Ideas: Thermal Bridge Free Rafter Connection

[Duncan62]

Could you please help me try to find a Thermal Bridge Free (TBF) connection detail?

 

It's for roof rafters as they connect to a lintel above windows in both cases.

 

I want to avoid metal hangers as these are a tremendous Thermal Bridge.

 

I suggested packing out both sides of the I-Beam (red line) with wood and fastening together, then fastening to the lintel (black lines).

 

SE said no.

 

I'm more of a yes man.

 

[joe90]

I am no SE but I cannot see why he said no, seems very practical to me, would be interesting to hear why he said no 🤷‍♂️. Mind you a thermal bridge only occurs when one side is outside insulation and the other inside!!!

Edited April 13, 2024 by joe90

[Duncan62]

1 minute ago, joe90 said:

I am no SE but I cannot see why he said no, seems very practical to me, would be interesting to hear why he said no 🤷‍♂️.

 

QUOTE:

"that wouldn't provide a sufficient connection as we'd be relying on nails through a plywood web of the beam and also the fixings would be running parallel to the grain/laminate of the packer which could lead to the packer splitting.

 

The rafter must be vertically supported and laterally restrained to effectively prevent the roof from dropping or spreading."

 

[JohnMo]

I am confused what thermal bridge you are trying to fix.

 

52 minutes ago, Duncan62 said:

avoid metal hangers as these are a tremendous Thermal Bridge.

Are they really? How is your roof insulated?

[saveasteading]

I can't see that your suggestion reduces thermal bridging. It increases the surface area in contact.

The SE is right. Some things you should just keep simple and work to standard,  proven methods.

The web is a weak material with only the role of keeping the flanges apart. It is not to be used for fixing things to.

 

The Original question was about the thermal bridging. Can you explain your concern.

[Duncan62]

1 hour ago, JohnMo said:

I am confused what thermal bridge you are trying to fix.

Are they really? How is your roof insulated?

 

32 minutes ago, saveasteading said:

The Original question was about the thermal bridging. Can you explain your concern.

 

Insulation: blown cellulose/woodfibre

 

The problem is that the metal hanger will span the I-Beam from inside the thermal envelope towards outside = thermal bridge. (THERM analysis would show as much)

 

Hanger as below but inverted in my case.

Also, as we're hanging an I-Beam the hanger would indeed span the whole section, not partially as shown on this solid rafter.

[Duncan62]

38 minutes ago, saveasteading said:

I can't see that your suggestion reduces thermal bridging. It increases the surface area in contact.

 

A connection made of wood will form far less of a thermal bridge than a metal one, whatever thickness wood is used.

 

The concern is in the direction: inside the building to the outside of the building.

 

You can hold onto the other end of even a small bit of wood in a fire.

You cannot hold onto a small bit of metal in a fire. The heat transfers. Very efficiently.

[JohnMo]

So your insulation is on top of the rafters?

 

That whole area will need to packed with insulation anyway, so still confused where any size of thermal bridge exists.

[MikeSharp01]

19 minutes ago, Duncan62 said:

Hanger as below but inverted in my case.

Can you actually get inverted hangers like that? You will need to pack the Web out to the hanger so you can get the full support to the sides of the hanger. Is there any insulation on the outside of the vertical beam the rafter cones down to. If not you could insulated along the outside edge with some pu and get a better overall performance for the overall detail so you don't need to worry about metal hangers.

[saveasteading]

The amount of heat transfer through the wood then through the metal will be small.

If somebody could work out that this might cost £5 per annum in fuel, would you relax about it?

Edited April 13, 2024 by saveasteading

[JohnMo]

1 minute ago, saveasteading said:

cost £5 per annum in fuel

I was thinking more like £5 over 10 - 20 years, that whole area needs to be insulated anyway, so no real thermal bridge exists anyway.

[ADLIan]

Can be modelled using 3D thermal modelling (not THERM) but frankly it's such a localised and discrete piece of metal and no doubt will surrounded by insulation it's not worth worrying about.

[Iceverge]

Do you have the original proposed drawings showing the metal hangers? 

 

Thermal bridging is really important once you get to high levels of insulation. I would encourage you to download THERM and have a play. It's not too user friendly and won't give you accurate results like @ADLIan says but it does give you a good feel for the situation. 

 

I suspect it'll put your mind at ease once you see how little point connections like wall ties matter and how much difference even 25mm of insulation makes to break a thermal bridge 

 

 

 

[ADLIan]

Therm won’t do it as it’s a 3D analysis rather than 2D.

[SteamyTea]

19 hours ago, ADLIan said:

Can be modelled using 3D thermal modelling

LISA will do 3D modelling.

And it is free.

 

[NailBiter]

On 13/04/2024 at 11:11, ADLIan said:

using 3D thermal modelling (not THERM)

What is the go to software package for doing this? I've been playing with the tools built into ArchiCad trying to learn how they work but it seems that there are a lot of capabilities missing (or I'm not able to find them due to lack of skill / training).