Floor insulation - under or over

[freddiemercurystwin]

True, but to be fair, I was responding to the claim that you actually did'nt need to use any insulation. The regs are also there to give peace of mind to a future purchaser and the confirmation that their house has been built to high standards and someone hasn't just plastered over the cracks, as it were.

Understood and apologies if my post came across in the wrong way. I agree with your and others comments later on in the thread and agree that insulation is a good thing. However, I guess that my concern stems from the fact that much of what is written relates to a 'new build' in which case it is relatively easy to implement all the various aspects of insulation.

But if you take the case of my property the rafters (made of oak) are about 80mm thick. The rooms are small. Previously there was no insulation between the lath and plaster and the sarking felt. Following the regs to the letter, my impression is that there needs to be an airgap of 50mm between any insulation and the underside of the felt? So that would leave me with 30mm of insulation before I start to encroach in the (already) small room.

If you then start talking about thermal bridging of the rafters (and does anyone know the relative thermal resistance of oak compared to softwood?) I get the impression that the BCO would like me to fix 125mm of Kingspan across the whole of the underside of the rafters.

Is that 'reasonable'?

There is a single document now that relates to work on existing buildings only (Part L1B), this refers purely to existing dwellings only not new, theres also a disclaimer in the said document that states you must do what is reasonable, if installing that thickness of insulation under your rafters would make converting the loft impractical this would be a clear argument that it is not reasonable. Have you actually read the regulations? The building regs are (believe it or not) not neccesarily cast in stone and most BC officers are a bit flexible. If you've got a real stickler thats a shame and unusual.

 

[londonman]

There is a single document now that relates to work on existing buildings only (Part L1B), this refers purely to existing dwellings only not new, theres also a disclaimer in the said document that states you must do what is reasonable, if installing that thickness of insulation under your rafters would make converting the loft impractical this would be a clear argument that it is not reasonable.

Have you actually read the regulations?

Not completely but I've had a good stab at them.

The building regs are (believe it or not) not neccesarily cast in stone and most BC officers are a bit flexible. If you've got a real stickler thats a shame and unusual.

I'm hoping that this is the case (that they are flexible). A telephone conversation to one of the surveyors (at least that was how the receptionist described him) did seem to quote the regs exactly.

Hopefully when the BCO comes out for his first visit, these things can be ironed out. Fingers crossed.

 

[freddiemercurystwin]

Fingers crossed.

And everything else too!

 

[londonman]

Sorry...one more question.

If the insulation is below the concrete slab then I can see the benefit of having vertical insulation around the perimeter of the room.

But if the insulation is on top of the slab then I can't see the need for any vertical insulation. Is this a valid statement?

 

[freddiemercurystwin]

yes

 

[DenzB]

High thermal mass (using the concrete slab as a heat sink) prevents overheating in summer and is more responsive to ambient conditions. Don't forget the floor slab is not being heated in isolation, the walls will also be charged during the heating period. Also the heat will not be 'lost', providing there is insulation below, the heated slab will release it's heat slowly during the cooling period. Buildings with low thermal mass i.e. timber frame walls, suspended timber floors gain and loose heat more rapidly making for more extreme temperature fluctuations.

 

[DenzB]

Edge insulation is as important as floor insulation elsewhere. This prevents a cold bridge at the most vulnerable point. If you omit edge insulation you undermine the performance of the floor insulation (whether under or over the slab). Don't skimp on this, the cost is minimal compared to the heat saving over the lifetime of the building.

 

[freddiemercurystwin]

Edge insulation is as important as floor insulation elsewhere. This prevents a cold bridge at the most vulnerable point. If you omit edge insulation you undermine the performance of the floor insulation (whether under or over the slab). Don't skimp on this, the cost is minimal compared to the heat saving over the lifetime of the building.

Do you work for an insulation company?