is it as the builder says ? Cavity wall V Block and cost ???

T

tedcrilly

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Hi

hoping someone may be able to offer knowledge and/or advice on how to deal with a builder on the following matter.

Issue is at foundation stage and may need to switch from cavity walls to Cavity block construction but builder not acknowledging that there is substantial difference in cost.

In the middle of work with a builder on an extension, plans drawn up by architect (who will do site visits and sign-off when complete but is not negoitiating with the builder directly on any costs ... that is may pleasure !)

The plans spec'd are for a cavity-wall construction for a 13 X 18 extension, velux, etc. Builder priced accordingly (45,000+) Builder hesitated on using cavity wall on the Party wall and favored 9" solids on the flat, quoting fire-proofing. Architect accepted as reasonable.

Now construction is under way, foundations are dug and we have found that the foundations for the gable end wall would require to be covering over the main sewer. With 4" cavity wall construction allowing for 12"/12"/12" foundations we would lose 6 inches on the internal length in order to pull the foundation back so as not to cover the sewer.

The builder is suggesting that we go with Cavity block construction throughout and insulate internally. With 9" cavity blocks foundation would be 27" allowing them to build wall 9 inches in from of edge of foundation/sewer, thus getting back 6+ inches internally subject to internal insulation.

I have been advised by others in the trade that the cost difference in terms of material is not that significant but the labour would - overall savings might be approx. € 2000 as less blocks and only laying one leaf of bricks. The builder is not acknowledging any cost difference (so I can presume it is going to him), saying internal insulation materials are expensive and cost to lay cavity blocks is more expensive (???). He says he will do what I want - either go with Cavity wall and lose the internal length OR get approximately what I want but use the cavity blocks.

He wants an answer ASAP.

The issue on the sewer is real and I believe this has jsut afforded him the opportunity to suggest the switch.

To sum up, the 13 foot length was specified so as to allow for a run of 6 units along one wall but no kitchen has been ordered yet so flexible there.

I feel like he is getting ready to play me like a violin on a range of issues to either save money on the job, which will go to him, or to add extras, despite the fact I think i am paying a very generous price to ensure a good quality build and finish?

I obviously want to avoid confrontation this early in the job.

Any comments or advice welcome

thanks

TC
 
Your build cost seems reasonably generous so the builder should be making a reasonable profit either way. He's given you a choice so the decision is entirely yours. OK that's my builder's hat off. There should be a cost differential in drylined hollow blockwork versus cavity wall. However this may not be reflected on a small job like this. As he says the insulation boards are expensive and there are other issues with grounds being required for fixing units etc. But I would be inclined to seek some small clawback, maybe you could negotiate for a more expensive kitchen unit or higher electrical spec etc. Builders don't like giving money back (who does?) but additional materials or labour can be used as part of the bargaining ploy.
 
Dry-lining of internal walls is the least preferable way of insulating external walls - you lose the thermal storage capacity of the wall among other things - others (e.g. Heinbloed etc.) would be able to tell you more about the merits of cavity wall insulation versus internal dry-lining.

Just something to think about.
 
Sherman said:
Dry-lining of internal walls is the least preferable way of insulating external walls - you lose the thermal storage capacity of the wall among other things - others (e.g. Heinbloed etc.) would be able to tell you more about the merits of cavity wall insulation versus internal dry-lining.

Just something to think about.
Click to expand...

All the above is true, but this principle will also apply to timber frame construction. Drylining and timber frame construction have a low thermal mass- which means that response time of heating is rapid but in warmer summer weather these types of buildings can be too warm and difficult to cool down. I don't think this is really critical though in the case of a small extension such as yours.
 
tedcrilly said:
Builder hesitated on using cavity wall on the Party wall and favored 9" solids on the flat, quoting fire-proofing. Architect accepted as reasonable.
Click to expand...

Your builder was correct here. Party walls should always be solid block. How did your architect specify this?

Now construction is under way, foundations are dug and we have found that the foundations for the gable end wall would require to be covering over the main sewer.
Click to expand...

How did your architect miss this? Or the builder when laying the foundation? Did the architect not determine position of sewers and drains before drawing up the plans? Also, your builder should have told you about the problem with sewer when the foundations were going in, not now.
 
Firstly thanks for all replies

carpenter - thx, what do you mean by "with grounds being required for fixing units etc. " ??

extopia - on the location of the sewers, its a 1920's house north dublin, varying lengths of extensions have been built in adjoining houses, some longer so must be built over the sewer - Corpo couldn't confirm exact distance so we did'nt know until we dug.

No foundations poured yet, pegged out and ready to pour now.

I am not looking for any overall reduction on price from builder, its more to know ...

1. whether to choose cavity wall or cavity blocks, pro's and Con's

2. if Cavity blocks am i correct in looking for builder to acknowledge that there is an element of saving here that could be applied to another aspect of the Job e.g. extra velux windows ?

TC
 
Timber grounds are required behind drylining if you wish to hang heavy fixtures off the wall. Normally this is done using MDF or softwood battens, fixed behind the drylining.

Cavity walls are a preferable construction system to hollow blockwork as cavity wall construction utilises a cavity and dpcs (damp proof courses) to prevent rain and damp finding its way through to the interior of the habitable space. Hollow block walls rely on the use of a drylining to create a warm dry space. However as already outlined earlier i think an acceptable compromise would be to offset any potential savings in cost against an increased spec. for finishes etc.
 
>> Corpo couldn't confirm exact distance so we did'nt know until we dug.

Poor performance by your architect IMO. It's easy enough to determine the line of a sewer by locating the nearest manhole, whether in a neighour's garden, back lane or whatever.
 
I would prefer to use cavity blocks and outside insulation.
 
Sorry for not looking into this post earlier, I simply missed it. Hollow blocks - esp. the 9" variety- are banned already in the UK for the use in public housing.The reason is that they allow to much moisture through. There was an article in the magazine "Construct Ireland" ( ). Anything fixed to such a wall from the inside would rot or/and loose it's heat insulation value. I cite the article: "This type of construction is banned and deemed unfit for humane habitation in other countries such as the UK " - well, we're in Ireland. See construct Ireland, No.8,vol.2,2005.
In principle it's right to use a loadbearing wall structure like blocks and to insulate them from the outside as Touch Wood has said. But the technical expertise here in Ireland simply hasn't reached the broader market. Allthough it's done here it's still not "standard". As it is on the continent.
Once a cavity wall is buid/in place it is difficult to increase it's U-value. On the calculation sheet - yes. But in practical terms these cavity walls have to be "aerated" -that means they have airgaps build in. To get the (rain)water from the outer wall back out again. And insulating an aerated wall from the outside makes as much sense as standing naked on the Northpole next to a fur coat.
Tedcrilly fell -as most home owners did - to an incompetent builder sanctioned by an incompetent architect/civil engeneer.
To get a good insulation the only way forward is either a non-breathable wall insulated to a zero energy demand standard or a breathable wall equipped with a rainproof cladding with a similar heat insulation property.
The rest is rubble to come I'm afraid.
 
9 inch cavity blocks are not good when you put a sand/cement plaster on the outside as they suck in the water like Heinbloed said.
If you put the insulation (preferably a rockwool breathable board) on the outside of the 9 inch block wall and put a Drywit type breathable plaster on the outside then you have a perfect job.
Insulation on the outside of a wall works a lot better than the same amount of insulation inside and the condensation line doesn't end up inside your house as it does with drylining.
I know some guys in the Dublin area who do this type of work.
The partial fill cavity wall system that uses Polysterene or Polyeurethene in the cavity doesn't work well because of Thermal Looping (also in the same issue of Construct Ireland).
If you want to build using partial fill cavity walls (2 four inch block system) then I would reccommend using Rockwool batts.
 
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