Compressive Strength - insulation behind ledger board

[jayc89]

I'm replacing a small-ish section of first floor joists with posi-joists (4.5m x 3.8m), I'm wondering whether I should insulate behind the ledger board before hand, but appreciate the calcs for doing so aren't simple. I came across a Green Building Store blog where they did something similar using Compacfoam 200;

 

 

I'd have thought the point loads in that set up would be quite high, opposed to running a strip the full length of the ledger board and 1.5x the height to distribute the load across the full wall. But, Compacfoam 200 looks to have 10x the compressive strength to EPS 300, which is pretty impressive, and probably explains how they were capable of using it like that. How would you calculate the compressive strength required of any insulation fitted behind a ledger board?

 

[Russell griffiths]

Wood is a fairly good insulator , don’t make that job more complicated than it needs to be. 

Just get that ledger board bolted on solid. 

[George]

The timber ledger acts as a very large washer so the force is fully distributed rather than being a point load. Using resin anchors would mean there's only a nominal tension in the anchor (and resulting compression in the supporting material).

[jayc89]

56 minutes ago, George said:

The timber ledger acts as a very large washer so the force is fully distributed rather than being a point load. Using resin anchors would mean there's only a nominal tension in the anchor (and resulting compression in the supporting material).

 

Presumably the compression behind that big washer (ledger) is great, and would be transferred to anything between it and the wall (insulation), so something like PIR would just get crushed by it? (Hence needing something specialised like Compacfoam)

[George]

Bigger washer = lower pressure

[Gus Potter]

Hi all.

 

On 29/09/2023 at 15:44, jayc89 said:

I came across a Green Building Store blog where they did something similar using Compacfoam 200;

I don't think this is safe for the following reasons. Mind you you can ask ten SE's and you'll get ten different answers.. here are my thoughts.

 

On 01/10/2023 at 08:19, George said:

The timber ledger acts as a very large washer so the force is fully distributed rather than being a point load. Using resin anchors would mean there's only a nominal tension in the anchor (and resulting compression in the supporting material).

Agree that the ledger distributes the load over a number of the fixings. Mainly to account for a few badly installed ones. Also agree that if you use EPS or foam 200 you will get initially nominal tension in the anchors (as the say load increases due to snow) as the EPS will compress as the ledger rotates (and the fixings start to bend)  due to the added eccentricity of the load. If the ledger was hard against the wall, it was 50 mm thick and with joist hangers then the eccentricity would be would be 50mm, maybe a bit less if the hangers are of wrap over type.. If you put a compressible insulation behind the ledger the eccentricity is now doubled to roughly 100mm and this completely changes the behavoir of the connection in a dramatic way. The insulation is not strong enough to deal with the forces until it compresses by a long way and by that time other things will fail.. probably suddenly which is dangerous.

 

An EPS or Compacfoam 200 insulation means that it will often carry a load of 200 kN/m^2 at 10% compression.. thus before it can carry that much compressive load it needs to compress by 50 x 10% = 5.0mm and by that time your ledger will have rotated a lot and dropped a bit. All the bending forces will by that time get transferred to the fixing which is into brittle masonry. The fixing will now be subject to the same downwards loads (call these vertical shear forces) but also a significant bending force due to the compressible nature of the insulation. This causes the masonry to not least crush as the fixing enters the wall and that can reduce the capacity of the fixing by more than 50%. I have only touched here on some of the basics.. the behavoir of these types of stand off connections is much more complex and you enter into this world at your peril.

 

I think your starting point here is to get a feel for the difference in load capacity of a fixing with a stand off (which is what you have) is to go to the Hilti website and see how much difference it makes when you have a a column base plate set above say a concrete pad stone. You can see this often on lighting standards next to motorways / railways where you can see the column base plates set above the bases with the bolts exposed.. you have something similar.

 

Now even navigating the Hilti or similar site and interpreting the load data is going to take a lot of knowledge, but just have a look.. they have some pretty user friendly diagrams. I hope you'll then see the big load capacity reduction and realise that what you are proposing is not safe unless you design for the completely diiferent behavoir.

 

However there is often more to this than meets the eye. When we design a building we want to tie different bits together for overall stability. In your case this could mean that we want to tie the rafters back to other parts of the house so the whole thing acts as a oner rather than individual elements.. we call this as SE's not least robustness (if one bit fails the rest hangs in there) and look at alternative load paths. We think.. just say the builder cocks that bit up.. is there another way the building can hang in there before it falls down? So rather than pushing the connections to the limit we need to introduce a bit of redundancy to account for human and material error.

 

In principle you could make you thermal break using say something like this https://www.armatherm.co.uk/thermal-break-materials/armatherm-frr/..

 

I think it is a waste of money.. just accept you have a bit of a cold bridge and beef up insulation elsewhere to compensate. Also remember that you are fixing into old masonry, maybe bricks with holes in them, for the fixings to work well they need to be away from the mortar beds etc.. you are asking too much here.. in theory it may work but can you build it in real life and make it safe?

 

So to conclude my thoughts are DON'T do this. Fit the ledger against the wall, maybe with a DPC behind.

 

 

 

 

 

 

 

Edited October 3, 2023 by Gus Potter

[NandM]

Is there any reason not to use a ledger board for the both sides?  

 

With my project, I'm thinking about following my getting my brickies to build all the walls and then for (hopefully) me to follow but using ledger boards for the floor joists. (Would this approach cause any problems??)