Insulated Slab

elfoley

Is the benefit to be gained in installing 50 mm insulated slab over 38mm slab very significant?Is it worth the extra cost? Insulation in cavity wall is 60mm kingspan.

sinnerboy

Yes . In a detached house ( assume this is your case ) the single biggest surface are of the building through which heat escapes is usually the external wall .

The extra thickness of insulation take you from a u value of 0.19 to 0.17
This is probably the easiest upgrade to your spec you can make - far easier than assessing the m+e spec ( solar/HRV/heat pump/ condensing boiler etc )

And unlike any of the m+e kit - the investment an additional insulation will not require on going maintenence and eventual replacement .

For a more complete answer , tailored to your project you should engage a BER assessor

gally74

folks,

i have 60 mm in the cavity aswell,

i have now being told that i should pump the remaing 40mm as it would be better than dry lining,

?

sinnerboy

Basic difficulty is that you can't visually check that the pumped in insulation is filling all the cavity everywhere

In high exposure areas - coastal for example or in highlands there is a risk of water pentration when you "lose" the cavity

Many certified systems insist on a min 50mm cavity

Now I'm not saying don't do it - just be aware of the pitfalls .

I admit I have no personal experience of using pumped in insulation

sydthebeat

sinnerboy wrote: »

Basic difficulty is that you can't visually check that the pumped in insulation is filling all the cavity everywhere

In high exposure areas - coastal for example or in highlands there is a risk of water pentration when you "lose" the cavity

Many certified systems insist on a min 50mm cavity

Now I'm not saying don't do it - just be aware of the pitfalls .

I admit I have no personal experience of using pumped in insulation

I have seen a cavity that has 60mm board insulation + 40-50mm of blown in bead insulation. To be honest, only that the price to do this is somewhat economical, i wouldnt be a huge lover of it. The beads certainly do not form an additional 'board' layer and appear to be very haphazard regards to filling the cavity. They join to gether with a glue like substance, but the beads still remain 'squishy' if that makes sense.

I have also seen blown in rockwool and in my opinion this would be a better product. Its pumped in under much higher pressures and this alone means that more area will be filled.

ardara1

gally74 wrote: »

folks,

i have 60 mm in the cavity aswell,

i have now being told that i should pump the remaing 40mm as it would be better than dry lining,

?

Lets do the maths.

you have a 40mm airspace there with a resistance of worst case 0.18 if it's an AEROBOARD EPS type insulaiton (Better if its a foiled insulation)

0.18 resistance is the same as 7mm of pumped insulation - so infact you will only gain 33mm of pumed insulation in reality. 33mm of bead or fibre pumped is the same as adding 18mm of Polyiso to the inside if it's directly on the wall - 13mm if you put it on dabs.

We're talking insignificant amounts of insulaiton here which ever way you choose.

If you're going to do it correctly add 50mm of insulation internally (reduced at window reveals etc) and make sure it's well sealed.

By the way - I don't think that the pumped insulaiton is allowed below 50mm cavity - doesn't fill properly. Be wary of filling the cavity of you have facing brick also.

HAVE YOU LOOKED INTO YOUR ROOF SPACE FIRST OF ALL? - Get plenty of fibre up there first.

sydthebeat

gally, assuming you have 50mm epanded polystyrene in your cavity...
you existing wall u value is 0.56 :eek:

this assumes 20 render, 100 block, 50 cavity, 50 insulation, 100 block, internal skim.

if you pump the remaining approx 50mm with bonded bead this brings the u value down to approx 0.36

if you leave what you have in the cavity and insulate internally with 62.5 composite plasterboard (50mm insulation and 12.5 plaster) then your wall u value drops to 0.3

You should be aiming to reach 0.27 anyway.....

when costs, degree of work, interruption to life etc are taken into account.... this are the factors you need to think about when choosing..

gally74

thanks all folks, yer the best espically syd,

had an insulation guy out the other day. im off to the show in the RDS tomorrow.

the insualtion is 60mm kingspan, i spoke to this chap about air tightness as i am defiently going for heat recovery.

so heres my options,

HRV + dry line exterior walls, 300mm in attic,

HRV + air tightness, 300mm in attic,

im starting to think option 2 is the way,

P

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