2022/3 changes to the regs and garden rooms

[jpadie]

I know that it will often be the case that a garden room will be exempt from the u-value requirements but I'm interested in seeing how I might be able to comply.  The construction will be a timber frame and suspended floor. 

 

When I use the calculators supplied by celotex and the like the results come back that there is no product capable of meeting a u-value of 0.13; and that's using joists that are almost double the depth that I need.   I could insulate over the floor-joists as well but they don't provide a calculator for that but working around the problem isn't really the purpose of the question here ... which is:  is there any practical product that meets the regs for suspended timber floors? if not are we in a world where all new construction will be on a solid slab (my house is 100% suspended timber flooring)?

 

 

 

 

Edited August 17, 2023 by jpadie

[Marvin]

Hi @jpadie

 

Basically I insulate suspended floors with 100mm PIR (Celotex or similar) under the floor joists and 100mm in between the joists with 22mm 22mm moisture resistant floor boarding above.  If the joists are 47mm wide by 97mm high at 400 centres then you should achieve a U-value of about 0.115. To work the lower PIR has to cover the underside of the joists from supporting wall to supporting wall. Obviously to allow the boards under the joists you have to cut the boards to suitable sizes.

 

The question is how do you fit the PIR under the joists.  I use 25 by 50mm treated batten screwed up with stainless steel screws. However this assumes there is enough free air space under the floor after fitting 125mm of extra materials. In extreme cases I have fitted the PIR this way retrofitting by making hatches in the floor and working like a potholer. As cutting the boards can be a bit hit and miss.

 

If I am putting new joists down then I run 25 by 50mm cut to 25 by 25 and screw it to the sill plates if a thin coat. This means the joints and the PIR between them will cross over the lower level.  Fill any gaps between PIR and walls/joists with foam filler to stop cold air blowing around.

 

 

Good luck

 

M

 

[jpadie]

Thanks @Marvin

I'd not considered another layer _below_ the joists.  

 

currently below the joists is just dirt.  I could dig down further to create the space.  the joists were going to be 4x2 but I've realised that they won't span the distance (3m).  so the choice was either to go to 8x2 or to put a couple of foundation blocks down the centre line and build four separate floor assemblies.  
 

feels like scenario 2 (the 4x2s) is better.  I can see how to make this work by building the floor upside down and then installing the panels (would glue be good enough to secure the boards to each other?    I'd lose the areas of insulation that would have to be cut out for the foundation blocks but gain the u-value of the concrete in lieu.

 

quick thought: i'd assumed that e.g. celotex 100mm would not fit into a 4x2 joist (95mm) as it was not compressible enough to be secured in that space.  is that correct?  

 

 

[Marvin]

Hi @jpadie

 

I think there are a mix of designs being discussed here, which depending on the requirement and ground conditions, dictate what is best.

 

If it is a garden room and building regulations don't apply I would go for a raft foundation.

 

M