Cavity insulation / internal boarding - HELP!

Madmak

Not sure where best place is to post this. Have reviewed all of posts but still none the wiser...
I’m the middle of a self-build - air-to-water heat pump, UFH downstairs and upstairs (posi-joist system with screed upstairs).
We have a 150mm wall cavity with 110mm Xtratherm board partial fill. It was originally spec’d with 150mm insulation but for cost reasons at the time went with 110mm, as difference in u-value was negligible. But wondering if we’ve possibly left ourselves a bit short all I seem to be hearing is 150mm full fill? What are the risks/watch outs with this 110mm insulation?

Due to so many different opinions...going around in circles on whether to do sand and cement finish on internal walls OR do we need to add an internal insulated board (35mm). Is this internal boarding necessary?
Very grateful of any guidance here.
Thanks

baby fish

Madmak wrote: »

difference in u-value was negligible

If you based this decision on a calculation then you have answered the question already? - you dont need it because the difference in u-value was negligible?

I have a similar setup in my house in relation to cavity insulation thickness however I have radiators in my house and have put 50mm insulation board on internally, mechanically fixed because I dont like the airspace left behind the slab with the dabb method.

But if I was doing my house again, I would probably fill the cavity just because sand/cement and skimmed finish is tougher on the inside...in saying that, my insulated slabs will last my lifetime and longer..there is nothing wrong with them. The insulated slabs allow my house to heat up fast, like under a half hour - the flip side it probably cools down bit quicker too... I usually have heating on for maybe 1 & 1/2 hours - 2 hours during heating period with a max of up to 3 hours if its very cold. That keeps my house warm.

I say I probably would use full fill cavity insulation , however, would I be happy with it? I dont really know to be honest, maybe id want my insulated slabs back... I know a person who cored a hole through her wall and full fill insulation - the beads came pouring out of the cavity - there was no adhesive used with the beads.

Personally I think a 35mm insulated slab is a waste of time, in reality its 25mm of insulation with a 10mm slab? almost pointless if you ask me.

I'd go 50mm if you going to put on insulated slabs.

Just be aware, if hanging something to the wall you need to consider your fixing when going through an insulated slab - whereas with sand and cement a plug and screw will most likely do.

I think in your situation, you made a decision to go without - stick with that.

Madmak

Thanks Baby fish appreciate that. Fair point on the 25mm insulation. Too little too late perhaps!
Are you aware of any potential issues with the partial fill cavity?
We’re finalising first floor block work at the moment so I guess just want to try ensure we’ve addressed any future issues (to the best of our ability) while there’s still a small window to do so?
Cheers

baby fish

There are no problems with this type of insulation.... your only possible problem is your block layer - he is installing it for you!

He must join it properly at corners, keep tongue and grooves clear of mortar , ensure it's installed properly around door and window opes to prevent a cold bridge, ensure continuation of insulation in cavity up to ceiling insulation(so don't put an old fashioned method of cavity closer block over the cavity creating a cold bridge)

So to ensure you are preventing future problems you need : employ a block layer who knows what he is doing and then know yourself what needs to be done so you can check the work or employ someone qualified to check quality of work on site. Do you not need a qualified person with current regulations to look after the build for you , ensure everything is done properly? If this is being done, there are no problems with this type of insulation.

I'm not sure what the minimum cavity space required is to pump a cavity with insulation is, I think it's 50mm, you could look at pumping the insulation into your remaing cavity if its possible if you are worried about the amount of insulation you have.

In short, make sure the dpc is properly installed around opes, make sure you have no cold bridges around opes, ensure you have continuation of insulation from cavity to ceiling.. ensure the insulation boards are joined together properly - then this type of insulation is perfect.

Calahonda52

baby fish wrote: »

.....

I'd go 50mm if you going to put on insulated slabs.

.

Without a dew point calculation this may be bad advice.

OP: this is the third different post on this topic

sydthebeat

insulated plasterboard internally on a new build is a bad idea. the reasons for this have been discussed numerous times on this forum.

your heat pump pulls your build beyond min regs which allows you to use partial fill board rather than full fill board in the cavity.

my own opinion is that your 110 cavity insulation more than enough to have a comfortable home.
concentrate on your air tightness at this stage