Limited height and ground floor Insulation Dilemma

[mickeych]

We're still finalising details on our barn conversion, but in conversation with our SE and we have a bit of a dilemma since we have a very restrictive amount of total height available to us and this appears to have a significant impact on the amount of insulation we can put in below FFL (see his sketch attached). We are trying to avoid having to underpin the whole building and so I really need some of your combined experience and opinions to consider how we can achieve maximum insulation under the FFL.

[Conor]

What's making up that 375mm build up? Is there an existing concrete slab you're building on?

[mickeych]

No. There is no slab at the moment, so we were discussing how much could be scraped out without causing a problem. Had trial holes dug to test the ground. It looks like the barn was built on a shallow layer of hardpacked rubble so not a lot of foundations to play with

[Dave Jones]

dig down as far as you need to get 300mm of XPS, conc in around the pillars at same time.

 

IT just shows the utter madness of the system, they actually want to prevent you insulating below ground because it will effect who exactly ? No one. Just a rule on a pieve of paper somewhere written by a faceless unsackable cretin.

 

Can see why the french shot en mass thousands of the ones in charge.

[markc]

You don’t need to underpin if you are not going below the underside of the existing founds. 

[Hastings]

I have twice now put new insulated concrete floors into old buildings that have no founds to the walls and both times we had to dig below the level of where the old walls 'stopped'.

 

With agreement from the structural engineer (who was initially prescribing underpinning) we stayed away from disturbing the solum under the old walls mainly by feathering/rounding the lower outer edges of the new inner slab/foundation. At the same time we also raised the new FFL (relative to the original) a little, which we worried about at the time would spoil the space but in practice we don't notice it now. The finished ground floor ceiling height is slightly less than 2.4m.

Edited September 17, 2023 by Hastings

[mickeych]

This sounds very logical. How did you practically feather-in the lower edge of the new inner slab and can you remember what sort of space you were dealing with? Our challenge is the height of the timbers in what will be the 2nd floor as even if we are super creative we are struggling to get more than 1.8M under the main timbers (which are the original elm beams and have some listed restrictions on so can't be changed)

19 hours ago, Hastings said:

I have twice now put new insulated concrete floors into old buildings that have no founds to the walls and both times we had to dig below the level of where the old walls 'stopped'.

 

With agreement from the structural engineer (who was initially prescribing underpinning) we stayed away from disturbing the solum under the old walls mainly by feathering/rounding the lower outer edges of the new inner slab/foundation. At the same time we also raised the new FFL (relative to the original) a little, which we worried about at the time would spoil the space but in practice we don't notice it now. The finished ground floor ceiling height is slightly less than 2.4m.

 

[mickeych]

22 hours ago, Dave Jones said:

dig down as far as you need to get 300mm of XPS, conc in around the pillars at same time.

 

IT just shows the utter madness of the system, they actually want to prevent you insulating below ground because it will effect who exactly ? No one. Just a rule on a pieve of paper somewhere written by a faceless unsackable cretin.

 

Can see why the french shot en mass thousands of the ones in charge.

I like the French analogy, but go gently on me as I'm new to this!

 

Are you suggesting stepping back from the outer walls to dig down, or just going down in line with the outer walls? Would you then create the pad on which to place the XPS and then creating the layers beneath the FFL?

 

[joth]

23 hours ago, Dave Jones said:

dig down as far as you need to get 300mm of XPS, conc in around the pillars at same time.

 

IT just shows the utter madness of the system, they actually want to prevent you insulating below ground because it will effect who exactly ? No one. Just a rule on a pieve of paper somewhere written by a faceless unsackable cretin.

 

Can see why the french shot en mass thousands of the ones in charge.

 

I won't argue with 300mm of insulation if you can do it, to add some retrofit context we achieved a certified passivhaus retrofit with "merely" 160mm of insulation under the new slab for existing floors and 200mm under the new extension floor. Our SE / BCO were a bit OTT in the amount of depth of new concrete demanded under the insulation - if attempting it again I'd definitely try and rebalance that as others have shown is possible.

 

[Hastings]

We went airtight but weren't attempting PassivHaus level of insulation just current (2018) regs plus a bit more, where it was doable hence only 100mm underfloor insulation.

 

The drawing shows roughly the average situation along 30m of existing 4 walls - but the sections where it was 'worse' were not long enough to worry about. It helped that the subsoil on which the old walls were built was gravel based (raised beach).

 

The internal stud frame carries the weight of the new upper storey, hence the need for thickening of the slab at the perimeter. The engineer originally proposed that the new roof also rest on the new inner stud frame and that would have required even deeper digging and concrete. I vetoed that proposal in anticipation of the old walls being very shallow and I am very glad I did.

 

The thickened reinforced concrete perimeter was wider than shown in my sketch. Sketch is not very to-scale.

 

(Airtight and damp proof layers not shown, for clarity)

 

Edited September 18, 2023 by Hastings
Afterthought

[mickeych]

OK got it. That looks and sounds remarkably similar to what we are discussing with the SE. The piece that we are struggling with at the moment is how far down we can dig as this dictates so much of the build. Do you remember just how far you dug down at the deepest point and what depth of concrete you maxed out at beneath the stud footings?

We did start to discuss how the roof is supported but not sure that we ever agreed which route to take. Why did you veto your roof being suspended from the inner stud frame since you were building the concrete base anyway? Did you have to beef up the old walls to take the new roof structure? 

 

Our SE is excellent (very experienced with old barn refurbs) and has already made a lot of valuable contributions but I suspect that the levels or airtightness you have achieved are going to be pretty tough for us to achieve in a 250 year old agricultural building, but I have a desire to get as close to PH as possible.

[Annker]

I'm working out a foundation design for similar cellar excavation, and its a struggle to get much detail online.

Two tibbits perhaps of interest to you

• The SE design needs to perform under 2 conditions. First during the construction phase and then during the final condition post-construction.
• Obviously subject to soil types, structural bearings and the difference between external GFL and internal FFL (existing and proposed). The general rule of thumb allows general cellar excavate (In contrast to "hit and miss" 1 metre sq pits when undermining/underpinning) is to permit ground excavated above a 30deg angle away from the base of foundation, (I recall some sketch suggesting as deep as 45 deg).     

[mickeych]

2 hours ago, Annker said:

I'm working out a foundation design for similar cellar excavation, and its a struggle to get much detail online.

Two tibbits perhaps of interest to you

• The SE design needs to perform under 2 conditions. First during the construction phase and then during the final condition post-construction.
• Obviously subject to soil types, structural bearings and the difference between external GFL and internal FFL (existing and proposed). The general rule of thumb allows general cellar excavate (In contrast to "hit and miss" 1 metre sq pits when undermining/underpinning) is to permit ground excavated above a 30deg angle away from the base of foundation, (I recall some sketch suggesting as deep as 45 deg).     

Thanks. That seems to make sense and aligns with @Hastings drawing