Extension wall

9"solids

Hi folks,

do extension external walls have to be standard 4" solid outer and inner leaf with a cavity or can the ext be built with 9" cavitys.

Are there many +/- to using 9" cavity with insulated plasterboard (dry lining) on the interior. Cost/time in buiding
The outside of the 9" cavitys will be rendered.

thx

gregos

9"solids wrote:

Are there many +/- to using 9" cavity with insulated plasterboard (dry lining) on the interior.

Yes. It's a shoddy way to build a wall. Builders get away with this in Dublin for some reason, but it has nothing to recommend it, and nowhere else in the country would accept it.

kadman

gregos wrote:

Yes. It's a shoddy way to build a wall. Builders get away with this in Dublin for some reason, but it has nothing to recommend it, and nowhere else in the country would accept it.

Ditto Gregos,

Builders that use this method of construction, are not builders. Chancers would be a better description.

Whether this form of construction is allowable or not, it is extremely poor. Any reputable professional would not use this method, as it invites all manner of serious problems in the future. :eek: :eek:

Avoid like the plague.

kadman

rooferPete

Hi,

Unlike my esteemed colleagues I don't have a problem with 9" cavity blocks, there are rules to be followed that make the difference between a good and a bad wall.

First make sure the outside is finished properly and a good scud coat is applied, the plaster must be two 10 mm coats with a proper working plinth.

The above applies to the twin leaf walls also but because of the seperate walls it is not unusual to find two coats of plaster that resemple the thickness of a thumbnail, of course the builder / plasterer can get away with this because the outer leaf can leak a little and drain away without interfering with the insulation.

The dry lining is critical with the cavity block, no blobbing on boards without insulation or you will have condensation spots on the wall.

You can fix treated laths to the wall and fit your insulation on top of the laths and fix the plasterboard through the insulation then skim the slabs.

There are thousands of extensions and houses built with the 9" cavity block the majority of which are performing to the owners satisfaction.

The ones that are giving trouble were usually thrown up without regard to the old rules of external plastering and many are built with an internal plaster finish that does not allow for condensation.

Done properly I have no problem the 9" cavity but then again anything done properly meets with my approval

.

gregos

Hi Pete. As you know, normally I wouldn't dream of disagreeing with you, and this time is not an exception. All I wanted to know was, given a choice, which method would you go for?

rooferPete

Hi gregos,

Please feel free to disagree anytime you see fit it's how we all learn and I still have some space left for new ideas or just to correct the old ones

Obviously my first choice would be for a twin leaf wall with high density insulation in the cavity and a 50 mm space between the insulation and the outer leaf.

That said it is not always feasible to use a twin leaf insulated wall, in my opinion it is not the end of the world to use a 9" cavity properly constructed.

.

gregos

Indeed. I suppose, all things being equal, there's a lot less to go wrong with a twin-leaf wall. With a cavity-block wall you still have to pay for the insulation, plus the plasterboard, and its fixing to the wall, the cost of which shouldn't be ignored. If the external render goes wrong, for whatever reason, water gets in. On the other hand, you'll have to pay for the internal plastering of a twin-leaf wall, but you'd have to do that with the plasterboard on the cavity-block wall anyway. Here in "the country" (where 71% of the population live), that would be regarded as normal. But in "the city" (the only one?) I'm told people have to pay incredible prices for this relatively simple task. I don't know. What do you save? A few blocks. What's quality worth to you? If the rest of the house is built to the Dublin standard, go with the cavity block.

rooferPete

Hi gregos,

What many don't understand and may never understand is the cost doesn't bother me one bit because when I'm spending it's not my money but the customers

I have noticed in your short but to the point post you have raised some very valid points.

The main subjects being the saving today is not really a saving at all due to the internal finishing and how unforgiving a 9" cavity block can be if the plasterer or the mix is even slightly below par.

Another item you didn't touch on is the potential loss of internal space due to the depth required for a quality dry lining job with what would normally be a lesser level of insulation.

The one factor that should be taken into consideration where city living is concerned is very often the space is tight and the homeowner may not have the space needed to pour metre wide foundations and lose the extra floor space they may gain with a 9" block.

I think anyone using a 9" block should have a very good foreman / supervisor to ensure the trades get it right first time because most will not get a second chance.

I do agree with you, the 29% are equally deserving of the best quality available to the remaining 71%.

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gregos

rooferPete wrote:

I do agree with you, the 29% are equally deserving of the best quality available to the remaining 71%.

Certainly. Particularly since they pay twice as much as everyone else!