Internal wall insulation around first floor joists

[Jdwood38]

Hi all,

Been deliberating over this for some time! Currently renovating a 1930s property with brick unfilled cavity. We've stripped back completely and all is now going back together, suspended timber floors downstairs have now been insulated and next step is the walls.

I was all set (again, after much deliberation!) to get the cavity filled but have bottled it at the last minute due to problems that people have and when I've inspected the cavity have found too many bridges for me to feel comfortable doing it. So as we were going to batten out the walls anyway, I've decided to go with internal wall insulation instead. We are able to insulate right the way up the walls (so 'through' the ground floor ceiling/first floor floor cavity).

I'm just wondering what the correct way is to insulate around the first floor joists? They are set on a wall plate within the inner leaf so the ends will be exposed within the cavity and I imagine will cause a thermal bridge through the insulation. The cavity is vented with air bricks at ground and roof level so I'm guessing there will be enough air flow to stop the build up of condensation on the ends of the joists but what happens where they penetrate the insulation?

Any advice greatly received.

John

 

[John D v2.0]

I guess your main problem would be warm air leaking out around the joists causing interstitial condensation rather than thermal bridges.
You can get all sorts of gaskets from Germany to seal around the timber, or just stuff the area with wool and hope for the best (keep the internal humidity down as much as possible)

 

[^woody^]

There is no thermal bridge where there is an air cavity

 

[Jdwood38]

Thank you both.

John, any idea what the gaskets may be known as (apart from joist gaskets of course!) so I can get searching?

Woody, I'm intrigued to know the reason why there would be no thermal bridge? I assumed the cavity would basically be the same temperature as outside because of the air bricks and that cold could transfer through the joist end?

Thanks again,

John

 

[endecotp]

Stone and steel are good thermal conductors, so they can "bridge" cold into a warm space.

Wood and air are much less good conductors, and having them bridge across your insulation is much less to worry about.

Thermal conductivities, W/mK, approx:

Steel: 50
Sandstone: 1.7
Wood: 0.1
Celotex: 0.022

So the wooden joist will conduct only 4-5 times as much heat out of the building would leak through the same amount of celotex.
For comparison, one steel screw with a cross-sectional area of 5 mm^2 will leak as much as 10cm x 10cm of celotex.

 

[Jdwood38]

Stone and steel are good thermal conductors, so they can "bridge" cold into a warm space.

Wood and air are much less good conductors, and having them bridge across your insulation is much less to worry about.

Thermal conductivities, W/mK, approx:

Steel: 50
Sandstone: 1.7
Wood: 0.1
Celotex: 0.022

So the wooden joist will conduct only 4-5 times as much heat out of the building would leak through the same amount of celotex.
For comparison, one steel screw with a cross-sectional area of 5 mm^2 will leak as much as 10cm x 10cm of celotex.

This is really helpful, thanks. It's certainly been a fascinating journey of learning doing this house up!

 

[John D v2.0]

Thank you both.

John, any idea what the gaskets may be known as (apart from joist gaskets of course!) so I can get searching?

Just search for tescon tape and you'll find somewhere that sells just about all the different airtightness stuff. I got some airtight adhesive from a company in Germany, basically sticks polythene to masonry or timber. It would do for you but I think your problem will be time taken for labour.
If you're skimming your ceiling and you don't have any holes left in it, you can use the ceiling as a VCL instead of putting it at the joist ends. That might be easier and more reliable.

 

[^woody^]

Stone and steel are good thermal conductors, so they can "bridge" cold into a warm space.

Wood and air are much less good conductors, and having them bridge across your insulation is much less to worry about.

Thermal conductivities, W/mK, approx:

Steel: 50
Sandstone: 1.7
Wood: 0.1
Celotex: 0.022

So the wooden joist will conduct only 4-5 times as much heat out of the building would leak through the same amount of celotex.
For comparison, one steel screw with a cross-sectional area of 5 mm^2 will leak as much as 10cm x 10cm of celotex.

You seem to be misinterpreting the concept of a thermal bridge. By that description, the whole wall will be a thermal bridge as one side is colder than the other.

There would only be a thermal bridge if a component extended right to the external air from the internal surface

In the OP's case the cavity is a thermal barrier and there is no bridge. The joists ends are in contact with the air cavity (ie a barrier) in exacty the same way as the internal leaf is, so there is no bridge and no problem.

 

[endecotp]

By that description, the whole wall will be a thermal bridge as one side is colder than the other.

His joists are bridging the new internal insulation; the bricks aren't.

the cavity is a thermal barrier

Not a perfect one, else he wouldn't be fitting the insulation.

To me, a "thermal bridge" is any relatively good thermal conductor that bypasses an insulator.
Perhaps you have found some dictionary that defines it specifically as having one end in the open air.

 

[^woody^]

His joists are bridging the new internal insulation

Thats not a bridge.

The cavity is a thermal barrier whether you think it is good or not. In the same way that say, Jeremy Corbyn is still a politician even though he is not that good.

Im just using the Common Construction Term Dictionary (unabridged version) 2017

 

[freddiemercurystwin]

But are you using common sense woody? There is a break in the insulation bubble, whilst it may not be a dictionary definition it is a thermal bridge all the same, that said the air in the cavity will be cold but it will still be warmer than the air on the outside face of the external skin though so I wouldn't worry about it, its renovation and often not ideal.

 

[John D v2.0]

The building regulations define a limiting value for thermal bridges, which to me is the most sensible way as it defines it based on what is important for preventing condensation.
So it doesn't matter if you have wood in a cavity, concrete connected to the ground, or steel with 10mm polystyrene covering it, as long as the u value is better than the limiting value it's not a thermal bridge.

 

[^woody^]

Common sense would indicate that the bubble is the existing air cavity, and that is not being bridged.

There can't be additional layers of internal insulation and then there be a claim of bridging of those layers, when in fact the whole structure on the inside of the existing cavity is isolated, so there can't be by any sense of the phrase, a "thermal bridge".

The issue is, is there a barrier between the extenal air or the outer most component that prevents heat travelling by conduction through the material to the inside? And the answer is yes, a clear air cavity. So there is no thermal bridge. That is the concept of a thermal bridge. No if's or buts ... and no googling for obscure reasons to the contrary.

 

[freddiemercurystwin]

Ok woods!

 

[^woody^]

Ok woods!

You always post that when you can't put forward a suitable counter argument