looking for some advice, putting down concrete floor of our new conservaory at the weekend, should i be looking to lay insulation into the floor slab? are there really any benefits? also where should insulation go, i understood it should be above dpm but a builder has suggested under dpm is just as good and will save using sand. any help appreciated
conservatory floor slab
- Thread starter ndmxp07
- Start date
Sponsored Links
2
2scoops0406
If it's any help, our insulation (ploystyrene slab) in the conservatory was laid thus :-
Hardcore / Soft sand / DPM / Polystyrene slab / concrete base / screed.
Hardcore / Soft sand / DPM / Polystyrene slab / concrete base / screed.
2
2scoops0406
Probably my memory that is defective
Sponsored Links
Hi boys. While you are at the subject of floor conservetories. I have inherted one three years ago and I have noticed some damp patches on the slaps in the winter. I have put damp proof membrane and under lay than carpet. Am I likely to have problems in the future with this excercise?
The slaps I think been put directly on to the soil,as the conserv was intended as a patio,than was adapted to a coneserve,also the walls have not got Damp Proof course. Your comments and segestions will be an intreasting read
The slaps I think been put directly on to the soil,as the conserv was intended as a patio,than was adapted to a coneserve,also the walls have not got Damp Proof course. Your comments and segestions will be an intreasting read
- Joined
- 23 Jun 2004
- Messages
- 178
- Reaction score
- 1
- Country
Eddie & jeds, you're both right. The dpm can go either under or over the polystyrene; it doesn't matter.
- Joined
- 23 Jun 2004
- Messages
- 178
- Reaction score
- 1
- Country
That just isn't true jeds, and the insulation can go above the concrete if wanted.
Not sure which bit you think I'm not telling the truth about?
Of course insulation might be placed above the concrete - or in several other positions - but that is not what we are discussing.
Where insulation is placed as in the construction given - i.e. below concrete - the membrane should be above the insulation. See Diagram 4 of ADC.
To comply with Building Regulations - not that you need to in a conservatory but good practice etc. - the floor should be protected from interstitial condensation.
I have specified repairs to several concrete floors where damp was diagnosed but the problem was in fact sopping wet insulation due to condensation.
About a year ago I prepared an independent report for an adjudication where the claimant was convinced that damp was penetrating (or rising) up through the floor. To prove this either way I had a polythene sheet laid under the carpet. Within just a month the carpet was visibly damp and, when lifted, there was visible water on top of the sheet - underneath the sheet was dry.
I am slightly baffled as to why anybody would place a membrane below, and risk condensation, when there is no good reason to do so?
Of course insulation might be placed above the concrete - or in several other positions - but that is not what we are discussing.
Where insulation is placed as in the construction given - i.e. below concrete - the membrane should be above the insulation. See Diagram 4 of ADC.
To comply with Building Regulations - not that you need to in a conservatory but good practice etc. - the floor should be protected from interstitial condensation.
I have specified repairs to several concrete floors where damp was diagnosed but the problem was in fact sopping wet insulation due to condensation.
About a year ago I prepared an independent report for an adjudication where the claimant was convinced that damp was penetrating (or rising) up through the floor. To prove this either way I had a polythene sheet laid under the carpet. Within just a month the carpet was visibly damp and, when lifted, there was visible water on top of the sheet - underneath the sheet was dry.
I am slightly baffled as to why anybody would place a membrane below, and risk condensation, when there is no good reason to do so?
- Joined
- 23 Jun 2004
- Messages
- 178
- Reaction score
- 1
- Country
Surely the floor is protected from condensation by reducing the U value so that the temp doesn't drop below the dew point? If you have the floor insulation designed to achieve this then I don't see that the position of the dpm is relevant. It's the temp of the floor that is relevant. I've got this pic from the building regs:
This clearly shows the dpm below the insulation.
This clearly shows the dpm below the insulation.
Where the insulation is above the concrete, it is less critical because the concrete itself has an insulation value. If warm vapour reaches the cold side of the insulation the temperature will still probably be above the dew point.
Where the insulation is below the concrete it does not have the thermal protection of the concrete slab. In this case the cold side of the insulation can be significantly lower than the warm side.
Therefore warm air, carrying a high level of water vapour, travels through the construction layers and as it passes through the insulation layer it rapidly cools. That is why the membrane should be above the insulation layer - to prevent the vapour reaching the cold side. This is standard practice.
I'm not sure where your drawing came from. Below is Diagram 4 from the current Approved Document C - Site preparation & resistance to contaminants & moisture.
As you can see, the membrane is clearly above the insulation in both cases.
Where the insulation is below the concrete it does not have the thermal protection of the concrete slab. In this case the cold side of the insulation can be significantly lower than the warm side.
Therefore warm air, carrying a high level of water vapour, travels through the construction layers and as it passes through the insulation layer it rapidly cools. That is why the membrane should be above the insulation layer - to prevent the vapour reaching the cold side. This is standard practice.
I'm not sure where your drawing came from. Below is Diagram 4 from the current Approved Document C - Site preparation & resistance to contaminants & moisture.
As you can see, the membrane is clearly above the insulation in both cases.
- Joined
- 23 Jun 2004
- Messages
- 178
- Reaction score
- 1
- Country
I've read that jeds and seen the drawings, but my drawing also comes from Approved doc C Section 5-Walls. I still say it doesn't matter. The insulation below the floor slab means the floor slab is at a higher temperature. Sorry if I am misunderstanding you, but you appear to be saying that the concrete slab is keeping the insulation warm but surely it is the insulation that is keeping the concrete warm? I always thought that the dpm is to stop damp rising up through the hardcore. You are saying that it is to stop damp going down through the floor to the insulation?
Yes the slab is at a higher temperature but the insulation is the key stage - as warm air passes through gaps in the insulation it rapidly cools and condenses - warm side is very warm - cool side is very cool.
Look at ADC floors - 4.4 - it clearly says that ground bearing floors should be designed to prevent interstitial condensation - that is why the membrane is shown above the insulation.
There is no good reason to place the membrane below - there is a good reason to place the membrane above where it provides both vapour control and damp control!
I can't see why anybody would not simply place it above? It makes no difference whatsoever to the buildability!
The same principle applies to any vapour critical construction and was the reason, along with the World in Action documentary many years ago, why timber frame buildings got such a bad name. These days vapour control is much more understood - except that many manufacturers have not yet caught up. And apparently the ADC walls writer!
Also read any BRE report and good build guide on the subject - they all say membrane above.
Look at ADC floors - 4.4 - it clearly says that ground bearing floors should be designed to prevent interstitial condensation - that is why the membrane is shown above the insulation.
There is no good reason to place the membrane below - there is a good reason to place the membrane above where it provides both vapour control and damp control!
I can't see why anybody would not simply place it above? It makes no difference whatsoever to the buildability!
The same principle applies to any vapour critical construction and was the reason, along with the World in Action documentary many years ago, why timber frame buildings got such a bad name. These days vapour control is much more understood - except that many manufacturers have not yet caught up. And apparently the ADC walls writer!
Also read any BRE report and good build guide on the subject - they all say membrane above.
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
