Extension - Cavity Blocks or Cavity Wall?

FrankGrimes

I'm about to start on an extension (found the Foundations thread very useful btw) and am wondering would it be possible to go with Cavity blocks as opposed to a cavity wall?

Its being built on the side of our end-of-terrace so the wider the wall the wider the foundations, the further in from the boundary wall we have to go - thus losing us space in the width of the extension.

If I go with a cavity wall is it usual to use two walls of 3inch solid blocks or is 4inch the norm?

Would cavity blocks meet building regs and just how much better in terms of insulation would the cavity wall be?

Neil_Sedaka

I don't think cavity blocks are the best option, we once had a relation build an extension and he used cavity blocks.
We had serious problems later with damp walls (cavity blocks were full of water by the time the build was finished)

Was later told by a "proper" builder that cavity blocks should never be used to build anything other than garden walls etc.

I don't have much experience with this and so am open to correction btw.

O-Ninio

Without a doubt cavity walls, use about 60mm polyurethane insulation i.e kingspan or xtratherm are both easilyy available. If you use cavity blocks you will have to dryline the walls and if you took this option your compromise would be between heat retention and space i.e the thicker the insulation you use and if you use insulation backed plasterboard you will lose more space around the room, the less insulation you use to do this the more space you will have but it will not be as warm.
4 inch or 100mm blocks are what you will need for this with a fdn 900mm wide and an equal spread of fdn either side of the wall. Do not forget to fill the cavity to below dpc level with a lean mix to prevent the forces of the earth on either side compressing the cavity together.
While it may prove more expensive initially it will be more beneficial in the long run to go with the cavity wall. With cavity blocks it will be hard to meet Part L of the building regs which rerlate to insulation and also on the 4th of January last the EU energy performance directive for buildings came in and this means over the next three years every house is to be energy rated by these assessors who will be going around each house. Not too sure what penaltys or action these will be able to take yet, anyone know?

narommy

I agree that twin leaf walls is the best but it is farily horrific to think most houses in the east coast are now built ing this way.

GreeBo

narommy wrote:

I agree that twin leaf walls is the best but it is farily horrific to think most houses in the east coast are now built ing this way.

Most dublin houses between about 75 and 95 are cavity block.
Im very interested to see what comes out of these assesors reports.

Regarding insulation backed plasterboard, would you still use laths with this or fix straight to the blocks?

stapeler

I've just finished an extension and used 9" cavities. On the interior I put in 2" kingspan, DPC and drylined. Upstairs I did the same but put in 3*2 battons and filled in with fiberglass for extra insulation. Very happy with the result so far..

narommy

stapeler wrote:

I've just finished an extension and used 9" cavities. On the interior I put in 2" kingspan, DPC and drylined. Upstairs I did the same but put in 3*2 battons and filled in with fiberglass for extra insulation. Very happy with the result so far..

THe above sounds better than the average standard house.

I suppose it depends on the quality of the work and materials used. Looking at the thread about cracks and painting demonstrates the quality of workmanship in many developments. How do you know if the DPC has been properly installed if you aren't there to supervise it. How do you know that the vapour barrier is correctly installed or even installed in the first place.

But using a twin leaf with an insulation board isn't a guarantee either. You could get a rough builder and he may leave gaps in the boards.

I will twin leaf use a blown product, probably beads. They have a new type of grey bead now available i think

Using kingspan or another rigid board is going to be better than wool which will sink a bit and leave a gap at the top of the wall for heat to escape.

rooferPete

Hi,

The question mark has hung over 9" cavity blocks for years, like most products if not used correctly they will fail, the very same can said for twin leaf, actually twin leaf can fail because of cavity ties and poor insulation, even Timber Frame in the wrong hands has it's problems.

Most traditional extensions have exposed walls and are liable to be cold, that is why I would recommend twin leaf but only when carefully supervised, no mortar left to drop in the cavity, top quality insulation and make sure enough ties are used, cavity closures and correct flashing details are essential.

A major item with 9" cavity blocks has always been the quality of the external plaster, ventilation because the cavity is bridged and of course the inside finish that being the most popular way to insulate.

Regardless of the work it is important to make sure the joints are properly rubbed up, the "It'll Do" attitude by some block layers can be a contributing factor to faults in all block or brick construction.

.

Reyman

GreeBo wrote:

Most dublin houses between about 75 and 95 are cavity block.

My house Was built 1960 with cavity block. Closest thing to an ice box you could get with no effort. Appaling building material - how was it ever allowed in the country?


Are they really going to send out inspectors to check out insulation of all houses? Sounds unlikely to me. Anyway my house will get last place in the country!

PoolDude

Now I'm worried (should I be?). My garage is being built using cavity and I will use the upstairs as an office when working from home.

The walls with 9” cavity blocks, then dryline upstairs i.e. place 1” insulation against the walls and then a plasterboard slab. This would then be hardwalled.

Reyman

I don't have any insulation inside the block. That will probably help a lot if you have some. Though I doubt if 1" is enough? i'd have thought 2" would be ideal

Some of the other builder guys here may know more about this than me !

fingal

According to the SEI site they will need 2,000 full and part-time assessors to run the energy rating system but there will be no direct financial penalties for poor performance. However the indirect consequences of poor energy performance may be reflected in the price you get for your house. Should the property market ease up then you dont want to be held up to ransom!!

Again the energy rating issue only applies to buying/selling a house and not to the rest of us who are staying put.

Reyman

Interesting and a good idea I suppose ! But if you're stuck in an older house with cavity blocks walls it's very hard to know how to improve the insulation.

You can add insulated board to the walls but then there's a problem with the windows, sockets etc and it's very awkward. I've 8" of insulation in the attic that's the best I could and it made a fair difference

coxy123

Interesting topic about blockwork etc. for extension. I am about to build a garage myself and would have thought cavity blocks would have been sufficient. However recently talking to a builder he said to steer clear of them cos. they would cause dampness.

Mr.Wonder

I was talking to my builder friend about these blocks not so long ago and he advised me that they should be avoided like the plague. According to him, its actually illegal to build houses using these blocks in the England. He tapped one block gently with a hammer to demonstrate why.It fell in bits with a gentle tap. You have been warned!!!

kippy

Jaysus,
I didnt think there were any houses built with cavity blocks!!
Unless they are drylined the dampness would be terrible I would say.
We built a shed with them but there will be noone living in it!
As mentioned above the Cavilty walls are a far better option if built correctly(as pointed out above)
Probably cheaper in the long run as well I would think.
Kippy

Reyman

I think we're the only country in Europe that uses Cavity blocks.

A long term deal between the quick buck builders and their Fianna Fail friends I suspect!

O-Ninio

The EU performance on the energy directive for buildings is not going to happen here realistically I don't think there are even any assessors appointed yet? However a machine for measuring u values in existing buildings is apparently being currently developed but even if this does evolve what can they do if they measure your floor and the result is a u value that does not meet regs, the place was proably built before regs what can they do?
The rise in fuel prices does however mean it is in every billpayers interest to invest their money on insulation rather than giving it to Bord Gais etc.
The fact is cavity blocks are still used frequently and cavity blocks are not the only shortcut taken, was there not war a while ago in some new development near the Phoenix Park because people in the apartments could hear vtheir neighbours talking. What is submitted on drawings for permission and what is actually built on site can vary wildly. A developerwill ensure all regs are met at planning permission stage but face it they are mad for profit and hae no prob building crap if it means more money for them.
What follow up checks are carried out? I know from working in the States before after we finished a job an inspector came out and measured the rise and going of steps height of handrail etc. to ensure compliance and if it was not o.k the people would not be moving back into the house.