couple of questions!

builder1234321

A couple of questions I am curious about. I am thinking about building in the future.


1) Is wide cavity a a necessary requirement these days in order to meet regulations regarding insulation?

2) if you were not to go wide cavity is it very difficult to meet regs?

3) Should any builder be able to handle wide cavity? is it just a matter of getting specialist wall ties to tie in walls?

4) If going for a 300mm cavity, so 500 mm total wall width, how wide does foundations need to be??

5) Is MVHR required by the regs?

6) Do you have to have solar panels to meet regs?

7) If going direct labour is 75 euro psf a decent ball park to get to builders finish? prob impossible to answer i understand.

beyondpassive

1) Is wide cavity a a necessary requirement these days in order to meet regulations regarding insulation?

using DEAP to assess compliance with new 2011 regs, you are looking at wall and roof U-Values of under 0.17 U-Value, depending on what windows are used and if MHRV is used, if hole in the wall vents are used then you're looking at 0.15 U-Value. Building regs isn't the biggest driver of thermally efficient construction anymore, fuel costs and occupant comfort will drive you towards at least 250mm of insulation throughout, treble glazed and MHRV. So factor in 15% less floor space to cover the extra cost and hire a good architect.

2) if you were not to go wide cavity is it very difficult to meet regs?

You should consider timber frame as there are 4 or 5 companies out there with very good products and will handle all the structural items, leaving you to handle the finsihes and simpler items. Theoretically at least you could go 150 cavity with drylining, but that wouldn't be ideal as drylining eliminate the thermal mass and dry lining in huge cold bridges. With wide cavity the inner leaf acts like a big thermal heat sink and keeps internal temps steady.

3) Should any builder be able to handle wide cavity? is it just a matter of getting specialist wall ties to tie in walls?

If the project has good construction details and a good contract then any builder can get it right. Get it right on paper first and it takes the risk out.

4) If going for a 300mm cavity, so 500 mm total wall width, how wide does foundations need to be?? Dosent necessarily need to be any wider than standard as the loads are the same, your structural engineer can calculate on th eback of an envelope after ground investigation.

5) Is MVHR required by the regs?

Its not strictly required, but MHRV is pretty much an automatic choice for all new homes. It makes compliance easier. hole in the wall vents in a low energy build are not advised for heat loss and humidity reasons.

6) Do you have to have solar panels to meet regs?

You have to provide a 'reasonable' amount of renewables, DEAP works it out as a usable 10KWH per metre squared per annum. 2 or 3 banks of Solar tubes are the most popular choice but heat pump and pellet boilers are other alternatives. The secret here is to look at the house as a system rather than the sum of its individual parts. By combining insulation, airtighness, MHRV and good windows, you can utilise some of the solar panel heat for space heating. Focus on running costs to drive all your decisions, rather than the building regs, the Passivhaus approach is a very detailed methodology to reduce the heat demand and work out what levels of insulation and window + ventilation solutions are best.

7) If going direct labour is 75 euro psf a decent ball park to get to builders finish? prob impossible to answer i understand.

€90 euro builders finish even in the Northwest, windows are a big item and you can get PVC 3G windows for under 20 grand where an Imported high end AluClad could be €70k. Finishes are another big ticket item, where you could use cheap materials like pine in place of Iroko for doors stairs and skirting, which would save you a fortune, but you'd have to look at it for the next 30 years.

Again the advise is, invest in a clever compact simple design and save some budget for good quality finishes. Do you really need 6 bedrooms and 5 ensuits and more corridors than Versailles. Investing in good design will save you a fortune in the end as you'll have less 'pokey out bits' and other visual detritus like dormers, porches and single elevation stone cladding, that bad designers like to add to tart up their carbuncles.

builder1234321

I cant thank your post mate (guessing as I am a new user its not allowed?).

so thanks!

I was under the impression that foundations have to be three times the wall width. This I assume only applied to standard cavity. From reading i am guessing it is something to do with the angle of shear in concrete or similar.


Anyone else have alternative perspectives to the above questions?

BryanF

you asked for another comment on this thread, but BP has said it all above

1. if we are only discussing cavity block walls then yes a wide cavity is the best option, dry-lining when you have gone to the bother of building a cavity makes no sense to me
2. no, but there is no longer a simple 'meet the regs' figure that i can give, its a holistic approach using the DEAP software and current part L regs
3. its just like that other post you just commented on, there are details to be worked out, that are not yet common practice - my view is if pay peanuts you get monkeys - and the correct way to do things is to have all the details and spec agreed and priced for prior to commencement on site
4. your eng will sort this out for you, its possibly not taken on that 3times the width rule of thumb - really if your going to this job properly you'll be thinking about thermal bridges at that junction which will impact on the detail also
5. no, but insulation levels as they are means it makes financial sense in the long run
6. no, they are just one of a list of options to comply
7. how long is a piece of string

Gluteus maximus

A 200mm pumped cavity with low conductivity wall ties should bet you a "real" u-value of about 0.135.

Cavities any wider than that and the wall tends to get oppressively wide, with very deep reveals.

It is unfortunate that DEAP is the methodology by which we are measuring compliance, but we are stuck with it.

I agree with previous poster - the 2011 insulation levels are, even the backstop values are great and will ensure thermal comfort but the key to performance is the airtightness and how one addresses the requirements of Part F of the Regs.

fclauson

Gluteus maximus wrote: »

....
Cavities any wider than that and the wall tends to get oppressively wide, with very deep reveals.
...

I am doing a 250mm cav - the reveals are actually very nice and not "oppressive"

If you look at and of the old statly homes they all had walls of this sort of depth - and it gives a very nice window cill for "stuff" to sit on

Think of the sill height in each room - we have gone for 2 blocks + granite cill - this allows you to sit on the window cill if you want to

Gluteus maximus

thanks for that - what is the wall build up inside to out? wallties?

thanks for your help.

fclauson

• plaster
• regular block (except at base of each wall where there are 3 rows of quinn)
• 250 fully pumped cav - with stainless wall ties every block (but check with you engineer for what you need)
• regular block
• render

All sockets on external walls have their boxes bonded into the wall to stop air leakage

Window detail is difficault - so DO GET IT DRAWN OUT in detail before, agreeed, signed off, understood, etc etc - we had to take out the window cills due to the wrong construction detail being implemented on site

Gluteus maximus

250mm cavity, with standard block on edge inner and outer leaf, WOW.

Is it single storey?

Have you started? I would recommend that you invest in a low conductivity wall tie - increasing the insulation and then compromising it with a steel cold bridge every block seems contradictory...

Thanks for your help.

fclauson

Gluteus maximus wrote: »

250mm cavity, with standard block on edge inner and outer leaf, WOW.

Is it single storey?

yes

Have you started?
I would recommend that you invest in a low conductivity wall tie - increasing the insulation and then compromising it with a steel cold bridge every block seems contradictory...
[/quote]
My mantra has always been
Passive
Pragmatic
Price sensative
and Practical

the low conductivity wall ties failed because of Pragmatic and Price Sensative - for the added value of reduced thermal bridging they did not stack up on price