Garage conversion - insulation / soundproofing

Postby Dice_man_ » Sun Sep 04, 2011 4:08 pm

Hello, I am in the process of having my garage converted by a company specialising in this. I have some concerns that they are not doing the best possible job as I was led to believe and would really appreciate feedback from others on a few points below. My garage is half integral with a flat roof for the outside section that will not be replaced.

1. The floating floor has been installed with DPM on the exisiting concrete floor + 50mm polystyrene (EPS) just passed the intergal section border and the rest with 50mm Celotex. When I had discussed with the project manager I had asked for all the floor to be Celotex (and the quote only specifies Celotex) but they just havent. Will this make much difference and would the relevant buliding reg require the full floor to be celotex regardless of any of it being integral. Note they have used 75mm Celotex on the outer walls which is the only bit I am happy with (no insulation will be put on the integral secion walls).

2. They have purchased loft fibre insulation for the flat roof. However, it looks like there is only about 130mm space there so I would think they wont get enough insulation (from reading on the internet I thought 250mm was the minimum to meet the regulation). When I had discussed with the project manager I thought we agreed they would use Celotex for the roof as well if there was not enough room - would this be better way to go and I should I insist on this (note I dont want the ceiling any lower than it is)?

3. I also asked about sound proofing one integral wall which joins to the neighbours house as I understood this is a building regulation. The response was that i do not need it without explanation. Is it not necessary or should I push them on it?

Since they do so many garage conversion they have said that a building inspector will only look at the job at the very end so will only really be able to see things such as the window meeting requirements and nothing with insulation or sound proofing. My concern is therefore that they are cutting corners to save money where possible and also that they are not bothered about strictly meting regulations since the inspector will not be able to know exactly what they have done.

Thank you very much in advance for any comments on this.
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Simply Build It

Postby xavi99 » Sun Sep 04, 2011 9:04 pm

1. The floor needs to achieve a certain U value, I think it is 0.2. With that spec, 50 mm Celotex and 50 mm polystyrene it will not be far away but polystyrene is not as good as Celotex and I would guess is cheaper to buy. I would have thought that 75 mm Celotex on the walls is a bit light to achieve the required U value of 0.28. You could phone Celotex and discuss with their technical dept. They are usually very good and will do U value calculations if you give them the specifications, but very busy.

2. 130 mm glass fibre will not achieve the required thermal standard. There is also the issue of ventilation. If they are stuffing the space between the joists there should be a minimum 50 mm air gap above the insulation with cross ventilation at the soffits, a continuous 25mm ventilation strip. Often this is not done because it is difficult to achieve. An alternative would be to line the underside of the joists with 500 guage polythene dpm to prevent warm air passing through ceiling to cold space and condensing.

3. I did not think that sound insulation was a requirement, as Part E of the bg regs applies only to conversions where new dwellings are created or new build dwellings.

My guess is that this company is using an approved inspector for the building regs. They usually agree a general specification and then only do a inspection at the end of the job. They use an approved inspector because they can deal with the same company nationwide and get a consistent approach. They can also get away with what they want because no inspections are ever made during the works.

Can you contact the approved inspector direct to discuss your concerns?

It is a difficult one, good luck.

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Postby welsh brickie » Mon Sep 05, 2011 6:14 am

sounds like your getting short changed,I am unsure of the of the exact building regulations regarding insulation properties,But if it was specified celotex insulation for the conversion they are not complying with the contract.
The building control officer should be informed,If they have a good reputation they wont mind you calling him,also take photos of everything so if its covered by the time the inspector turns up he has a view of whats been installed
welsh brickie
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Postby Dice_man_ » Mon Sep 05, 2011 9:27 pm

Hello, thanks for the quick replies it helps. The project manager is now doing the work after I voiced some of my ocncerns so I feel a bit better as he is willing to listen and act (unlike the builder I had to deal with last week who was really difficult to talk to and just wanted to do whatever is easiest).

1. I complained that they had used polystyrene but he convinced me it was adequate and he will also make it up (at least the cost) from other extras he as offered to do on other things.
Celotex have an online U value calcutor. For 70mm Celotex (GA4000) on a single skin brick wall it gives a U value of 0.24 which is (perhaps surprisingly) within the 0.28 suggested. I did however ask if he could put some fibre insulation behind the celotex since there is a gap anyway and he said although not necessary he would do it on my request.

2. He said he will use Celotex for the flat roof as well and is aware of condensation issues (I will keep an eye on it). Please could you also let me know the required U value for a flat roof or where I can find this info?

3. One of the other builders who quoted me said I would need sound proofing and I also found this guidance note on my local council website:
Google ""
'Where the garage is semi-detached, sound insulation should be provided to the party wall by the provision of a sound absorbent quilt and two layers of plasterboard fixed to an independent stud wall with a minimum cavity of 13mm'
I'm not sure about this one now.

He also said he was happy for me to talk to the inspector and is confident they will have no problem wih what is being done (I will leave that for now). I must say I'm still surprised by some of their comments regarding building regs and it sounds a lot easier to go through an approved inspector (I feel like I am turing into the inspector).

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Postby xavi99 » Wed Sep 07, 2011 7:24 pm

Flat roof should achieve 0.18 I believe. You can find out for sure in Approved Document L1B, 2010. If you google it then it should come up. Sorry no time at the mo to trawl thru to check.

Sound insulation is probably required. I did do a quick search and most council's seem to be saying it is required.


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