DPM tears

[WWilts]

New build in progress. 

 

Ground floor:

Hardcore
Blinding

DPM taken up to lap DPC at finished floor level

100mm concrete

To be followed by 100mm Celotex (with perimeter insulation), polythene, UFH in 50mm liquid screed (Cemfloor)

 

Problems:

1) DPM torn in at least 30 places over a ~75m length of DPM. Some large enough to put a fist in. Most large enough to insert a pinkie or pencil. A couple of places where DPM is loose from DPC, no lapping.
2) DPM & DPC cut or mutilated where internal block walls were knocked through in preparation for insulation/screed.

 

So far the only attempted repair has been some duct tape which has mostly come off.

Question:
Is this a problem? If so, what is a pragmatic solution? 

 

[Jilly]

I guess this is giving you sleepless nights I had something similar. Someone will be along for a proper answer but I have some bitumen repair tape if you would like it. PM me for details to see if you are nearby enough to collect it.

[Russell griffiths]

Buy a new roll get some of the black dpm joining tape get some scissors and get on your hands and knees and patch it all up. 
fairly simple really. If it wasn’t important then why put it in in the first place. 

[Jilly]

I put a double layer in mine under the screed (over old concrete). The builder accepted was belt and braces and didn't mind. It's my house after all. 

 

BTW I think Steamy Tea will tell you the physics and to to put a lot more insulation under your underfloor heating. Because of retrofit, I only had 100mm space so used Kingspan Kooltherm K103. It seems to be working, but I don't know/ can't measure how much old concrete I am heating up. Most people on here try to use 200mm PIR, I think.

 

Builders will tell you you don't need much insulation under UFH 'because heat rises, luv'. But it isn't what happens in this case.

 

Coincidentally, our heating conked out last week, and using a fan heater confirmed that convection heat warmed the air but the slate/screed became a freezing cold sink. 

[Mr Punter]

I am laying the DPM on top of the concrete slab instead of under it, just using extra wide DPC for the blockwork so we have a decent lap.  I have not done it this way before but it makes sense as it would be awkward to lay before concreting.

[WWilts]

16 hours ago, Russell griffiths said:

Buy a new roll get some of the black dpm joining tape get some scissors and get on your hands and knees and patch it all up. 

Builder proposes to use damp proof paint. He doesn't want call backs for rising damp & mould.

[PeterW]

Just now, WWilts said:

Builder proposes to use damp proof paint. He doesn't want call backs for rising damp & mould.

wont work - just buy a roll of tape and use that.
 

And rising damp doesn’t exist unless you’ve just built below the water table. 

[WWilts]

3 hours ago, Jilly said:

put a lot more insulation under your underfloor heating

from the web:
About 25% of the heat produced by your boiler will escape through the roof of your home.

About 35% of the heat will escape through the walls and through gaps, in and around windows and doors, and about 10% of heat will disappear through the floor. 
 

Invested the limited funds in expanding cavity insulation from 100 to 125 average. Only 100mm available for insulation from highest slab surface to bottom of 50mm screed.

[WWilts]

2 minutes ago, PeterW said:

wont work - just buy a roll of tape and use that.
 

And rising damp doesn’t exist unless you’ve just built below the water table. 

Comforting re water table, which stays several m below ground through the year.

Why won't the waterproof paint work? Can have a conversation with builder.

[PeterW]

Paint won’t stick to the plastic DPM. 

[Jilly]

57 minutes ago, WWilts said:

from the web:
About 25% of the heat produced by your boiler will escape through the roof of your home.

About 35% of the heat will escape through the walls and through gaps, in and around windows and doors, and about 10% of heat will disappear through the floor. 
 

Invested the limited funds in expanding cavity insulation from 100 to 125 average. Only 100mm available for insulation from highest slab surface to bottom of 50mm screed.

This is an explanation of the physics behind needing as much insulation as poss under ufh. The 10% losses refer to when using normal radiant/convection heat. That said, I've only got 100mm too. 

[SteamyTea]

Sounds like a case if patch or replace.

Water vapour behaved like a gas, because it is a gas. Liquid water is more affected by gravity, the DPM deals with both.

Holes in it means it fails for both.

 

Now go and find what ham fisted plonker broke it. Give them a building physics book and test them on it.