Hi,

we're (hopefully) soon to have some renovation work start on our home. As part of the work all rooms that are involved will have the floor insulated (100mm K3 Kooltherm) and UFH (Polyplumb overlay) installed. The work does not include the kitchen however which is where the condensing combi boiler is situated.

Our plan was always to rip out the old kitchen (already in place when I moved in 8 years ago) and replace with a new one of our own choice and use this as an ideal time to install the UFH along with the manifold etc in there first. At the moment the floor in there (suspended timber) is not insulated so i'm planning on doing this part of the work myself, ie prior to UFH installation. I'd looked into how to do this about 12-18 months but just wanted to run my plan of action by people who are far more experienced when it comes to this kind of thing.

First up, the K3 Kooltherm is expensive and with the boards being 600mm wide there will be a lot of wastage, ie joists are approx 370mm apart. I was told I couldn't glue spare pieces together to make a new one so I figured i'd be throwing 1/3 of the stuff away which seemed like an awful waste. Because of this I looked at Space Blanket as an alternative. The 200mm rolls have an R Value of 4.5mK/w which if I understand the sums correctly would equate to a U value of 0.22 which is what is specified in the building spec for the 100mm K3 Kooltherm. So in effect I get the same level of insulation for far less cost. One thing that I also wondered about was which way to face the silver coated side of the space blanket. In my loft it faces up so I imagine it would be the reverse under my floor, ie facing down to the ground. Anyone know if this matters either way?

So..... I remember reading about suspending the insulation with netting (nailed to the underside of the joists as support) but have heard that this can sag and reduce the effectiveness of the insulation. With this in mind I wondered if there was any reason I couldn't use the more rigid plastic type fencing (usually green) you get in a roll from garden centers etc to prevent any sagging? I was thinking that wouldn't sag at all if firmly secured to the underside of the joists meaning the 200mm of roll would be slightly compressed into the 175mm joists height and hence touch the floor surface above it.

Finally the UFH install guide says to install the damp proof membrane on top of the flooring prior to laying the overlay boards and piping. I think some people have put this membrane below the floorboards and above the insulation. Would having this membrane over the floorboards rather than under it cause any longterm problems with damp/condensation or do I even need to do both?

Thanks in advance......

Tony, why do you advise against overlay?

I'll have a look at the multifoil thread when I get time later, but the foil on the space blanket is basically cooking foil over 200m of rockwoll/glass fibre insulation. I was opting for this over regular glassfibre/rockwool insulation due to the glassfibre being enveloped and being far more installer friendly because of it.

I did wonder about using the smaller pieces of K3 but was told that wastage was a fact of life with the 600mm boards. I've even rang the kingspan techline and they say there's no way they recommend to join the boards. I did ask if you could compress two pieces between the joists and they said yes BUT supporting it could be a problem, which makes sense. I've since been looking at Thermasafe styrofoam (PET) insulation. I can get even better R & U values @ 200mm with that so i'm also tempted to go down that route instead.

Thanks for the comments Tony. Our ceilings are 2.6m so we're happy to lose approx 23mm due to the install. I'll have a chat with the plumber regds costs though. As for underfloor maintenance, regular UFH would be even harder to take up wouldn't it as the piping is laid in screed between the rafters isn't it?

To follow on from Tony's last comment. A UFH supplier we have spoken to suggests batons fixed to the joists to carry EPS insulation with the UFH pipes fixed to it as per tony's sugestion. They then recommend 20mm screed to level with top of joists to evenly distrubute the heat. This is in place of the diffusion plates used by other suppliers I would imagine?

20mm of sand and cement screed will crack up. The minimum I have seen used is 65mm. It is more sensible to use 75mm with fibre reinforcement, though I do not see the point in any screed at all

Sorry, too much alcohol last night, I did not fully read the thread I assumed we were talking about insulating completely over the top of the joists, which would be just as daft.

Tony/Mike
had similar questions regarding the screed and will be asking them about it.

Having just read moogaloos comments, checked the Chelmer website and they recommend 'a dry sand and cement' fill around the pipes in some situations?

This site has some picks of lytag being used for UFH over PUR board with pipes from chelmer

It is the last third you are interested in

http://www.staff.ncl.ac.uk/roger.broughton/newhouse/miniblog/miniblog.htm

strawberry netting

I would use polythene not a breather membrane laid over the joists and trapped behind the skirtings.
I dont think it is right to use a breathable membrane on the warm side of the insulation you want no chance of condensation in the insulation.

Yes it is just thick polythene, but so would much thinner polythene

<blockquote><cite>Posted By: sweevo</cite>

I was intending to suspend the insulation using netting unless someone had a better suggestion.

</blockquote>

Would it not make the insulation more effective if you used something that stopped air movement across the underside of the insulation, but still allowed any moisture to get out? Maybe geo-textile fabric - it's fairly cheap.