# UFH, depth of screed, insulation and radon barrier



## BMD (14 Nov 2010)

Have a few questions which I would like some help with;

1. I am putting in UFH upstairs and downstairs and was wondering how much insulation and screed I should be using.

For the ground floor we have spec'd 150mm of insulation (under the UFH pipes) and then and 75mm of screed. Does this sound ok?

For the 1st floor we have spec'd 25mm of insulation (under the UFH pipes) and then and again 75mm of screed. Should we change the screed to 50mm here or use easy screed to get a better response times in the bedrooms etc? Also is 25 mm of insulation sufficient upstairs? The 1st floor will be pre-cast suspended concrete

2. On our ground floor, one half of the house will have an in situ poured concrete subfloor while the other half needs pre-cast slabs (as one half is bog and we had to go down very deep to get foundations). The builder is proposing to put the radon barrier below the pre-cast slab however I am not sure about this as it could rip the barrier. what do you think?


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## onq (14 Nov 2010)

1. Your archtiect who did your overall house details or your BER assessor who informed you of the likely rating for the house should be asked to specify.
Just remember, a house is an integral unit - just becuase rooms aren't used doesn't mean air is not migrating from warmer areas of the house like kitchens and bathrooms to colder rooms above.
Water vapour migrates even through vapor checks and leaving cold rooms overhead is never a great idea.
With underfloor heating, response times are pretty slow so you should ideally keep a minimal level of heat on at all times to avoid condensation build up.

2. The builder can try to do that, but it is likely to sag as the fill compresses. Your architect should advise you on the correct detail to use uncluding how best to drain the underfloor space to avoid moisture gathering there which may othewise eventually cause smells in years to come. I would ask him to consider placing the Radon Gas Barrier above the PC units but protected by a screed. This gives support to the RGB and allows firm support for collar details at cored penetrations - but equally I'd try to avoid these if you can.

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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## BMD (15 Nov 2010)

Was really just checking whether the insulation was sufficient (or excessive) and the same with the screed (i.e., is it excessive for response times). Obviously the architect has spec'd it but its always worth getting a second opinion.

Regarding the second question, I'm not worried about the floor being half pre-cast slab and half pour (this is standard practice where necessary), I was just enquiring about the radon barrier under the pre-cast slab.


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## onq (15 Nov 2010)

BMD said:


> Was really just checking whether the insulation was sufficient (or excessive) and the same with the screed (i.e., is it excessive for response times). Obviously the architect has spec'd it but its always worth getting a second opinion.



Your comment here in this thread appears to contradict this.
Did you engage an architect on site who specified a different floor build up?
Regardless of what has occurred to cause this apparent confusion my comments are relevant.

Self builders tend to focus on the matter at hand, not the consequences of the decision for the building in later use.
This is why its important to have your architect involved on site, not just pay him for a few drawings which migh be fine and detailed enough for planning stage, but which need a shedload of work to bring them up to tender and then working drawing stage.

My comments are intended to de-focus you from the specifics and direct you towards getting a comprehensive review of your details.
You can bet that if something like this is left out, there will be other things arising, eg. Part F ventilation of the roof is a recurring query.

I have to admit I am surprised that you need to ask such questions if you retained a competent architect to prepare working drawings and advise on site - he should be able to answer all your questions.



> Regarding the second question, I'm not worried about the floor being half pre-cast slab and half pour (this is standard practice where necessary), I was just enquiring about the radon barrier under the pre-cast slab.


Your knowledge of building matters may or may not be limited, but believe me all this stuff is related.
Its the way an architect looks at detailing, because the buck stops with us and we have to consider it all.
If you want the "limited answer to a specific question" approach, post your query on Boards.ie on the Planning forum.
But be wary about answers that don't review the global issues affecting a particular detail - problems arise from ignorance, not competence.

In relation to your comments above, I wasn't talking about the floor sagging, but the radon gas barrier.
Pre cast floors are required for depths of fill greater than about 1200mm.
This is because excessive fill can lead to compression and cracking in the slab I understand it.

My understanding is also that this fill, on which the radon gas barrier rests, will will compress naturally over an extended timeframe.
This will tend to lead to a gap between it and the underside of the PC Unit, with the possibility that moisture may gather there and cause problems.
Detailing and draining of this area is also important, and needs to be considered at an early stage, as is the continuity of the radon gas barrier.

This is particularly important at the point of changeover from one form of construction to another and cannot just be left to a certified installers [or worse, an uncertified builders] "lap detail".
Any membrane, unless properrly supported, will run the risk of sagging and tearing with time, especially if not properly protected from site traffic.

You also need to talk to your local *Building Control Officer*.
Those who take their job seriously may have their own ideas about what membrane you should use.
I'm not talking about a preference for brand, I'm talking about durability on site and workability of the material in use.
And remember, once the floor goes on, if a problem arises at a later date, it'll cost a fortune to correct.

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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## BMD (16 Nov 2010)

Thanks ONQ, very informative.


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## onq (16 Nov 2010)

You're very welcome BMD, but don't take my word for it, talk to the guys on boards, some of whom can think globally and talk to a BER assessor.

After a while you may develop a feel for the role played by the envelope and you'll realise that in the good old days the thermal resistance of an external wall and an intermal space divider might be equivalent amd both were quite low.

Isolating some rooms seemed logical, but still needed care because of the problems of moisture migrating around the house and condensing in colder areas.

Nowadays with the external envelope becoming more and more impervious to the passage of heat, there is less reason to see a benefit to heating only some of the rooms, because there is little benefit to the resulting temperature imbalance.

I hasten to add I am not a BER assessor, and they may advance a contrary argument, but the approved details on the DOE website or so poor [what fool approved them?] that you cannot really assume the people who have set their terms of reference have a clue.

What is needed urgently is a set of typical details for say four typical forms of construction:

1. Cavity Walling with fill fill and internal insulation.
2. Timber Frame with integral insulation
3. Externally Insulated concrete walls

And then a commentary on 

4. Passive Haus Construction

These should show compliacne with ALL the regulations, not just address the sealing and insulation requirements under Part L Conservation of Heat and Energy and at the same time fully integrating Part F Ventilation and Part B Fire Safety in terms of the MVHR.

Afer all, what prevents a fire using the ducts to bypass the 30 minutes fire resistance of the 1st Floor construction?

I leave it 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.


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