. You can trade off some heat loss/gains. I have a south facing Sun lounge on my new build. This room is open plan to the kitchen/diner and works fine as hot temperatures are easier to control in bigger spaces and it doesn't get too cold either.
    • CommentAuthorGuest
    • CommentTimeMay 8th 2007
    Surely you want to do a good job for the lady?
    If asked, she will no doubt confirm that using the existing conservatory is impossible when the sun shines, because of the heat and when it rains because of the noise. In all probability it is only comfortable for a short period in the mornings and evenings during the summer and too cold during the winter. To extend the existing set up will be unkind, especially if you install a radiator or underfloor heating without providing insulation. After all she will have to pay for all that wasted heat. Another point, the imminent Council Tax review will be facility based, they will take into consideration all improvements to the property meaning extra Council Tax. I would recommend a solid roof to attenuate the rain noise and to provide shelter from the sun (as well as easier access to the upper part of the property) or as much of one as is permitted under the present regulations.
    Yes, you have hit the nail on the head there Guest i.e. the conservatory is boiling hot in summer and freezing in winter. That's why it is being pulled down and replaced with a proper extension (see first post on the thread).
    • CommentAuthorken davis
    • CommentTimeMay 8th 2007
    see my comment above. most conservatories will have been built on a ground slab and minimal or no foundations. to comply with build regs you will have to convince building control that the slab is sufficient to take the extra weight and not to move relative to the house. dig a couple of holes and see what you've got. of course the slab could be reinforced (unlikely) in which case you will need a diamond core drill to cut out a sample.
    • CommentAuthorGuest
    • CommentTimeMay 11th 2007
    Two areas of comments:
    To create this room at minimal cost and building inconvenience you could tell the Council's building regs dept that you are building an entrance porch. As it is below 30sq m floor area it will be totally outside their control. However you MUST NOT PUT IN ANY HEASTING SOURCE as that will bring it back into B/R control.
    If you want to heat it another way of complying is to carry out calculations showing that the area weighted heat losses (total of U-values of elements x areas of elements) from the existing house + the proposed extension will be LESS than those which would have come from the existing house + the same extension built to the minimum specified standards. This means the extension's elements can be below the insulation standard usually required PROVIDING you improve the insulation of the existing house to compensate. (I recently obtained approval in this way for a 2-storey extension with a glass roof way off the 0.2 U-value usually required by insulating the existing attic). There are special rules for calculating the openings areas of the "official" extention not being built which may help you (25% of floor area + area of any windows in existing house being enclosed if I remember correctly), but all this is in the Building Regs Part L1B.
    Good luck.
    Thats for that Guest. That is what I thought the position was - can anyone confirm that this is still possible under the current regs? I was thinking we could put more insulation in the walls and roof of the extension and also go to 300mm in the loft of exisiting house (currently 100mm I think) and the overall package would be far better insulated than just scraping through with the minimum, even without any insulation in the slab of the extension.

    If we built the structure of the extension in timber frame with cedar cladding and a cedar shingle roof covering, I doubt it would be any heavier than the current uPVC conservatory with dwarf wall so I should have a fair chance of convincing the building inspector that the footings are adequate given that the current structure hasn't suffered any movement.

    I think that's the line I'm going to take anyway.
    • CommentAuthorGuest
    • CommentTimeMay 11th 2007
    Hi Chris.
    Glad you found my comments useful. Yes these are current regs - see Paragraph 18 of L1B (downloadable/printable for free) for details. You need to do the calculations to prove it works before getting too far ahead. I have created a simple Excel spreadheet which makes the calcs much easier and allows you to experiment with different U-values to try and achieve the right answer! I can E-mail the speadsheet to you if you can let me have an address. Mine is charlieduke@tiscali.co.uk
    Had a result today with the building inspector. He is happy with the footings (even though only 12" deep) and has no problem with the lack of insulation in the existing slab as long as I can prove the SAP rating of the extended house is better than the current house with the extension done to current building regs. As the loft only has 100mm of insulation in it at the moment that shouldn't be too tricky...

    He has also agreed to let me build the extension out of a externally insulated 100mm thick concrete block wall finished with a timber rain screen. A couple of points that I would like some info on:-

    1 Existing house is a mid to late 1980's bungalow with a concrete beam and block floor. Will the floor have any insulation in it?
    2 Is is worth insulating the extension any better than the current house or should I just try and scrap the SAP rating condition? The wall I will be covering with the extension is sheltered and south facing.
    3 For the external wall insulation, I'm thinking of 50mm EPS fitted between timber batons with counter batons and a further 50mm between them. Any comments? Can I fix the timber rain screen directing over the insulation or do I need a air gap?
    4 The roof will need to be mono-pitched and hipped at either side to keep it below the maximum 4m allowed under the permitted development rights. How can I find out what size timbers to use and what arrangement of insulation is preferable? I'd prefer to stick to EPS rather than Kingspan on cost grounds.


    • CommentAuthorMrT
    • CommentTimeMay 23rd 2007
    Interestingly Chris I'm planning on doing something similar with my badly constructed single leaf brick extension.
    I've yet to goto building regs but have been thinking along similar lines to yourself. I agree with reusing the slab
    and this what I intend do. Its seems to me that you are tying to make good with minimal disruption. To put in an a new slab would be a waste of concrete.

    I plan on using thermalite blocks with and EPS with sheets . Orignally I wanted to use diffutherm boards but the detailing looked difficult and the extra weight may put the current foundations under to much strain.

    There was a good feature in the winter 2003/4 edition of Building For a Future. In this example 2 layers of 100mm Polystyrene sheets were fixed using screws, plugs and penny washers. They were staggered to avoid cold bridging. You could use fischer insulation fixings to avoid conduction through the metal fixings. In this case lime render was applied over a metal lath. You could add timber battons and add the cedar boards. If you use thermalite turbo blocks the 2 layers of 50mm insulation would bring you below the current requirements.

    I'm a bit behind you but plan on doing something similar so you are not alone. As for insulating the floor, cork tiles will help a little.
    One point I would like to add.

    1 metre of dry ground (underslab) should give you 0.25 u-value!

    If you can insulate the edge vetically or horizontally (external wing type insulation) for 1 metre then you should comply?

    By insulating well enough for your local climate and keep extention temperature above 16-18 degC then all that ground underslab will warm up over time and create a nice comfortable sun room (year round).

    Passive solar sun space.
    I was thinking of running the external insulation down to the top of the footing. Are you saying, Jeff, that I should run it out a metre horizontally also? This would involve some digging and my back ain't too clever...
    My back is shot too! so I don't like recommending digging.

    The horizontal insulation can go close to the surface under paving etc or like http://www.thenaturalhome.com/earthtube.htm:
    I've found a table (nos. 23 & 24) in the 1992 version of part A of the building regs that refers to roofs constructed with "Purlins supporting sheeting or decking for roofs having a pitch more than 10 but not more than 35 degrees". Can't find same table in the 2004 version. Is this still a valid method of constructing a roof?

    It would suit this extension as I could leave the purlins long and create an overhang on the south facing side of the building to give some shade. I presume it would also allow the insulation to go between and over (or under) the purlins and hence leave the roof "open" up to the ridge i.e. with no ceiling ties.

    Any advice before I speak to the building inspector would be appreciated.


    Timber size tables have been dropped from Building Regs, but are published by TRADA (Timber Research and Development Association) for a range of angles and loadings. I see your depth is only 2.4m so purlins can probably be ommitted, but I don't have the tables at home; I'll try and look at them tommorrow and post in evening.
    Hi Chris. The TRADA tables Start at 15 deg pitch so I've used the flat roof table, which gives the requirement for a span of 2.5m of 122 x 47mm of C16 (normal) grade timber at centres up to 600mm. Thats without purlins. At that thickness I would put 125mm Celotex/Kingspan between rafters and 30 on top to produce a warm roof with a U-value of about 0.2.
    Good luck.
    Charlie, thanks for that. However, I've had a change of plan which I perhaps haven't properly explained. The extension will now replicate the shape of the existing gable end of the house so it will have a pitched roof with a ridge in the middle. Clear span from side to side will be 5.2m and 2.2m clear span from the gable end to the gable end of the extension. Roof slope will be about 30 degrees. I was hoping to leave a good overhang over the front of the extension as it is south facing and will have a good deal of glazing, so I thought a decked roof with no rafters just purlins built into the existing gable end, angled at 90 degrees to the roof line and sitting on the gable end of the extension over hanging a metre, say.

    This seems to have be allowed in the 1992 building regs but my building inspector was probably about 10 when they came out and he doesn't seem to know much about it... I was thinking about using 6x2s at 600 centres with OSB decking, insulated with 150mm EPS between the purlins and 50mm EPS over the top of the OSB, plasterboard on the underside.

    How does that sound?
    Yes, the 6X2's sound fine. 5x2's would probably be OK, but as the ends of the purlins will overhang/cantilever off the gable the larger would be sensible. Make sure everything is strapped down to structure and structure to masonry/floor slab to counteract uplift from the overhang.
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