A-rated house construction

E

eoinsgaff

Guest
Hey there, hello to all here. I'm been an avid viewer for some time but I now need some help with an issue.

We're currently about to start an house in a very exposed part of Co. Kerry. We hope to get pretty close to passive standard but are limited by budget so we will probably end up about 95% of the way there. Theoretically anyhow.

We have settled on the following external wall, from inside to out:
  • 225mm AAC block - quinnlite or similar
  • 140mm of EPS
  • breather membrane or vapour barrier or neither. Unsure if necessary.
  • 40mm cavity
  • 100mm of regular concrete block
  • Sand/cement render
This has given us a theoretical U-value of 0.185.

We have a number of issues to iron out but the biggest issue is that my Fiance is concerned that condensation will develop between the EPS and the AAC block. While we know that the dew point is on the outside of the insulation, there is some logic to this concern. Apparently there is a slight pressure difference between inside and out resulting in an air movement from inside to out. Moisture could then get trapped at the insulation. Given the concerns with AAC blocks and moisture (they are very porous and suck up moisture like a sponge) we would like to put this concern to bed.

This then leads to the next problem of how to render the inside. First things first though.

Thanking you all in advance

Eoin
 
Hi Eoin,

Welcome to askaboutmoney.

An external leaf and EPS means the breather membrane serves no function.
Sheet membranes cannot be installed in a cavity wall situation - the wall ties prevent this.
Vapour barriers are placed on the warm side of the insulation to prevent water vapour affecting the insulation.

You appear to be using huge amounts of insulation in an exposed location.
This will result in a 180mm cavity, which requires wall ties which are non-standard.
You need to appoint a competent architect AND a competent structural engineer to advise you.

ONQ.

[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.
 
Your build up doesn't make a huge amount of sense.
Why not go with external wall insulation only?

Be aware of just how difficult AAC is for everyday use afterwards - hanging curtains, kitchen units, pictures etc.
The simplest way to "plaster" AAC is to cover it with plasterboard and skim that. (possibly adding insulated plasterboard - but be aware of dew point issues).
 
There is some logic to our approach. The price for acrylic render on the external insulation alone exceeds the cost p/m2 of a single leaf of external block work. We have done a Cost Benefit Analysis (CBA) on a wide range of a approaches and for our particular location, exposure, cost, readily available labour as well as work we can do our selves, this is our preferred option. We may be suffering from information overload so we are open to correction.

We have also been advised to fill the cavity with pumped insulation. We are not keen on this approach for two reasons. Firstly the concern about a moisture path to the inside leaf. Secondly, workmanship and trying to ensure the entire house is properly filled.

If we could remove the concern about moisture we could overcome the workmanship issue. And by overcome we mean either a physical solution or a correction on our 'assumptions'.

Thank you all so far.

Eoin
 
Quinnlite or other similar blocks are known to have issues in relation to their durability in use, particularly as materials into which to fix mechanically - for example, hinges for heavy doors, like fire doors used for sound sealing and or heavy external doors security. I would have some concern about the point loads they can carry, particularly if you carry out extensions in later years.

Quinnlite will advise you on specialist methods of fixing required and extol the virtues of the ease of workability of the material, but I would still have concerns and I would get an engineer to check the adequacy or otherwise of any structural solution.

I would have fewer concerns about a dwelling using external insulation, but the money and detailing here goes on eliminating relative cold bridges, especially the more insulated the house is. Items like window sills have to be specially designed and may need to be specially made or detailed. Also you really have to consider solar gain in a super insulated house, not to avail of it, but to avoid getting too much of it or else the insides can become extremely hot on a sunny 31 degree summer's day in Ireland.

Finally think about your wallet on initial costs and heating bills and realise that the A-Rating you require is not an indicator of the likely annual fuel bill. It rates a given house, but a 3,000 sq.ft A3 rated house will cost more to heat than a 1,000 sq.ft. A3 rated house.

FWIW, yes you appear to be suffering from a lack of technical competence and reading too many brochures and websites can lead to information overload even for me :). We tend to see this a lot on AAM, both here and in the Law forums, where a little knowledge can be a dangerous thing and everyone seems to think its easy to do the work of professionals. Designing, specifying and building a compliant building isn't easy, and if something goes wrong you have no one to blame but yourself and no-one from whom to seek reasonable redress. The buck stops with you.

ONQ.

[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.
 
eoinsgaff said:
We are not keen on this approach for two reasons. Firstly the concern about a moisture path to the inside leaf. Secondly, workmanship and trying to ensure the entire house is properly filled.
Click to expand...

Regarding workmanship - generally speaking if someone has been in the field for a long time and is well regarded, they will be pretty good. With fullfill the issue is generally how close together they put the drill holes for the cavity filling - the smaller the spacing the better.
After the full fill insulation is done, you can have thermal imaging of the house done to confirm that all locations are filled. Make sure all opes are reasonably sealed, stick in a heater, wait a couple of hours and take pictures.

There is a guarantee regarding water penetration from full fill suppliers - so I wouldn't be too worried about that, unless you are really exposed. Another possible solution would be to use a full fill board such as Aerobord - this also cuts down on the thermal looping involved in a partial fill board.
 
onq said:
FWIW, yes you appear to be suffering from a lack of technical competence and reading too many brochures and websites can lead to information overload even for me :).
Click to expand...

Hmmm, i probably should have come clean from the start. I'm a chartered engineer and my wife-to-be is an architect so I don't think we're lacking technical competence. However, when one is doing it for themselves it's never perfect and it is probably this that leads to the information overload.

While I am carrying out the structural calculations - there is a cantilever causing more problems than expected - we have still appointed another practicing engineer to sign off on the final design. We are acutely aware of how personal circumstances can cloud judgement. In this case an example might be the very tight budget (and I worked 10 years with contractors so I know tight).

While I have read most of the threads here on AAC block-work, and I am aware of the concerns highlighted, I am probably still looking for feedback from someone who has used the Quinnlite blocks. As stated, moisture is of particular concern, not so much in the construction methodology, but in the overall life cycle of the product. I would be relying heavily on its thermal conductivity to give the required U-value and unexpected moisture would really put an end to this.

Also, are my concerns over a pumped cavity and moisture passing through unnecessary? Thinking as I write, I may not have carried out as much research into this topic as I should have.

Regards

Eoin
 
Eoin,

If the details had made sense, you wouldn't have received the answers posted here.

I started out with a good grounding having studied building construction for two years in Bolton Street, then developed details on my own projects and finally my own and other people's theses.
I spent eight years in a Commercial practice working alongside competent architectural technicians learning how not to do certain things.
Then four years from 2001 working with a Building Control Officer on remedial works to dwellings.
I have spent the past two years re-learning my detailing to include the current concepts to get myself up to speed.

As I stated at the start, the placement of the membranes in the initial build up above does not appear to show an understanding of the way moisture migrates through a composite build up.
Superman appears to concur and he is a poster I have a great deal of respect for, although we might sometimes differ detail and reasoning.

You claimed there was some sense to your proposal then seemed to justify it in terms of cost benefit analysis.
This is far removed from the realm of building physics.

If you want to learn something about this new and developing field may I respectfully suggest:

(i) a three day Passive Haus course near you (although there are problems with some of their details too) and/or
(ii) talk to Joe Little Architects and/or
(ii) read his articles on Construct Ireland website.

I should point out that I am not affiliated to Joe Little Architects or Construct Ireland in any way - I met Joe at Plan Expo 2009 and I have subscribed to Construct Ireland.

I trust this is of some use.


ONQ.

[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.
 
Thank you ONQ.

So what's your view on the moisture being trapped in the AAC blocks and moisture moving through pumped insulation?

Thank you superman for your contribution so far.

Eoin
 
You're welcome Eoin.

I wasn't making the point that I know everything and thanks for not taking me up wrong.

I was trying to show that its taken a lot of years to acquire the experience I have by applying what knowledge I do have and testing it against houses (not mine I should add) which developed defects (did the theory fit the empirical data) together with often intense consultation sessions with a competent building control office.

As for your materials selection, most engineers I've worked with prefer use of durable materials - steel over timber, in situ over brick, etc.

Thus I find it incongruous that an engineer would use a block with low compressive strength because it is 80% air - I already offered my comments on Quinnlite blocks.

I know you're focussed on insulation factors, but still...

http://en.wikipedia.org/wiki/Autoclaved_aerated_concrete

I think a once off built up construction like yours needs careful modeling and analysis using a HVAC programme to test for dew points under all humidity conditions and temperature ranges.

I think external insulation on standard construction with special attention paid to the detailing of opes, thresholds, cills could be a good way to go.

[broken link removed]

Think again, is my best advice to you.

============================

Some general comments.

Using materials in a super-insulated situation requires a means

- to minimise water vapour infiltrating the construction and
- of allowing water vapour to pass through the sandwich (passing through a one-way membrane) and into a situation (vented cavity) or material (render) that allows any condensate to dry off.

Its a bit like having a bath with a drip - you still need an outlet or the bath will eventually fill up.

And the outlet needs allow a volume more than the drip rate to leave the bath to cater for those situations where internal flooding or leaks can occur.

This usually means allowing ventilation through a material that "breathes", which is where insulated timber frames seem to do well.
However you have to "seal" this insulation so wind and air pressure within the interstitial cavities cannot affect it detrimentally.

The effects of weathering using the external render only (i.e. on a metal mesh either close against the insulation or with a 50mm cavity) is something I have no empirical long term data on.

I would be worried in exposed conditions in either case that insulating the house too well means the external envelope suffers badly in below-zero temperatures and may not dry out fully above this.
Houses need to be kept warm too but I don't have access to detailed empirical information.

A durable outer leaf in masonry - whether rendered or not - may have a significant enough thermal mass to get through all but the most persistent cold spell.
Unfortunately, persistent cold spells seems to be where we are heading, and I have seen a lot of frost spalling damage to brickwork garden walls around our estate following last winters endless cold.

============================

A brief note on fire safety.

Previous to this insulation was kept away from fire by a half hour rated construction minimum, with protection at opes in cavity walls and the like.
What will occupants have to do to escape if the external insulation is hycrocarbon-based and it becomes part of a fire emergency.
What happens with flammable external insulation - does the whole house go up in a ball of flame - How do you get out?

I know of two fires that occurred during construction where the insulation went up like wildfire - one was a timber frame where the torch-on caught fire, part fell into the cavity, the other was a concrete frame where a welder's spark ignited the insulation.

So despite it needing more space/depth than HD insulation and because its less thermally efficient I would tend towards external insulation or in-frame using Rockwool or another fire resiting mineral wool which does not absorb moisture.

BTW - and it may seem like a non-sequitur but its a fire thing - recessed downlighters need careful detailing to avoid starting fires and/or breaching the required fire enclosure of the internal floors in a house.
In addition the heat build up in floors due to lighting was negligible before now, but with people insulating floors too, this could be a bigger problem.
So consider using proprietary fire hoods on recessed lights, properly sealed to the ceiling.
Also consider specifying downlighter transformers that cut out if they overheat.

============================

Finally a note on durability.

I have seen a few externally insulated buildings - all looking good and carry out their functions.
I would be a little concerned about the durability of the external finish in terms of mechanical - as opposed to frost damage.

A tip with a car will not usually damage a masonry outer leaf, but rendered metal mesh will surely buckle.
Same goes for external slates or timbering - neither are known for huge compressive strengths when used in such situations.

No I have to go and do a bit or I'll be "kilt". :)

ONQ.

[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.
 
Thank you again ONQ. A lot of food for thought there.

I've really gone full circle with this build. I started with a few sketches of a block passive house, went to timber frame, SIP house, ICF, AAC like described and now I'm seriously looking at just pumping the 200mm cavity and using regular blocks inside. While I want a perfectly insulated house, that is eternally durable and still very economical, something just has to give somewhere. Add a retaining wall, a wide span first floor with a load bearing cantilever and our apparently simple rectangular house - with a few little flourishes - has been quite a challenge.

Going back to the Cost/Benefit Analysis it may just be worth the 'moisture path' risk and simply pumping the 200mm, regular block cavity. Maybe this risk can be managed with a good quality render to the outside leaf, by ensuring the workmanship is generally sound etc. Maybe one could also say that risk with the moisture path is less than the risk with the AAC's.

More study me-thinks with ground due to be broken on Tuesday. Hmmm...

Eoin
 
The issue with loadbearing masonry is fixing the position of the load bearing wall so the line loads and point loads are centred.

Assuming the overall wall width is maintained

  • internal insulation pushes the line load out and increases the span
  • external insulation pushes the load back in and decreases the span
In both cases you are reliant on a render.
I'd be uneasy with that in an exposed area in Kerry.

You can, as you have just found out, tie yourself in knots with all the different specifications.
Sometimes the best course of action is to do something fairly traditional, but very well.
I'm not trying to set your path, Eoin, but don't abandon the tried and tested yet.

=========================================

As a comparison, I have completed an A3 rated dormer bungalow in Castleknock.

For the ground floor I used

  • 100mm outer leaf in brickwork
  • 100mm cavity with full fill cavity insulation
  • 215mm inner leaf in solid concrete blockwork laid on the flat
The superstructure and other details included.

  • 215mm solid concrete blockwork laid on the flat inner walls supporting
  • Composite steel and concrete floors with fireproofing
  • MHVR system
  • FD30 doors
  • 200mm timber joist roof structure
  • 200mm Rockwool insulation
  • 2st floor studs filled with Rockwool.
Not cheap:
Needed a great contractor (to put up with me), together with a good team of bricklayers and steelworkers and a competent and experienced engineer.

Even if water somehow penetrates the brickwork outer leaf and then gets through the cavity fill, it is unlikely to get through the 215mm solid concrete inner leaf.

So far, the results speak for themselves:

  • Quiet.
  • Warm.
  • Great finish.
  • Solid as a rock.
  • Economical to heat.
You get what you pay for.

Nuff said.


ONQ.

[broken link removed]

All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.
 
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