Help with thermally broken lintel.

[Lemna gibba]

I feel that I'm spamming these forums with different questions, but have really appreciated the advice that I've been given. 

 

We are building a single story extension. It will be block and brick with a 150mm cavity. We will have large 3.8m sliding doors. The extended part of the room is about 5.9m wide and extended by about 2.4m. In case it's important it will have a 10 degree roof with a 1.2 x 0.8m roof window. We've got triple glazed sliders with a uValue of 0.8.

 

We are trying to be very efficient with this build. We have specified Celotex Thermoclass cavity wall insulation, good insulation in the roof and lots on the concrete slab. Our SE made the calcs ages ago and specified a 152x152x30 UC with 8mm bottom plate.  At the time I didn't realise about cold bridging. We're at the stage where we can address this. The rest of the house is mostly solid brick. As we will wrap EWI around this, I'm not concerned about the beams for that part, but I am thinking that having cold bridging above the expensive sliders may well negate the effect of the very expensive triple glazing. 

 

I asked our SE about this. We paid him for the calcs ages ago, so he didn't have to come back on this. He suggested that we reached out to a specialist lintel provider. I reached out to Catnic, and they said the catologue price would be nearly 4k! They specified an extra heavy duty one.  I'm going to try Keystone next. But at this rate I think this is going to be too expensive. I understand uvalues well, but am having a problem understanding how much of a difference a thermally broken beam would make. If anyone has any input, either on how worthwhile or the best way to get one specified then I would really appreciate this. 

 

 

 

[Lemna gibba]

As an update Catnic have a thermally broken lintel but at the cost of 2k from a builders merchant. This seems beyond our price range. Keystone have suggested  this beam XCFS/K-150 , but looking at it I can't really see the advantage of it over the UC with a bottom plate that was originally proposed. Birtley don't have thermally broken lintels that would make a 3.8m span. I understand that the detailing around split lintels is complex, but this remains an option. 

 

[George]

Would you be able to have 25mm internally insulated plasterboard on the underside reveal? This would mitigate much of the small thermal bridge and unless it's a passive house I wouldn't be overly concerned. Even triple glazed windows are a pretty big thermal loss!

 

If you having 100mm cavities you will likely need internal insulation anyway to meet the newer regulations.

 

Edit - noticed the image shows 145mm cavities.

 

Can you get a outer leaf catnic CCS channel channel lintel to work? No need for brick slips then.

Edited August 15, 2023 by George

[joe90]

With my bifolds (not quite 3.8m) I used two concrete lintels, one in each leaf and cut brick slips to stick to the outside lintel, you would never see they were slips and no thermal bridging at all 👍

Edited August 15, 2023 by joe90

[Big Jimbo]

Also, my understanding is that a window frame should be sited as far in from the outside, as to have most of the window frame within the insulated cavity. If the window frame is 70mm thick, and you are using a standard 150mm cill, you are going to want a 20mm cill overhang to the brickwork. That means that at most your window will be 30mm into the cavity, and 40mm outside. Now if you are rendering, you will have even less, and if you are using a batten and cladding, even using 180mm cills, chances are none of your window frame will be within the cavity. A huge cold bridge. We need to start making window cills a lot deeper.

[Big Jimbo]

2 minutes ago, joe90 said:

With my bifolds (not quite 3.8m) I used two concrete lintels, one in each leaf and cut brick slips to stick to the outside lintel, you would never see they were slips?

I've done that before. Stuck them on with Sticks like shite.

[joe90]

Just now, Big Jimbo said:

I've done that before. Stuck them on with Sticks like shite.

I used floor tile adhesive.