Why do you need a Vapour Barrier Under Concrete Slabs - The Structural Why Files aswers the question

[@MyFortressConstruction]

I have a concrete guy putting a plastic vapor barrier down for a detached metal building/garage, but he's not running it down in the beams, only down the sides of each section. Doesn't that defeat the point of the plastic since water will seep up through the 2' deep beams and create a criss cross pattern of higher moisture on the slab in those areas? We are in clay soil about 20' higher than the flood plain with a clay sand fill under the slab. Interested to hear your thoughts, thanks.

[@dekkerlundquist5938]

Hello Matt. I have a question about my garage. I have a garage with a concrete floor and it seems no vapour barrier was used under the concrete. I have not noticed any real problems with moisture coming up through the concrete floor, but I have seen the occasional appearance of efflorescence in one small section. I am contemplating grinding the concrete floor and sealing it with an epoxy flooring product. The makers of the epoxy flooring product say that its impervious to water, and I'm not sure I believe them, but I am definitely doing my homework. Apart from my concern about the epoxy failing down the track because of moisture rising up through the slab, my bigger worry is wether the slab will be more prone to deterioration in the longer term as a result of its exposure to this moisture without any barrier under it? Thanks in advance for any thoughts you could share about this.

[@CornellEngineers]

Hi Dekker Lundquist Once the concrete has cured, a vapour barrier is more about health and amenity of enclosed spaces (ie protecting against undue moisture, damp and mould) than it is to the protection of the concrete. So sealing the slab surface with a product that bonds well to the concrete should solve the localised patch of efflorescence (once it is cleaned off the concrete as you say). It's a tough job but identifying where moisture is getting under a slab is my preferred first step. Are there broken pipes, poor drainage, external drainage issues? I haven't heard of a floor finishing product failing because of rising damp but that doesn't mean it doesn't happen. Keep up the good work with the research. Matt Cornell

[@dekkerlundquist5938]

@@CornellEngineers Thank you very much for taking the time to send your helpful reply.

[@vitalilisau6181]

Will it be good to put such moisture barrier under a driveway? I think it might prevent from mold growing on top of driveway. I have to pressure clean it every year or two.

[@CornellEngineers]

Hi Vitali I have heard some concreters say not to put a moisture barrier under a concrete driveway slab I have seen driveway slabs done both with and without a vapor barrier. My personal preference is yes, have a vapor barrier under a driveway slab. However, it is not mandatory. Matt Cornell

[@jass7899]

Have you got a video how to properly tap around the plumbing pipes? To not allow moisture through

[@dollie436]

good clear information

[@CornellEngineers]

Thanks for your feedback David. Glad you liked it.

[@MichaelaBenson-oi2pz]

Thanks for confirming that the vapour barrier goes under the foam insulator.

[@llanzafa]

Great work thanks mate

[@Griggsy2529]

Great content, just a quick question is it a requirement to continue the membrane over the form work to top of slab or it can be terminated at ground level, Cheers Adam

[@CornellEngineers]

Hi Adam Good question. The membrane has to terminate somewhere and it should be terminated in such a way so it does not catch and direct water under the slab - that would be counter-productive. So it depends on the circumstance and type of slab. For raft slabs, the code says that the membrane should extend under the edge beam and terminate at ground level. Where local experience shows that it is ok it may terminate at the internal face of external raft slab beams. For waffle slabs, the membrane often runs past the formwork and only lately have I seen it being turned up and being temporarily affixed to the edge formwork. Clause 3.2.2.6 of NCC2019 requires the vapour barrier must extend under edge beams to finish at ground level in accordance with their sketches. The code also allows for it to turn up and lap with waterproofed edges but this isn't often done well. So my preference would be to terminate it on the outside of the slab at ground level and ensure excellent site drainage on all sides so that water drains away from the dwelling and doesn't sit on the membrane so that it can't run under the slab. If your question is in relation to slab edge dampness then the answer is good site drainage more than slab edge membranes. Check out my new video on site drainage. kzbin.info/www/bejne/a3TYaYOOg5iKopY

[@lionreyez2416]

Styrofoam placed in or under concrete does it weaken it structurely? Does that Styrofoam degrade over time? What is the reasoning behind it use? Questions for the public?

[@1goblingreen404]

Are vapor barriers required, or recommended for concrete slabs for accessory buildings (sheds) no attached to a residence?

[@CornellEngineers]

Clause 3.2.2.6 on NCC says: A vapour barrier must be installed under slab-on-ground construction for all Class 1 buildings and for Class 10 buildings where the slab is continuous with the slab of a Class 1 building as follows.... then some explanation of what a vapour barrier must be. So your unattached shed is a Class 10 and doesn't 'require' a vapour barrier - but what are you saving by not installing a vapour barrier? Not much. If you have the choice I'd put one in. Hope that helps! Matt Cornell

[@danielbass09]

@@CornellEngineers seems most concrete contractors don’t know this requirement. And I’ve seen contractors laying down the orange plastic and then deliberately putting holes in it in between the reo squares. Not sure what they think this is achieving… Would this be an issue at all for a pergola area that will have tiles over the concrete slab?

[@CornellEngineers]

@@danielbass09 The vapour barrier is primarily to prevent rising damp coming into an enclosed, unventilated (inside) space. As far as I can tell, it wouldn't be the end of the world if there was no vapor barrier under a tiled pergola area slab. Matt

[@tommikehappy]

Good video. Just wondering how to seal around all services?

[@CornellEngineers]

The plastic is a vapour proof membrane, not a waterproof membrane. AS2870 calls for penetrations to be taped or sealed with a close-fitting sleeve. I normally just see tape at penetrations.

[@dtran21k]

Do you recommend to use vapor barrier for footpath and concrete steps?😊

[@CornellEngineers]

Hi @dtran21k Interesting question! I don't NOT recommend it. For the reason that the plastic prevents the ground sucking moisture out of the concrete I like it - but I recognise that a barrier isn't required under footpaths and external concrete steps to prevent rising damp. If the plastic is available and on site, then I'd suggest using it. If it is 'too hard' to get a roll of 0.2mm polyethylene membrane on your site then I'd question that answer but accept that it isn't really required. Matt Cornell

[@kakalutangitangi9869]

isn't the vapor barriers will cause plastic shrinkage cracks on your slab? cause your concrete will dry from top to the bottom instead of bottom to top?

[@CornellEngineers]

Thanks for your question. This is a common thought - but plastic shrinkage cracking is caused by the concrete surface hardening faster than the concrete underneath. We use curing methods to prevent the surface concrete from hardening too quickly before the full depth of concrete has gained strength. In fact, the bottom of the concrete with a vapour barrier is similarly protected from the surface hardening too quickly which is pretty much perfect! Matt Cornell

[@kakalutangitangi9869]

@CornellEngineers thank you. would you mind sharing what are those curing methods?

[@nigelknight1435]

Why do you only have a DPM and no DPC in Australia, in the UK you have both and they are connected at the perimeter to seal the building from moisture, I have not seen a DPC used in QLD, just a DPM which is allowed to go under the external footing, rather than the footing being in direct contact with the ground?

[@CornellEngineers]

I think you'll find we do have DPC in Australia. It is used widely and well. You're right though - we don't tend to lap out DPM to our DPC. Maybe that is food for thought!?!?

[@sikandarazam698]

It's a rather naive question. If there is a plastic sheet(vapour barrier) between the ground and the concrete slab, doesn't that put up a risk for the concrete slab to slip/slide? Because the concrete is not in direct contact with the ground any more. How does the concrete slab get in touch with the screw pierings?

[@CornellEngineers]

Hi Sikandar It is actually a good question. Lateral sliding of a dwelling is pretty unlikely. The concrete to ground roughness and footing contact area is quite high despite the plastic sheet. Screw piers aren't required to stop a house moving sideways - they are there for vertical support. However, the polyethylene vapour barrier is stopped at screw piers (and bored piers) to allow the steel pier of the screw pier to be embedded in the concrete of the footing. The risk if moisture rising up through a thick concrete footing is lower than the risk of moisture rising through the thin slab. I hope that makes sense? Matt Cornell