Fitting DPM when walls are already built

[Chreag]

Hi
I am ready to put the DPM down in my extension, but the walls have already been built, so I can't tuck it over the DPC.
I know I can tape it to the wall, but I don't know how far up the wall (and what sort of tape).

Can anyone help please? Do I need to use double sided tape or tape over the ends with single sided, and should the DPM go higher than the DPC or level or what?

Also I've seen people saying to use a mastic strip instead of tape. Would that be something like this;
http://www.toolstation.com/shop/Adh...+Gutter+Mastic+300ml+Black/d180/sd2350/p11002
and is that better than tape?

Thanks for any help!
Chreag

 

[jeds]

Tape is a non starter. Silicone mastic is the way to go. Just lay a couple of good thick beads on the dpm and stick it to the wall. The dpm needs to be clean and dry. Give it a good wipe with a damp cloth and let it dry before you start. If the wall is very dusty I'd go round with the henry as well.

 

[Chreag]

Where do I stick it on the wall? Is it anywhere higher than the DPC, or do I mastic at the DPC level?

Should I use tape to join the sheets of DPM with an overlap or is mastic best for that as well?
Thanks

 

[ban-all-sheds]

Tape is a non starter. Silicone mastic is the way to go. Just lay a couple of good thick beads on the dpm and stick it to the wall.

Cut a slot in the wall to push the DPM into?

The dpm needs to be clean and dry. Give it a good wipe with a damp cloth and let it dry before you start. If the wall is very dusty I'd go round with the henry as well.

Worth sealing the surface with anything, e.g. stabiliser, PVA?



[EDIT]Spleling misteak corretced[/EDIT]

 

[tony1851]

deleted

 

[jeds]

Where do I stick it on the wall? Is it anywhere higher than the DPC, or do I mastic at the DPC level?

Should I use tape to join the sheets of DPM with an overlap or is mastic best for that as well?
Thanks

Mastic it to the wall cut off level with the dpc or the top of your finished floor. The purpose of the upstand is to prevent lateral penetration so it's not essential to chase it into the wall. What's going on top of the dpm - screed or concrete or what? Are you fitting an insulation upstand?

Lapped sheets can normally be taped. It's taping to a wall that won't work. Again the poly needs to be clean and dry.

 

[Chreag]

Thanks John
I will be putting the concrete slab over the dpm, then the insulation and then a 2 or 3 cm pug mix enclosing the UFH pipes.

I'll put 25mm thick celotex upstand around the perimeter to counter thermal bridging.

 

[jeds]

That's fine. Just leave the poly high and trim it off after at finished floor.

 

[theoldun]

Thanks John
and then a 2 or 3 cm pug mix enclosing the UFH pipes.

Screed is to thin. Min 65mm thick on insulation. Also throw some fibres into screed mix
Regards oldun

 

[Chreag]

Thanks oldun

I thought screed had to be 65 - 75 mm, but an 8:1 pug/biscuit/dry mix is fine at 25mm.
I like the idea because it would warm up and cool down quicker than a full screed.
Is that not the case?

Related to the pug mix, how wud I put the final flooring on. I'd prefer tiles and the wife wants wood. Can we lay a floating wood floor straight on the pug?
How would i lay tiles?

Thanks

 

[noseall]

I thought screed had to be 65 - 75 mm, but an 8:1 pug/biscuit/dry mix is fine at 25mm.

I have no idea what the latter part of this sentence means particularly in relation to the first part.

As Old Codge has said - minimum thickness for an unbonded screed is around 75mm.

 

[Chreag]

I'll try and do the sentence better sorry!

From what I've read, a screed mix is 4:1 sand to cement and is mixed with a bit of water (but not much).
A pug/biscuit mix is 8:1 with no water and seems to be mainly laid upstairs and to keep the weight down is optimally laid at 25mm. I figure if it works okay upstairs then why not directly on the insulation over slab downstairs? I'm not too interested in having a big slab to retain the heat.
Here is one web site's info;

The tried and tested method of installing UFH in suspended floors is using a pug mix. High quality insulation boards are placed between the joists and held in place with battens, so they are fully supported and the top of the board is approximately 20mm below the top of the joist. The UFH pipe is then clipped into place, typically at 200mm centres onto the insulation board. The joist must be notched to allow the pipe to pass from one ceiling void to the next. Once the pipe is laid, a dry mix of 8:1 dry sand and cement is smoothed over the pipe work and tamped level with the top of the joist so when the finished flooring is laid, the sand and cement mix is in contact with the underside of the flooring.
This system allows good heat transfer from the UFH pipework to the floor surface. The method is particularly suitable for buildings with higher heat losses and also for combination with lower temperature heat sources such as heat pumps.

 

[Chreag]

and so if i lay a pug/biscuit mix, can I lay engineered wood straight on it, or if I choose tiles do i need something like a decoupling layer first.

Or am I barking up the wrong tree and should be saving my pennies for a proper screed job / liquid screed? The main advantage I can see for those is the higher thermal mass to retain the heat longer.

 

[ban-all-sheds]

Thermal mass.

This is what it does:

When you need the heating on, it takes several hours to get up to temperature, and in the meantime you are cold.

When you want the heating to go off it takes several hours to cool down, and in the meantime you are too warm.

Fine when the weather is predictable. An absolute nightmare when in spring and autumn the temperature can be chilly one day and warm the next, or cold overnight and fine by mid-morning.

You will at some point find you need to open windows because it's too hot in the room because the floor is staying hot.

 

[Chreag]

Yes thermal mass

That's why the thin pug mix sounds appealing as it has a much lower thermal mass and should hopefully be quicker to react.

The only alternative I can think of is the aluminium spreader plates but a lot of people seem to say they are noisy and don't work too well unless they are fitted really well.

I guess electric is another option, but i'd rather hook it up to the existing heating.