# Perimeter floor insulation-request for advice



## TheBoy (15 May 2009)

Hi guys

Any advice would be a great help.........

Basically I have a concrete sub-floor on a new build which has been put on top of the floor insulation but no perimeter floor insulation has been installed around the edges. 

How important are the strips of perimeter floor insulation to prevent heat loss/cold bridging? I guess it's a big issue with UFH but I am going for standard rads which it is maybe less important for (most heat travels upwards?)?

Is there anything I can do (within reason, esp cost!) that would salvage the situation? 

Can I put a very thin layer of insulation between the sub-floor & screed?- I have read somewhere that this can lead to condensation problems as it prevents the sub-floor from 'breathing' & drying out. Is there any type of insulation that would not cause this condensation issue whilst stopping heat penetration (maybe that's a contradiction?)

Is it still worth my while putting strips around the perimeter before the finished floor screed is put in?

Maybe I am best to cut my losses & focus on other details?

Sorry for all the questions but would love some guidance.

Cheers.


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## baldyman27 (15 May 2009)

TheBoy said:


> Basically I have a concrete sub-floor on a new build which has been put on top of the floor insulation


 
This is completely the wrong way to do it and is hugely lessening the effectiveness of the floor insulation. The subfloor should be laid first, then the insulation, then the screed. If you had a contractor do this, then I would have some serious questions for him.


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## TheBoy (16 May 2009)

baldyman27 said:


> This is completely the wrong way to do it and is hugely lessening the effectiveness of the floor insulation. The subfloor should be laid first, then the insulation, then the screed. If you had a contractor do this, then I would have some serious questions for him.


 
Thanks baldyman27. Just wondering what the reason for this is? Is it something to do with the insulation getting wet? Surely it will still get wet when the screed is laid on top of it? Is it stupid to heat such a big mass of concrete? However would the 'thermal mass' theory for blocks not also apply to concrete in floors? The Kingspan booklet for the K3 floorboards shows this type of construction as does the Homebond book (that doesn't necessarily mean it's correct).

Unfortunately I can't do anything about that now. Re my original qu- do you have any advice?

Cheers


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## baldyman27 (16 May 2009)

Apologies, but I find it very hard to answer your questions as the job seems to be a bit of a mess! Why was there a subfloor poured with an allowance for a screed when there was no UFH? The simple answer, and this is just my humble opinion, is to just pour the screed and forget about additional insulation.If you are using rads then the floor isn't going to be heated anyway and any heat loss to the walls will be negligible. I have to ask, out of curiosity as I am in the business and wonder about the cowboys out there, did you have an engineer and/or a contractor involved? Again, if you did, then there are serious questions to be asked.


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## sydthebeat (16 May 2009)

baldyman27 said:


> Apologies, but I find it very hard to answer your questions as the job seems to be a bit of a mess! Why was there a subfloor poured with an allowance for a screed when there was no UFH? The simple answer, and this is just my humble opinion, is to just pour the screed and forget about additional insulation.If you are using rads then the floor isn't going to be heated anyway and any heat loss to the walls will be negligible. I have to ask, out of curiosity as I am in the business and wonder about the cowboys out there, did you have an engineer and/or a contractor involved? Again, if you did, then there are serious questions to be asked.




the OP has said he is installing a rad system.. therefore its perfectly acceptable to pour a concrete floor on top of the insulation.. i would certainly state that perimeter insulation is vitally important, and shouldhave been installed..!!!

the typical build up in this instance is:

floor finish
powerfloated concrete slab floor
insulation
DPM or Radon membrane
blinded and compacted hardcore

this is a typical homebond detail


if you are in the business... i cannot understand how or why youve never come across this build up...


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## baldyman27 (16 May 2009)

sydthebeat said:


> the OP has said he is installing a rad system.. therefore its perfectly acceptable to pour a concrete floor on top of the insulation.. i would certainly state that perimeter insulation is vitally important, and shouldhave been installed..!!!
> 
> the typical build up in this instance is:
> 
> ...


 
Syd, the OP has stated that a subfloor has been poured on top of the insulation and that there is now to be a screed poured on this. This makes the build-up;

Hardcore
Radon barrier
Insulation
Subfloor slab (100mm to 150mm deep)
Screed (min 50mm)

So you have two slabs of concrete, min. 150mm, potentially up to 225mm. Of course perimeter insulation should have been installed, though it is not as vital when an OFH system is not being used. The effectiveness of the floor insulation already installed is reduced given the depth of concrete that will eventually be on it. The build-up that you have described above is correct, but is not the build-up that the OP has described.

Please don't pass judgement on my professionalism without first reading the thread properly.


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## sydthebeat (17 May 2009)

I did read all the posts, and what i posted above was the 'typical' build up.. .as you can read....

again, this build up.. .whether it be a powerfloated slab, or  screed on slab finish, is not at all unusual..

to correct you, you do not have two 'slabs' of concrete. the subfloor is teh structural element, and teh screed is simply a finishing element. The depths of both elements should be signed by by teh site engineer. It is common to use this method where there is risk of damage to floor finish due to construction work. How do you know the depths of these elements... and thus your assertion that teh effectiveness of teh insulation wil be reduced??? the op doesnt post them... and if you are familar with u value calculations, you will know that the thicker the elements in teh build up, the lower and better the u value....

therefore if teh OP will actually end up with a 225 thick floor, which i doubt, he will have a better u value than a 150 floor...

i most certainly was not questioning your professionalism,.. i do not get where you picked that up from... i was asking how or why youve never come across this build up , given its commonplace usage.

i will assume your located in the west where raft foundations are more common, therefore most floors are made up of insulation on subfloor with screed over...

however in teh midlands and east it is a lot more common to use teh build up as mentioned by the OP.


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## baldyman27 (17 May 2009)

Syd, we may have picked each other up in the wrong way. I'm in Cork, very few rafts here either. I do find it strange to have a subfloor and a screed poured on top when there is no UFH. It is an unnecessary and expensive build-up IMHO. I will respectfully defer to your knowledge of u-values. The most sensible build-up in this case would have ended up with a 125mm structural slab on top of the insulation. This would be the FFL. I just do not see the sense in pouring a separate screed on this again.


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## TheBoy (17 May 2009)

Apologies if I didn't make the build up clear. I take it then that the perimeter insulation (when a traditional rad system is used) is not absolutely vital & that there's nothing that can be done about it now?

Just out of interest is there much difference between heat moving from the room space into and down through the blocks & heat from the floor moving in this way? 

Cheers guys


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## sydthebeat (18 May 2009)

theboy...

perimeter insulation is required to conform to building regulations... see TGD Part L 2005 or 2008, whichever is applicable to you...

therefore your builder is not building in accordance with building regulations and should be made remove the floor. Your architect / engineer should enforce this point, and if he/she doesnt, then its time to think about hiring a new one.

baldyman, im sorry if my posts seemed facetious... they were not meant that way.
Pouring a finishing screed, at teh end of teh construction process, is one way of doing things. Its a good idea actually if theres danger of teh floor being damaged during the construction process. This method was common before the advent of powerfloaters.


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## galwaytt (18 May 2009)

You're both right !!

There's nothing wrong with insulation under the subfloor/raft.  This is becoming more common, especially in passive-spec houses and the like.  However, the screed above that, should always, imho, be on another layer of insulation, over that.   I have seen houses in the UK with 300mm of XPS insulation under the raft, over the hardcore.

Tesco in Waterford also uses this method, and theirs is the first passive-spec supermarket in the EU (iirc..)

In all cases, floor screed should be isolated from the walls.

My 0.02, as they say.


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## baldyman27 (18 May 2009)

sydthebeat said:


> baldyman, im sorry if my posts seemed facetious... they were not meant that way.
> Pouring a finishing screed, at teh end of teh construction process, is one way of doing things. Its a good idea actually if theres danger of teh floor being damaged during the construction process. This method was common before the advent of powerfloaters.


 
Syd, my misunderstanding as much as anything else, no apology required or expected.

It's my experience since I started in construction c.10 years ago to build up the floor as described above. The very odd time we get a base to price that would have a sand/cement screed at FFL on top of a subfloor slab and I always try to get this changed to just a powerfloated single slab. Only once has this change been rejected by an engineer and this was because there was marmoleum flooring as a floor finish. I just try to reduce the cost to the client and most engineers are quite happy with this. 

Back in the days before powerfloats, before the tiger thing, builders had more pride, more time to do a job properly. The OP in this case seems to have been caught by a bit of a cowboy. There is no disagreement between us, just a misunderstanding.


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## sydthebeat (18 May 2009)

thanks baldyman, my misunderstanding as well.

I think the OP is being let down badly by their engineer / architect. Personally, i wouldnt allow a floor to be poured without seeing and photographing the under slab construction, including the perimeter insulation.

Cowboy builder?.. yes maybe, or just maybe too 'old school'....
Cowboy engineer?..... probably....


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