Airtightness In Refurbishment and Extensions

[Adrock]

Prior to my finding this place I embarked on the lower ground conversion and extension. The extension is a masonry cavity wall, the original house is a solid brick construction.

 

In the lower ground I've installed a type c cavity drainage membrane and I'm going to be installing internal insulation throughout the lower ground, overlapping slightly the new cavity walls. The extension has maybe 30m2 of bare thermalite blocks after the waterproof membrane was installed, from what I've read I around here they'll need some kind of airtightness detail to help with that.

 

What I'd like is some pointers for how best to achieve an airtight finish within the environment I've got. Any areas in particular I'll need to look at closely?

[Redbeard]

I like drainage membranes better than I like trying to make tanking slurry work, but they can have their issues. Firstly, before lining-out consider 'closing' the top of the membrane. The training I went on with a supplier didn't suggest this, but I found that the % moisture content of some of my joist ends increased after the installation. External ground level was a little below the bottom of the GF joists, so for some of the area I was able to use primer and butyl tape to 'close the environment' behind the membrane, so that the only route would be down for mainly liquid water, not upward (towards vulnerable timbers) via the open top of the membrane.

 

That's one issue, and another (formerly addressed by at least one system provider) is that failure to provide full vapour control around the full perimeter of the insulation can leads to condensation on the inner face of the membrane behind the insulation. If the wall membrane is taped to the floor membrane (again, as suggested by a number of system providers) there is a risk of a 'pond' that your board floor might sit in. The alternative suggested by the system provider was to run the wall membrane down below the floor membrane so that any moisture on the 'wrong' (inner) side of the membrane can at least escape into the perimeter drainage channels. But this in effect 'legitimises' interstitial condensation which is, at very least, a bit hard to get used to.

 

Air-tightness here can be tricky to achieve. Floor is easy - VCL laid over the insulation - and that can join the foil on PIR or A.N. Other VCL, but a conventional VCL at ceiling level will be a VCL on the wrong ('cold')  side of the insulation sandwich for the (GF) room.

 

1 hour ago, Adrock said:

The extension has maybe 30m2 of bare thermalite blocks after the waterproof membrane was installed,

 

Installed where? Has the extension got a drainage membrane? Surely not? Can you clarify?

 

Can you provide more detail please - words and drawings?

 

More responses when I have a full understanding of the circumstances.

 

 

 

[Adrock]

21 hours ago, Redbeard said:

Installed where? Has the extension got a drainage membrane? Surely not? Can you clarify?

 

I thought it might be difficult to explain on my phone without access to the drawings.

 

 

The above shows a snapshot of the extension, the back garden is circa 1.7m lower than the street level at the front of the house. We have constructed a reinforced concrete retaining wall and built on top of it. The waterproofing membrane has been installed to be above the DPC for those walls. My intension is to install the internal wall insulation to "overlap" the cavity wall insulation, as best as I can.

 

The elevation facing the back garden is within a small lightwell. Those walls do not have cavity drainage membrane on the majority of them (snapshot below)

 

 

21 hours ago, Redbeard said:

I like drainage membranes better than I like trying to make tanking slurry work, but they can have their issues. Firstly, before lining-out consider 'closing' the top of the membrane. The training I went on with a supplier didn't suggest this, but I found that the % moisture content of some of my joist ends increased after the installation. External ground level was a little below the bottom of the GF joists, so for some of the area I was able to use primer and butyl tape to 'close the environment' behind the membrane, so that the only route would be down for mainly liquid water, not upward (towards vulnerable timbers) via the open top of the membrane.

 

This was something Delta Membranes detailed within their instructions as an option. I have already sealed the tops of all of the membranes and introduced airbricks to ensure there is some ventillation to the cavity, as per their instructions.

 

21 hours ago, Redbeard said:

 

That's one issue, and another (formerly addressed by at least one system provider) is that failure to provide full vapour control around the full perimeter of the insulation can leads to condensation on the inner face of the membrane behind the insulation. If the wall membrane is taped to the floor membrane (again, as suggested by a number of system providers) there is a risk of a 'pond' that your board floor might sit in. The alternative suggested by the system provider was to run the wall membrane down below the floor membrane so that any moisture on the 'wrong' (inner) side of the membrane can at least escape into the perimeter drainage channels. But this in effect 'legitimises' interstitial condensation which is, at very least, a bit hard to get used to.

 

I am intending to install a VCL to the warm side of the internal insulation and have already installed one in the floor, with enough left to join it onto the wall VCL.

 

21 hours ago, Redbeard said:

 

Air-tightness here can be tricky to achieve. Floor is easy - VCL laid over the insulation - and that can join the foil on PIR or A.N. Other VCL, but a conventional VCL at ceiling level will be a VCL on the wrong ('cold')  side of the insulation sandwich for the (GF) room.

 

We have a warm roof on the extension, which doubles as a terrace for the ground floor. The VCL has been installed to the warm side of the insulation on this already.

 

Hopefully some of that clears up the lack of detail in my original post.

 

[Adrock]

I thought maybe a photo might help illustrate what I'm attempting. 

 

You can see the reinforced concrete and block wall behind the cavity membrane. I'm mildly confident the cavity drainage membrane will help with air tightness even before the VCL is installed.

 

My concern is the block wall you can see to the left of the picture. The joists are in joist hangers set on top of the blanks, so they'll also need some attention. 

 

 

Any thoughts?

[Redbeard]

1 hour ago, Adrock said:

Any thoughts?

 

Only that I don't understand what 'the blanks' are in 

1 hour ago, Adrock said:

The joists are in joist hangers set on top of the blanks,

 

Should 'blanks' be 'blocks'?

 

And since you feel that the bubble membrane should be OK for air-tightness are you simply asking how you make the joist ends airtight? If so the best you can do, probably, is to tape them into the membrane, with either a/t tape or the butyl tape which you probably bought from Delta. (Of course you still don't necessarily know what any residual moisture will be doing to the joist ends - if they are behind the membrane - I'm not sure because you say 'on top of the ...?blocks/?blanks'). If I have misunderstood please let me know and explain how I have misunderstood,  and I'll comment further.

[Adrock]

Blanks should have been blocks. The joists sit in the hangers so aren't touching the wall. I'm thinking maybe air tight sealant and then paint with air tight paint on the block wall and around the hangers.