Cavity Block build for Extension?

mccracko

Hi guys, Dublin based and I've PP for a double storey extension. My builder is suggesting using standard 9inch cavity blocks with internal vapour barrier and then a 112mm insulated plasterboard. Reading up this method seems to be frowned upon alot and a more traditional cavity wall should still be used?

I have had a few other quotes from builders and some others have also quoted with 9inch cavity blocks ( not cavity wall).

Anyone have thoughts on the above and is there still alot of risk of damp and airtightness using cavity blocks?

Thanks

Gusser09

https://www.boards.ie/discussion/2058336409/cavity-block-build-for-extension

9inch hollow blocks?

We went for that with 92mm insulated board. Engineer was happy with it.

No issues in 5 years.

Calahonda52

I suppose it depends what you want, if the render is good quality ie 3 coats, scud, base and final and you are not too exposed then..

My guess is the render id only going to be scud and final

lmk123

Don’t do it, cavity blocks are rubbish, you wouldn’t even get a great fixing to them for the slabs in comparison to a solid block, I’d suggest a using the traditional menthol with a 150mm cavity with 110mm insulation in the cavity, no need for any internal slabs then, it’s by far a superior job.

muffler

https://www.boards.ie/discussion/2058336409/cavity-block-build-for-extension

What is specified on the plans? Are you listening to the builder rather than the designer?

monseiur

Forget cavity block, they're bad news. Go for a standard cavity wall, use Quinn Lite Thermal Block or similar in the inner leaf of the cavity wall. Pump cavity with bead insulation or fit insulation board - no need for insulated plasterboard.

mccracko

Hi guys. Thanks for the feedback..I'll have to go back and assess this further so. But sounds like as thought cavity block feels like scrimping and go with traditional cavity wall. Yes since its a two storey the new gable end would be north facing and fully exposed..

10-10-20

https://www.boards.ie/discussion/comment/121774678#Comment_121774678

There are a few builders at that game here in Dublin. I saw plans drawn in May last year up for a kitchen extension with double-leaf cavity and associated partial-fill insulation, except the builder told the owner that he could do it cheaper and went and built it from cavity blocks with insulated plasterboard and no vapour barrier. Householder just paid for very rudimentary plans off the architect and didn't have an engineer either. What's more, while a vent was fitted there isn't an extractor fan, just a carbon filter jobbie. Householder wouldn't listen, (low) cost was king. 😣

They'll be on here telling us about the mould in the years to come.

muffler

https://www.boards.ie/discussion/comment/121777750#Comment_121777750

A good post.

Unfortunately there are too many people taking the word of the builder over the instructions or advice given by the engineer or architect. And before I get some builder coming on here accusing me of this, that and the other I will say that they are are not wrong all the time. But generally it is better to follow a detailed plan and/or specification and where necessary talk to the professional involved.

Dr.Tom

Insulated plasterboard is one of the worst things to come to market here in Ireland. So called builders love it as it's quick and easy to install incorrectly.

I've lost count of the amount of people that have come to me during my renovation to say "you should have installed insulated plasterboard".

standardg60

https://www.boards.ie/discussion/comment/121774262#Comment_121774262

My detached house is built with same and no issues in 20 years.

At the end of the day if done right it should be fine. I've seen atrocious mould in cavity wall built houses where the cavity was closed at roof level, so the main thing is to have a competent builder who knows what they're doing with either method.

whizbang

You should balance up the thermal heat retention of the insulation filled cavity approach Vs the cavity block and internal insulation.

If your existing structure is fully lined with internal insulation, then you are probably more used to the fast heat and fast cool cycles of internal insulation. The insulated cavity approach could then be a usefull thermal store for your whole house. But it may not suit you.

On the other hand if you already have a very solid block structure that retains heat, then this will reduce the need for thermal store in the extension. So the cheaper (but more easily managed ) cavity block approach would work well here.

Dr.Tom

https://www.boards.ie/discussion/comment/121785226#Comment_121785226

Would that mould issue be related to poor ventilation as opposed to poor building practices?

40 year old standard 4 inch cavity build here with a cavity closure at wallplate level and zero mould.

Ginger83

Run that chancer

standardg60

https://www.boards.ie/discussion/comment/121858849#Comment_121858849

There were no vents installed initially which i thought the installation of same would solve the issue but no joy.

The expert view was the pumping of the cavity would help and it has but there was still a clear line between the cavity wall and the closure blocks in terms of where the mould occurred.

Mentioned above but it would seem that regardless of construction method that the waterproofing aspect of the plastering is key.