Extension - insulation very expensive!? alternatives?

[pixiepoo]

Hi Theoldun, thanks for your reply

I really do appreciate your help and feedback on this, spent most of the weekend trying to work out alternatives for the uvalues and my brain is now fried.

The extension is two-storey which comes off the rear of the existing cottage with a pitched roof (with two dormer windows).

Our house is electric-only so think we would benefit from extra insulation but maybe just a few points below the maximum uvalue would do just fine.

External size is: 6.75 x 5.7m

Cavity walls: our spec said 120m cavity with 65mm celotex and the rest as air space. Problem with this is the brickie has already laid up to damp as he is away for the next few weeks, so we've got to stick with 120mm but not sure how to fill it!? We have got brick outer leaf and thermalite innerleaf with plasterboard (dot dab) internal. I can see from LABC that maximum is 0.28, our spec is suggesting 0.19.

Ground floor slab: I've worked out p/a to be 0.5.
LABC says max uvalue 0.22 and our spec says 0.20 to include the following:
150mm slab on 500 gauge VPL on 70mm Celotex FR5000 on 1200 gauge DPM on 25mm sand blinding on 200mm well compacted hardcore. The insulation depth is as you suggest (albeit the more expensive product) - does the hardcore etc sound about the right depths?


Roof: warm deck pitched roof. LABC says 0.18 and our spec says 0.13 at rafters and 0.14 at ceiling to include - Nilvent breather membrane, 150mm FR5000 celotex between rafters plus GD5040 celotex 40mm under rafters
Ceiling is FR5000 120mm between joists and FR5000 60mm across joists. This seems an awful lot to me!!

Thanks in advance for any help/advice!

 

[tony1851]

All over the top IMO, but have you checked the glazing area? If your glazing is considered excessive, you have to compensate by upping the U-values of walls and roof. This might be the reason the designer has gone overboard.

 

[pixiepoo]

Hi tony1851

The extension consists of one large room downstairs with three windows (1200x1200) to provide a uvalue of 1.6W/M2k each

Upstairs consists on spare room with dormer window (1200x1200)

Master bedroom with dormer window (1200x1200 ) and normal window (750x1000), ensuite with normal window (750x1000)

In total: 7 windows and one door

 

[freddiemercurystwin]

Nothing excessive there, maybe a green architect!

 

[SimonH2]

The way I see it regarding insulation levels is this :

BRs specify a minimum, which is (I guess) to force a long term improvement in building "efficiency". LABC only care that you meet (or at least appear to meet) those minimum levels.

Apart from BRs, insulation is a trade-off - better insulation will cost at construction time (materials, space taken, labour), but will reduce heat losses and so reduce long term heating bills. SO if you choose to put in much better insulation than the minimum required by BRs, then long term you'll spend less on heating and/or have a 'warmer' house. Where to draw the line on this is down to individual priorities and preferences.

Back to the OPs situation. Since the experts here seem to think what's been specced is well in excess of what's needed for BRs, then the OP should be able to cut his costs by using seconds - as long as he can get roughly the right products.

 

[theoldun]

Okay, take cav walls first. U value 0.28.
Thermalite turbo (lanbda) Kvalue 0.11. Therm Shield K value 0.18, so with Turbo use 100mm Dritherm 37 ,P/B D&D. With Shield use 100mm Drithern 34 Super. P/B D&D. Tell brick layers to set inner block wall over to 100mm wide cavity at DPC.
Oversite 0.22. We never lay insulation below concrete. Our spec for ground bearing slab is. 100mm mot, NOT HARDCORE, sand blinded and wacked, 1000g polythene turned up and over DPC.100mm cony. 75mm GA 4000 Celotex and 75mm Sand cement screed.
Roof 0.16. Assume close coupled flown off of existing roof, If so forget all about warm roof, as it will cause you a number of problems down the valleys, Go for Cold Deck Pitched Roof with breathable felt under tiles and 100mm of Rock wall with a K value of 0.044 between the joists and 170 to 200mm over the joists.
See how much that has saved.
Any problems, then come back.
Never buy seconds. False economy
Regards oldun

 

[pixiepoo]

That really is so helpful, cheers!

Will take that information tonight and do some sums.

Yes the roof will be off the existing roof so interesting about warm deck, we shall change to cold deck with breathable membrane.

Purely out of interest - why always insulation over the concrete? Does that make it warmer somehow? Being nosey now!

With regards to the roof, I will price that up for the rock wool. On our project that will work in the loft, but the upstairs rooms have sloping ceilings and dormer windows due to lack of height. Would i put rock wool as described in this area too, and just cover in plasterboard afterwards?

Many thanks

 

[theoldun]

We put insulation under screed to keep heat in the room and not going down to heat the concrete and form a heat sink
In the same way a warm pitched roof is a waste of space as you want the heat retained in the first floor rooms and not going up into the roof space, unless of course you have a fetish and you want to live up there.
You have a raised tie roof, and no you can not use fibre glass up the slope. We did not know it was raised tie, that is why we said your roof insulation was from another planet/
How ever it is no problem if you give us a bit more information.
Is the 6750 the external width of the roof or is it 5700?
What is the height of the external wall plates and the storey height of the top ceiling?
What is the pitch?
If we have that information we can work out your roof timber sizes and tell you how to insulate the sloping rafters.
All this information should be on the comic
Regards oldun

 

[pixiepoo]

Ah apologies... I should have realised that it would affect the spec.

It says on the spec that the rafters are to be 175x50mm at 400mm centres.

The width of the roof is 5700
Pitch is 49 degrees
Wall plate height it 3650 from DPC to soffit
2300 from 1st floor to ceiling, total of 4750 from ground floor to upstairs ceiling
It works out on the plans that there will be 1750 of the sloping roof visible upstairs across the length of the extension (6750)

Just going back to the ground floor, I was hoping not to screed it due to the awkward site we are on we have to get a concrete pump, with 2 call outs @ £360 a time plus concrete cost it gets quite expensive. But if there is a considerable difference in heat retention then i will swallow the cost. The finished floor will be wet-laid natural stone so we weren't so bothered about having a perfect screeded floor. However, Havent done this before so appreciate if it was a pie in the sky idea!!

 

[ban-all-sheds]

In the same way a warm pitched roof is a waste of space as you want the heat retained in the first floor rooms and not going up into the roof space, unless of course you have a fetish and you want to live up there.

Don't most people use dungeons for that, rather than lofts?

 

[theoldun]

Okay 5700 over all brickwork, less two external walls and cavities = overall plates = 5300 = half run of 2650 at 45 degree pitch = 3748 slope to rafter from top plumb cut to birds mouth heel cut plus overhang as required.
47x175 C16 timbers are of no use to you and you will need C24 and even these are on the bread line. I am only an old bricklayer, but in our life time we have cut and pitched 400 odd roofs and we have our doubts about this one for a number of reasons.
Any way we will work to your figures Forget 270mm thick fibreglass, and if you use C24 47x175 at 400 centres then clunk a 47x50 to the bottom of all exposed slopes. Use good quality permeable roofing felt and fit 165mm Celotex GA4000 between rafters and joists. Fit 100mm aluminium tape over all rafters and celotex to seal joints, Use 12.5 foil backed plaster board and skim coat.

Oversite concrete. Okay in your case now lay Celotex under concrete on top of DPM.. Any decent ground worker should be able to lay a slab level enough to wet bed coursed natural stone on. If you have barrow access round to back , we would batch up on site as you only have 3.5 cube of concrete. Any probs, then shout.
Regards oldun

 

[theoldun]

Don't most people use dungeons for that, rather than lofts?

Knew and old matelot who slung a mick of his rafters in roof space and slept up there.

 

[pixiepoo]

Cheers again for all your help Oldun, I will get pricing up for this.

Just so I am clear, are you saying: roof tiles - battens - rafters - permeable roof felt - 165m celtox inbetween rafters - foil - 12.5 foil backed plasterboard?

I am assuming I only need to plasterboard the sloping roofs and the ceiling, and leave the loft exposed?

The hubby and I are doing the build ourselves, no contractor in place so we could get a few mates round to help us with the barrowing of concrete but we have to get it up a 45 degree slope to reach the house, we'll see how fit we feel nearer the time!

 

[cjard]

The hubby and I are doing the build ourselves, no contractor in place so we could get a few mates round to help us with the barrowing of concrete but we have to get it up a 45 degree slope to reach the house, we'll see how fit we feel nearer the time!

By the time you've finished buying as many rounds as is necessary to compensate them for their efforts, you might have been better off getting a concrete pump


Incidentally, the "insulate under or over the slab" argument largely depends on how the house will be used. If it's occupied less often, and heating comes on an off at certain times then insulation over slab avoids heatsink problem that oldun describes. If however, there's going to be someone in the house all the time and a degree of temperature stability is desirable, then using the floor as a store of heat(thermal mass) can work out better

 

[pixiepoo]

Incidentally, the "insulate under or over the slab" argument largely depends on how the house will be used. If it's occupied less often, and heating comes on an off at certain times then insulation over slab avoids heatsink problem that oldun describes. If however, there's going to be someone in the house all the time and a degree of temperature stability is desirable, then using the floor as a store of heat(thermal mass) can work out better

It will occupied less often... monday to friday after 6pm and sat/sun all day. We are an all-electric household and dont have any form of central heating. We will be relying on our two stoves to provide heat over the next couple of winters before we buy some oil-filled radiators (yes we know they will be a fortune to run but we don't have the cash flow to install a brand new oil tank and system)

We did look into to electric underfloor heating but we've been put off by the fact that it doesnt heat the room well.

By what you've put cjard it seems we may be better off with insulation over concrete - will have to make some decisions