New outbuilding, cavity wall or single block and celotex, what would you do?

[steveinoz01]

Hi All

I am having a new outbuilding built next to the house to use as a utility room with central heating off of the house. It is about 2.5m x 3m so below the size requiring building regs.

The builder is suggesting that instead of building a cavity wall with blocks and quilted insulation between it would give us a bigger internal space if we have single block walls, battened and insulated with 100mm celotex then OSB board, plaster then skim. Apparently this meets building regs even though regs are not required.

Anyone see any concerns with this approach or does it make sense?

I appreciate any input.

Cheers.

 

[John D v2.0]

if it's got central heating off the house it would need building regs.
The internal insulation would certainly save space and 100mm of celotex is plenty thickness, but also external insulation is another option although would need greater thickness. With internal it makes it harder to hang shelves etc also there would be less thermal mass so it would heat up quicly with the sun and cool down quickly in the evening.
Also you can't plaster directly on OSB as it would move with the seasons and crack off, you would need a layer of plasterboard first.

 

[^woody^]

I'd say that the builder's proposal is a recipe for damp and mould.

How will the walls be adequately braced?

Does this structure really not require building regulations?

 

[steveinoz01]

Thanks very much for the reply. I should have said 'plasterboard then skim' is the builders plan.

If I heat the room using electric heating independent of the house in some form does that remove the need for building regs?

We do plan to put in kitchen units but most will be tall full height floor standing units so just anchoring to the wall at the top rather than supporting the weight of wall units. The other wall will be the sink and washing machine with base units below the window.

 

[Millwright]

All buildings need to meet building regulations, regardless of the size.

 

[steveinoz01]

All buildings need to meet building regulations, regardless of the size.

I totally agree. This is why I want to be sure what my builder is proposing is acceptable under building regs as I was expecting cavity walls with insulation.

I want the building to be safe and usable as a utility room. This is not being done on the cheap so if cavity walls are the best way to go that is what I will tell the builder to do.

 

[Millwright]

If it's going to be a part of the actual house, a regular door from the existing house to enter into it and with heating, it will require a cavity wall. Also think of the headache which will be dragged up by a conveyancing solicitor come the time to sell the house. Spend the money now, save the aggro later and get a builder who'll not entertain trying to cut corners and standards.

 

[steveinoz01]

Does this structure really not require building regulations?

I thought we were fine not doing building regs as this is what the architect said (we are having a kitchen extension done on the main house which needed plans) but you have made me think twice. I have just asked the builder to get the inspector out this week to do a building notice inspection as he has only just started on the footings. I would rather pay a few hundred pounds and do it right.

Thanks for your input Woody.

 

[^woody^]

Yes it's a bit vague from your description. If this is truly an independent outbuilding, building regulations wont apply. But your mention of connecting it to the house heating system seems to imply some sort of connection.

What the builder proposes is just like a garage conversion, so is possible, but it's all in the detailing for waterproofing, thermal bridging and structural supports of the walls, and you need to ensure that the builder really knows how to construct this properly

 

[steveinoz01]

Thanks.

What we are doing is knocking the old kitchen and utility within the house into one large kitchen and extending that as a separate project. We are then building a new utility room/building one metre away from the house opposite the back door. The outbuilding will have a central heating radiator and hot and cold water connection from the house which is possibly what makes it a fall within building regs, I am not sure. Safe option is to get the certificate just in case.

The builder has described it as 'like a garage conversion' too. I have used this builder on other projects and he seems to be competent. I will discuss the points you mention here with him and come to a decision.

Thanks again.

 

[Deleted member 292770]

pay a few hundred pounds

You’re talking a difference of thousands not hundreds.
A single skin structure with insulated timber internal frame will be as good as anything.

 

[platforminc]

Not sure if this helps, but what I have done in the past is to use a single skin concrete block (6 inches) and then insulated plasterboard on the inside making sure its tight with no gaps etc and then skim it.

Separate 25mm power connection from the main consumer unit in the house, electric heating and its on separate consumer unit, what you are doing sounds small size wise, so a garage consumer unit should do depending on the kind of appliances you intend to use. I should also say that in my case, the building is a good 10m from the house, so fully detached from the house.

 

[steveinoz01]

Thanks for everyone's input. The building inspector came out today to start the process of building regs sign off so we will end up with a fully compliant solution which is the main concern I had.