Cavity Closer / Thermal Bridging

qazxsw

Hi folks,

Was talking to my block layer the other day and he mentioned using cavity closers or a block on the flat at the top of our walls in a new build.

Am I correct in thinking that a cavity closer block stretches the full width of the wall, i.e. that it could lead to thermal bridging -

I was thinking of going with the block on the flat option and keeping the block shy of the inner leaf i.e. a "step back" so that I could put some insulation covered with mesh against the block to avoid creating a cold bridge.

The mesh to hold the plaster in place in case anyone wondering...

any opinions ??

gregos

You're thinking right, but instead of making something yourself you can buy insulated closers which will do a much better job. L-shaped closer blocks are really no longer acceptable.

rooferPete

Hi,

Unless I'm reading this wrong you should not have a problem with thermal bridging at the top of the walls.

My theory being that part of the wall will not be exposed to the elements because your facia and soffit should be below the top of the wall.

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gregos

I actually read it wrong. Thought he was talking about cavity closers at opes. Fool. However, I have to disagree slightly with Pete on this. Even though the very top of the wall is not directly exposed to the elements, the outer leaf is going to be cold. You don't want it to have any contact with the inner leaf because it will present a path by which heat can escape from the house. (Or by which cold can get in, depending how you look at it.) This is the way things are going: we can no longer get away with stuff like casting big ignorant band-beams across the cavity. It's all about the Kyoto protocol.

qazxsw

will be going with the "hand rolled option" as it will be faster + probably cheaper - blocklayer has started with the blox on the flat stepped back from inner leaf, I'll just get a few sheets of kinspan/xtratherm and rip them to correct size with circular saw and fit them myself later after the roof is fitted

I can see the point of both the previous posts, the header block wouldn't be directly exposed to the elements but would be colder due to ventilation slots in the facia/soffit ?

gregos

qazxsw wrote:

the header block wouldn't be directly exposed to the elements but would be colder due to ventilation slots in the facia/soffit ?

Would be colder due to the fact that it's the outer leaf.

qazxsw

pics of what I'm talking about are on http://www.garrettshouse.blogspot.com/

gregos

The fact that the inner leaf is in intimate physical contact with the header block means that you have a pathway to draw heat out of the house. The inner leaf will be colder than it should be near the top, and might even display condensation and mould growth later.

rooferPete

Hi gregos,

I find this interesting, so I am not arguing with you just interested in your theory as you know I love the theory especially when it has to be converted into practice

As I read it this is your theory :

If the cavity closer is bearing mainly on one leaf that would mean the the block is bearing approximately 25 > 30 mm on the other leaf.

If the main bearing is on the outer leaf which is the colder of the two then the cold will have to permeate through 225 mm of 100 mm concrete block before it effects the inner leaf.

Taking into consideration the inner leaf is warmer that would mean there will be a dew point within the cavity closer ?

If the load bearing is reversed and the main bearing is on the inner leaf then the heat from that leaf will will travel across the 100 mm thick by 225 mm block, effectively just moving the dew point.

I believe that if good building practice is followed the dew point would be so negligable it would not register on an infra red scan because the roofspace which begins below the top course of blocks must be ventilated.

Therefore the ambient temperature outside the building is going to be transferred into the roofspace by means of correctly designed ventilation.

The temperature inside the house should not really have any bearing on what happens within the roofspace provided the insulation is fitted correctly.

Also the insulation in the wall should be tight to the inner leaf of blockwork preventing any heat from within the building meeting with the colder air outside.

The only way I can see condensation forming is if the ventilation does not have a true flow and / or the insulation is insufficient or there is an area like a bathroom / ensuite / kitchen ventilating into the roofspace instead of to the outside of the building.

For example the extractor from a small ensuite is vented by way of a tile vent using a PVC pipe connected between the fan and the vent but we don't see condensation forming on the inside or the outside of the pipe within the roofspace.

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gregos

Hi Pete.

I wasn't actually thinking of condensation at all. I was just saying that if a part that's warm (the inner leaf) is physically connected to a part that's cold (the outer leaf), there's going to be a transfer of energy from the warm to the cold. And then I was saying that if this can be avoided, so much the better.

Also, as I understand it, condensation can occur at a range of critical temperatures, depending on the temperature of the ambient atmosphere and its degree of saturation. Therefore, if there is a cold surface in a warm room, it's likely to give rise to condensation.