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Rendering and airtightness


ToughButterCup

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That's the thing about building a house;  there are too many choices. And I'm at the bottom of yet another really steep learning curve. Hence this post.

 

We need to render the internal space between the ceiling and first floor to make sure this area is airtight before we fit the POSIs.

 

At this stage in our decision making process, I usually turn to the online version of SPONS and look at what's available. Comprehensive though the section on rendering is, there's no mention of rendering to provide simple air-tightness. For this small section, the look of the render and surface finish is irrelevant, and  thickness is only important in so far as air-tightness is concerned. Waterproofing isn't relevant.

 

I've come to understand that this process, however humble it may be, is key to keeping our future living cost down. So it's important to get it right.

 

Could you just give me some starters to help me widen my research, please?

 

So far I have

  • Looked through SPONS for trade names and researched those that I found : Tyrolean Finish, Cement and sand, SIKA waterproof render, Sto External render, Armat Classic Basecoat, K-Rend.
  • Done an internet search and come up with this interesting article: maybe we should simply render over an airtight membrane.
  • Visited the GBF and read this article, but it's mainly about air-tightness tapes: it raises important issues though about finding plasterers who are keen on attention to detail. Knotty one that.
  • Found out that there's stuff called undercoat plaster.........

 

This hill is steeper than I thought.

Ian

 

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Is there a section in SPONS about AT? That's where I'd expect it to be.

 

I find it incredibly frustrating this kind of best practice info isn't easily available.

 

AFAIK, S+C does the job over most of the wall, then tape onto penetrations like joists.

 

Can't say I'm overly keen on S+C lasting a long time, in which case you could use a membrane e.g. Intello.

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30 minutes ago, PeterW said:

Sand and cement will do it, smeared on with a hessian sack. 

I knew the answer would be high tech.

 

6 minutes ago, gravelld said:

Is there a section in SPONS about AT? That's where I'd expect it to be.

I find it incredibly frustrating this kind of best practice info isn't easily available.

AFAIK, S+C does the job over most of the wall, then tape onto penetrations like joists.

Can't say I'm overly keen on S+C lasting a long time, in which case you could use a membrane e.g. Intello.

AT means...?

S+C means sand and cement doesn't it?

 

Both @ProDave and @gravelld have mentioned using a membrane (Intello or Protect Barriair)

 Why use one when sand and cement might be all we need ? What does a 'long time mean'? Years, weeks, a generation?

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Sand and cement will cover the pores. Seal around your joists with expanding foam then tape over that. 

 

Put your ceiling noggins in around the edge and tape that joint to the wall  - when you put your ceiling and wall boards on then nothing is getting into that gap ....

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I know it's a refurb job in my case but the same principles apply. Step 1 is tape around any joints or joists.

Step_1.thumb.jpg.b8ca02c9404779ad29a7e458dd64d537.jpg

Step 2 is use a very wet sand and cement mix and apply like it's a very thick paint with a sweeping brush or similar. It can be messy as it's wet and can go everywhere. It will only be 2-3 mm thick but that's all it needs to be.

Step_2.thumb.jpg.c4753f70060fe057f7f9d5f8d001bd06.jpg

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OK folks next step...

I've been told that I need to make sure that the wall must be as flat as I can get it (where it butts up to the floor boards)  so that when they come to fit the flooring, the fit will be really flush. Hmmm.

It looks as if the workflow can be;

  • Fit the joists (POSIs)
  • Tape round the edges,
  • Sloppy sand and cement wash over the area
  • Fit the floor boards

 

 

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10 minutes ago, recoveringacademic said:

OK folks next step...

I've been told that I need to make sure that the wall must be as flat as I can get it (where it butts up to the floor boards)  so that when they come to fit the flooring, the fit will be really flush. Hmmm.

It looks as if the workflow can be;

  • Fit the joists (POSIs)
  • Tape round the edges,
  • Sloppy sand and cement wash over the area
  • Fit the floor boards

That's right. The last step I'd suggest is putting in some acoustic mastic sealant (or similar - could be tape) between the floorboards and the wall. The timber floor will expand and contract at different rates to the blockwork and (presuming these are first floor joists) you'll want to keep the noise transfer down. While the gap might be covered by skirting to correct it visually it's not a great seal for reducing noise transfer. You don't want to hear a TV downstairs when trying to sleep.

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It seems to me that the only challenge to the sand and cement scenario is the likelihood of cracking if there is any movement / flex so anywhere it might move / flex, like round the ends of joists (differential expansion / contraction) use a membrane and tape to ensure cracking does not harm the air tightness. 

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3 minutes ago, MikeSharp01 said:

It seems to me that the only challenge to the sand and cement scenario is the likelihood of cracking if there is any movement / flex so anywhere it might move / flex, like round the ends of joists (differential expansion / contraction) use a membrane and tape to ensure cracking does not harm the air tightness. 

 

This.

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If rendering/plastering onto something like aircrete blocks (e.g., Thermalite), then consider starting with a thin PVA mix beforehand (something like 25% PVA to 75% water, I think, but don't quote me)

Edited by richi
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Parging with a wet sand/cement mix (lots of cement) is a standard detail.  For big areas use a soft broom.  It works well, and has been used for years to seal up leaky brick/blockwork.

 

A web search on "parge coat" will reveal more than you ever wanted to know about it...................

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44 minutes ago, recoveringacademic said:

Hmmmm, a bit of me is wondering how I test  a test patch? I mean, how do I know the test patch works?

 

Get a 12" long piece of 32mm waste pipe, a camera, stopwatch and piece of purple fabric. 

 

Ask Debbie to hold the camera and the stopwatch. 

 

Cut a hole in the fabric and place it over your head with the hole over your cheek. Place your mouth firmly over the pipe and then put the pipe on the wall test patch. 

 

On Debbie's count, deep breath and blow hard into the tube. At 10 seconds, Debbie should take a photo of the colour of your cheek against the cloth....

 

Post the photos on here and we can provide the expert view of which is best based upon the colour differences.......

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