Screed Vs. floating floor in basement

[weegiebiker]

Hi all, I'm really hoping for some good advice here as I'm at my wits end with all the conflicting views from individual contractors:

I'm currently trying (very slowly) to convert my basement into a living space, it's approx 7.5m x 6.5m. I've recently had a contractor in to install a cavity drainage membrane across all walls and floors. Before they started I did ask if it would be best to level off the concrete slab first but they said prob best to leave till after which has now left me with this nightmare problem, over the 6.5 m the height falls about 15cm coming to the door which is only an 2.5cm above current floor level.

I was initially advised to screed (75mm ins and 75mm screed I believe to be normal?) but dont have much height to spare esp at the door (opens inwards) and was then put off it as i was told it could crack etc and a floating floor would likely be better. I'm also having massive issues just getting people to quote me for it!

With the floating floor I'm of the understanding that there needs to be a 150mm air gap for ventilation (even in a completely waterproofed room?) plus insulation,joists and T&G CP?! Also with the span I imagine I would need to build a dwarf wall- even possible on top of the membrane?

I'm on a pretty tight budget but want a durable finish with no short-term problems so dont mind paying for the more expensive option if need be, any thoughts on the best option?Thanks

The pic hopefully explains a bit better and is how i think the screed would work-any suggestions welcome!

Untitled

• weegiebiker
• 16 May 2010
• 1

 

[tommyl18]

i normally put insulation down first and concrete on top of the insulation then screed it 3 inch of concrete and 3 inch of screed so you might be a bit tight with the screed option

 

[mac391]

Basements are notoriously difficult to convert as you are finding out.

And building reg's don't have any info on it at all, in fact the info is owned by a private company that you have to pay for. That can't be right as it's useful to read through their recomendations.

Your second to last paragraph totally lost me, about screeds, joists and insulation....

If you have a height issue, have you tought about raising the door threshold and height so you can level the concrete first ? If not, what about removing the existing concrete and re-levelling.

On a garage conversion for instance. The layout to raise a sub-floor is: Sand blinding 10mm. DPM (1200 gauge visqueen), 100mm celtox/kingspan insulatiion, minimum 75mm screed with steel reinforcement.. Or no screed and waterproof chipoard ( i.e floating floor)

Ventilation is a different matter though. If there is the mention of a 150mm air gap beneath the floor then joists will be required, with insulation placed between them, but for this method you need aribricks!! to allow moisture to escape !!

 

[weegiebiker]

Hi mac, about the second last pararaph: Those are the specs for a floating floor in a basement as per building regs, which I was considering. If I was to follow them to spec (i think) I would require to build a single brick high dwarf wall to support the joists beacuse of the span of the room and this would also give the required 150mm gap, insulation would then be fitted above this gap but between the joists and then 22mm chip board on top. I'm concerned though cause if I do need to have the 150mm gap its double the height loss of the screed method.

We live in a 4-in-a -block flat and there are major issues raising the door lintl which is unfortunate as this would be a really easy option. Also, the waterproof membrane has been laid so ... no chance to flatten the existing concrete.

Btw the membrane is Delta MS 500, 8mm studded polyethelyne

To raise the level we have the base and membrane sorted so just insulation and the screed, dont think a floating floor is now poss due to above height loss.

Thats were the confusuon arrises, in building regs you appear to need ventilation if you have a wooden floating floor but not if you screed. If it doesnt need to be ventilated as its already fully waterproofed then we can lose the 150mm gap and the floating floor becomes feasible again. There is drainage installed under the membrane and i do not expect any water ingress as ive paid alot to prevent it :s.

Its a bit of a pain! Anyone fitted over a membrane like this before?

I think I need to contact building control.....again!

 

[weegiebiker]

Hi Tommy, why do you use 3 concrete and 3 screed? The basement is about 50m2, think that would run into about 10m3 of concrete/screed in total to level, which is quite scary.

And what is the insulation sitting on? Is it concrete and a DPC/membrane ie concrete-dpc-isulation-concrete -screed.

Also, I hope I dont sound too stupid but whats the difference between concrete and screed hence why do they need to be used seperatly?

 

[mac391]

Hi weegiebiker

Yes it looks like the screed method might be the best way forward but how you are going to tackle the height at the door i don't know. I suppose you could do away with building reg's on this issue and just put in a shorter door if you have to raise the threshold giving you a door height of approx 1800mm (71"). Can you not fit a sliding door or change it to open outwards?

Concrete slabs are called a screed and we also have bagged screeds. Often a bagged screed will be placed over a concrete screed to level any imperfections and create a smooth surface for the application of new flooring or liquid DPM's etc.

Talking of which, if you lay a 75mm concrete slab, where will all the moisture go? As it is, a slab of this thickness would take 3/4 months to dry in normal conditions and you probably don't have normal conditions.

I would double check with building reg's if there is any other insulation that's thinner but with the necesarry thermal values or ring kingspan/celotex. And does the delta membrane have any thermal value that might be deducted from the insulation value, allowing you to use less ?

 

[weegiebiker]

Hi Mac, can you believe it actually gets worse?

Just called building standards who are now saying I need a building warrant including U values for the complete conversion which really requires an architect and plans!?! I mean we do have scale drawings for the project no U values etc, suppose thats what I get for asking about the insulation thickness

I could change it to open outwards I suppose and bring the screed level in line with the new threshold but not sure if even that will be enough as the door is currently 1770mm short, what size would meet with regs?

As for drying-what do you mean by normal conditions? I assume the moisture would just collect in the air, increasing the humidity but I could vent it by leaving door open,the basement is completely dry, there was only ever hydrostatic water seepage from the earth retaining walls. What could be done on the screed up until the 3/4 point ie I assume no flooring but anything else?

The membrane has a value of 0.12m2Kw horizontally and 0.1m2Kw but not sure how to apply this.

Its all so very confusing, thanks for your help!

 

[mac391]

Now you're just making me feel bad lol !!

OK, kingspan will give the the relevant U values and tell you the thickness thats required. If you give them the U values of the membrane they should be ale to work out what is the best product to use in terms of thickness.

Instead of applying to building Reg's directly you can use an "approved" inspector.
You can have any drawings done by anyone who offers "architectural services" (not architects but do the drawings) at a fraction of the cost. This person will also know an "approved inspector" who ensures all the Reg's are passed !!

Good Luck