Cold Bridge at eaves

T-Dog

Hi all, in the process of a self build house project at the moment and we are approaching the wall plate level. I have tried to minimise cold bridging around openings and at floor slab etc. in as much as is possible. I had instructed my blocklayer to close the cavity at eaves level with a slate closer but he and my carpenter are concerned that without using a block to close the cavity, the walls will not be rigid enough to take the roof. We are using a 150mm cavity. Would a lightweight block with lower thermal conductivity be an option?

Any direction/opinions would be greatly appreciated.

T-Dog :rolleyes:

FergusD

Talk to your engineer, it's the only solution! I've done exactly that, closing a 150mm cavity with a slate, but had a detailed discussion with the engineer about it.

Regards,

Fergus.

beyondpassive

This is a detail used by a poster to this forum on his house. The inner leaf holds th ewallplate and the allplate is straped down 2 or 3 courses. Get your engineer to do the sums on the lateral loading. The outer leaf shouldn't take any loading.

mickdw

If you end up with the situation where you have a single 100mm block with the wall plate sitting on top of it supporting the entire roof structure, you would be very lucky not to end up with a cracked joint under that block.
THe calculations may say that the loadings dont require the stability of the double wall but imo & being a civil Engineer, the construction method & ease of construction needs to be looked at too. A single line of 100mm blockwork built upright will in many cases work loose throughout the construction process due to wallplate being fixed to it, joisting being knocked against the side of the wallplate when being left up against the side of the house in preparation for installation. Its is absolute lunacy in my opinion & I wouldnt be surprised in a few years to see problems arising from this detail. Perhaps if the roof is designed with an apex beam or whateverto take out any outwards forces at wallplate level, you would get away with it. I will certainly not be using it anywhere.
Whether you go with complete internal insulation or perhaps something like a foam glass block to cross the cavity (although I dont know if these are suitable at that location), anything but that suggested detail. Whoever came up with that idea was imo only looking at it from an insulation viewpoint & I would question whether they ever set foot on an actual site.

sydthebeat

mickdw wrote: »

Whoever came up with that idea was imo only looking at it from an insulation viewpoint & I would question whether they ever set foot on an actual site.

the irish acceptable details are basically a carbon copy of the UKs accredited details.

It is very interesting to note that on the UKs details, every single sheet has the following disclaimer:

"the above indicative guidance illustrates good practise for the design and construction of interfaces only in respect to ensuring thermal performance and air barrier continuity. The above guidance must be implemented with due regard to all other requirements imposed by the building regulations"

Those that published the irish version obviously didnt see the requirement to include such a disclaimer, and now we have building control officers demanding compliance with these details with no regard to the structural integrity of the junction. I wouldnt worry to much when the joist can be tied to the rafter at the eaves, but in storey and a half details this is dangerous.

sinnerboy

mickdw - I am not a structural engineer but I have spoken to a trusted 3 or 4 . You all say much the same thing . I fear for future reports of structural failures . Not so sustainable .

beyondpassive

mickdw wrote: »

anything but that suggested detail. Whoever came up with that idea was imo only looking at it from an insulation viewpoint & I would question whether they ever set foot on an actual site.

Thanks for the feedback guys. The single row of 100 block on the hollowcore does need to be very well strapped down. The strapping takes the lateral loading the block is merely a spacer. True the orthadox block closer detail is redundant because of energy, but lets not get hysterical about alternative approaches.

A Structural engineer can model the scenario mathematically and work out if the detail will or will not fail. I should have mentioned that Varty Eng wall ties which are good in compression also, are needed every block width on the upper most course and straps are needed at the wallplate every meter extending down 3 blocks from the 100x75 wallplate. Bridging should be put above the wallplate and Strapping is also needed. If someone is going to put on an engineers hat to question my site experience can they at least provide some empirical evidence rather than anecdotes and fish wives tales.

mickdw

sydthebeat wrote: »

the irish acceptable details are basically a carbon copy of the UKs accredited details.

It is very interesting to note that on the UKs details, every single sheet has the following disclaimer:

"the above indicative guidance illustrates good practise for the design and construction of interfaces only in respect to ensuring thermal performance and air barrier continuity. The above guidance must be implemented with due regard to all other requirements imposed by the building regulations"

Those that published the irish version obviously didnt see the requirement to include such a disclaimer, and now we have building control officers demanding compliance with these details with no regard to the structural integrity of the junction. I wouldnt worry to much when the joist can be tied to the rafter at the eaves, but in storey and a half details this is dangerous.

Thats very interesting that the uk regs state it like that. Seems there may be some clarifications ahead. Homebond seem to had jumped on board with detail too without querying it.
If there has been 1.5 storey built to this detail & the roof design allowing any lateral force, it will fail.

sinnerboy wrote: »

mickdw - I am not a structural engineer but I have spoken to a trusted 3 or 4 . You all say much the same thing . I fear for future reports of structural failures . Not so sustainable .

There may be trouble ahead.

sinnerboy

And there is also the small matter of the spread of fire within cavities

from here

http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1640,en.pdf

3.3.1 Hidden voids in the construction of a
building provide a ready route for smoke and flame
spread. This is particularly so in the case of voids
above other spaces in a building, e.g. above a
suspended ceiling or in a roof space. As the spread is
concealed, it presents a greater danger than would a
more obvious weakness in the fabric of the building.

Look to Diagram 17 page 87 and think very carefully about the guidance here