radon barrier above subfloor?

sydthebeat

in the attached detail.... i would like to get the radon membrane between the insulation and the conc sub floor...

is there any reason i cant??

does a conc subfloor have to be protected from rising moisture for structural purposes??

sydthebeat

ok.. im happy enough with this ammendment...

but i suppose the question remains....

archtech

syd,

I would be of the opinion that the radon barrier on the outer leaf needs to be above ground level, to avoid the randon making its way into the building via the cavity.

I don't think its necessary to protect the conc sub-floor from dampness, unless there was possible a very high water table level. others might disagree.

sydthebeat

thanks for the reply archtech...

im not too worried about the cavity issue are its a timber frame and the cavity is ventilated at this level.

There will also be an airtightness membrane internally which will be sealed to the conc screed.

muffler

Are you proposing to use the radon membrane as a DPM?

sydthebeat

muffler wrote: »

Are you proposing to use the radon membrane as a DPM?

yes

muffler

In theory I wouldnt see an awful lot wrong with the detail. In fact I recall a lot of builders doing this with a DPM years ago (pre bldg. regs). The problem they had then was to get a lap on the wall DPC.

In practice though, as it will be heavy gauge, I can see on-site problems with getting a tight fit to the inner leaf but then again this is nothing new I suppose.

sydthebeat

muffler wrote: »

In theory I wouldnt see an awful lot wrong with the detail. In fact I recall a lot of builders doing this with a DPM years ago (pre bldg. regs). The problem they had then was to get a lap on the wall DPC.

In practice though, as it will be heavy gauge, I can see on-site problems with getting a tight fit to the inner leaf but then again this is nothing new I suppose.

yeah, im expecting a 355mm upstand :eek: :pac:

but this detail is going to get more and more common when the 0.15 u value with UFH is commonplace...

Slig

sydthebeat wrote: »

yeah, im expecting a 355mm upstand :eek: :pac:

but this detail is going to get more and more common when the 0.15 u value with UFH is commonplace...

I started specifing this buildup when underfloor insulation got bigger. The problem with the old system now is that the DPM/radon barrier is too far below GL and gets risky if there are any leaks in the membrane.

The major problem with this new method is protection of the membrane during construction. Its going to be exposed while all the wet trades are being carried out

sydthebeat

Slig wrote: »

I started specifing this buildup when underfloor insulation got bigger. The problem with the old system now is that the DPM/radon barrier is too far below GL and gets risky if there are any leaks in the membrane.

The major problem with this new method is protection of the membrane during construction. Its going to be exposed while all the wet trades are being carried out

so how would you feel about keeping the radon OVER the conc subfloor???

im not too worried in this case as its a timber frame build....

i will expect the UFH system installed and screed poured before the frame is erected on site.

Slig

sydthebeat wrote: »

so how would you feel about keeping the radon OVER the conc subfloor???

im not too worried in this case as its a timber frame build....

i will expect the UFH system installed and screed poured before the frame is erected on site.

maybe I'm mixing myself up again, just to clarify
from the top:

screed
insulation
DPM/radon barrier
reinforced conc slab
hardcore

is that what we're talking about?

sydthebeat

thats the make up i was asking about.. .whether its a good idea or not...

if i keep the radon membrane over the subfloor the upstand is a lot less (150 less )

otherwise i have a 355 upstand before the subfloor is poured.

sydthebeat

having spoken to a builder on this i feel its not a good idea to place the radon / dpm layer over the subfloor as.... condensation problems arise.

in the typical buildup if condensation forms on the cold face of the insulation it is soaken into the conc... keeping the insulation dry...

if the radon / dpm is above then the condensation has nowhere to soak to so it stays within the insulation layer thus reducing the insulation characteristics....

makes sense to me..

RKQ

sydthebeat wrote: »

if the radon / dpm is above then the condensation has nowhere to soak to so it stays within the insulation layer thus reducing the insulation characteristics....

makes sense to me..

Whats stopping the condensation drying into the conc slab and soaking down into the layers of 225mm hardcore. Even whacked stone has air gaps that will soak away water.

How much condensation do you forsee?
Condesation from slab drying out or Rising damp?
(foil backed insulation)

Just to be clear, I'd always prefer the insulation over the radon barrier too.

Slig

RKQ wrote: »

Whats stopping the condensation drying into the conc slab and soaking down into the layers of 225mm hardcore. Even whacked stone has air gaps that will soak away water.

How much condensation do you forsee?
Condesation from slab drying out or Rising damp?
(foil backed insulation)

Just to be clear, I'd always prefer the insulation over the radon barrier too.

ditto:D

sydthebeat

the insulation was always going to be over the radon barrier in both suggstions..

the question was whether the conc subfloor could go under the radon.

Slig

I dont see how condensation is going to cause a problem. Maybe thats just me but I find it a much better build up. The membrane is much better protected
(maybe I'm going mad but I remember writing something similar in a similar thread before?).
I'm sorry but with the current buildup I would imagine that the months of rain falling on the exposed slab would cause much more damage than condensation ever could.
The steel in the reinforced slab should have the same cover as the foundations in any case and surely they are the worst case senario.

As Regards asking the builder, I have met some very experienced ones and some cowboys but both have been reluctant to progress from the "tried & proven" construction methods as its them that will get the blame if something unforseen happens.