Dot & Dab or just plaster?

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Hi All,

We've got an extension that I'm doing with my brother & some other assistance.

Our walls are 100mm cavity, with a brick outer, 100mm rockwool full fill cavity and 100mm Thermalite Hi-Strength 7 blocks for the inner skin. We were planning on doing dot & dab, however, we've currently got a out of work experienced foreman helping out, who's suggested we just skim the Thermalites rather than doing dot and dab and then skimming that. With one less layer we get less insulation, but it'll save time and money.

How much of an effect would this have on U-Values? Would we need to dot & dab to meet building regs? It's about 21m2 of external wall (3mx3m room), and I'm trying to decide if money and time savings are worth the insulation tradeoff an the slight loss of room size.
 
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You'll never notice the difference between the two. Its a bit late to be worrying about meeting the regs with the cavity's already full. Hasn't your Inspector given you the nod that what you've already done is OK?
 
You will not beat d&d on economy.

However, you do lose moderately more space i.e. 18mm+ for wet plastering as opposed to 25mm+ for d&d.

The sparky will be your friend though because he won't have to chase in the 25mm deep back boxes, if you go with d&d.
 
Thanks guys.

I never discussed skimming the Thermolites with the BCO, we just said we'd dot & dab and that was that.

According to this:
http://www.british-gypsum.com/technical_advice/u-value_calculator.aspx

It looks like going with skimming the Thermolites we we'd increase our U-Value by 0.02, which isn't a huge amount, and I have no idea what that'd equate to in financial terms.

What's the required U-Value for walls? I can't remember off the top of my head.

Anyone fancy a stab at working out the likely energy wastage over the course of a year with a 0.02 U-Value increase? I wouldn't know where to start!

Thanks
 
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I thought I'd have a play with the heat loss calculations, and I think I'm right is saying:

UValue x Area (m2) x TempDifference (C) = heat loss

So if we're looking at 1m2 over the course of a year and the cost of a 0.01 U-Value differences with an average temperature difference of 10 Celsius over the year.

0.01 * 1 * 10 = 0.1 Watts

The cost of that in kwh = 0.1 / 1000 = 0.0001

So the usage over the year will be

0.0001 * 24 * 365 = 0.876 kwh

I'm not sure of the cost of gas in kwh, but let's say 7p which is about half the cost of electricity.

We're therefore looking at

0.876 * 0.07 = £0.06132 per year per m2 for a U-Value of 0.01

So the cost for me to heat 21.6m2 of exterior wall with U-Value of 0.28 and 0.30:

28 * 21.6 * 0.06132 = £37.09
30 * 21.6 * 0.06132 = £39.74

So dot & dab will give me a saving of £2.65 per year on heating costs.

Does that look correct to anyone with a mathematical brain?
 
Well i'll be.....

I built a 22sqm extension, with thermalite as the inners and never knew you could just skim them. I dot'n'dabbed the whole lot.

Live and learn people.
 
When I had a single storey extension built some 6 years ago the builder finished the inner block with what he described as 'sand and cement' and then skim. As a result I have a layer about 10mm thick on top of the block.

He said it was his preference and back then I took little interest in such matters.

Is this an old fashioned way to do things or does it have its advantages?

Thanks
 
Is this an old fashioned way to do things or does it have its advantages?

Thanks

Yes, and yes, it's nice and solid.
ie: When your team loses and you fling the remote across the room, it won't end up punching a hole in your plasterboard.
(I actually know someone who did this once!) :LOL:
 
Is this an old fashioned way to do things or does it have its advantages?

Thanks

Yes, and yes, it's nice and solid.
ie: When your team loses and you fling the remote across the room, it won't end up punching a hole in your plasterboard.
(I actually know someone who did this once!) :LOL:


Thanks. It is indeed very solid. It is the finish in my family room and has had a hard life from my two small kids but still looks good.

So why has it gone out of 'fashion'?
 
Cos plasterboard goes up quicker and thus is cheaper. Probably thermal issues involved as well with using PB, (which a 6 Y.O. build shouldn't have if it was done properly)
 
Thanks. It was built to a good standard and passed all BR insulation requirements.

I'm having another extension built soon and am wondering whether I should ask the builder (a different one) to use this method instead of plasterboard.
 
Proper plaster is always easier/better for the end user when it comes to fixing things to the wall and/or repairing.
 

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