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Join our Property forum for renovation, DIY, and house selling advice.

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Property/DIY

urgent help needed with loft insulation!

19 replies

Thandeka · 08/02/2011 19:44

We are boarding out our loft but the builder says currently the joists will only give space for 70mm insulation which is obviously inadequate- so he proposes building up the joists with 2by4 and getting it up to 150mm insulation better but still half of the optimum 300mm they reccomend. (currently there is 200mm in there. Alternately he proposes using some stuff called tri iso super 10 which is 30mm thick but gives equivalent of 210mm insulation on top plus using some of our existing insulation on top to give optimum insulation before boarding over.

Price wise- building up joists etc wood and labour is going to be £330 or tri iso stuff £500-600 (but no labour charge). So what to do?

Does anyone know of any similar product to this tri iso super 10 stuff which is cheaper!? help me decide! Any other pitfalls I need to think about?

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Thandeka · 08/02/2011 20:24

anyone? builder needs decision by tommorrow morning. SIgh!

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ILikeToMoveItMoveIt · 08/02/2011 20:27

hang on, will ask dh

Thandeka · 08/02/2011 20:34

cheers ma dear. Am so befuddled by it all...

not reading great things online about multifoils....

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ILikeToMoveItMoveIt · 08/02/2011 20:49

DH here. I'm a builder for my sins and I'd go for fibreglass as its easier to get into all the nooks and crannies.

The regs have recently changed - but used to be for 275mm of fibreglass for what its worth. Also the Tri-iso is a bit contentious with Building Control (the LA body who are responsible for building standards)as it's efficiency is measured in a different way to all other insulation - including fibreglass.

Also Tri-iso (or any other multi-foil) works best on sloping roofs and this is the only situation that Building Control would accept its use.

There is another product which is cheaper for the materials but more expensive to install; Celotex (or PIR board. There are lots of different brands of which Celotex is to PIR what Hoover is to vacuum cleaners)this is a rigid foam board sandwiched between layers of foil and is very efficient - being nearly half the thickness of fibreglass for the same insulation value.

In summary: Fibreglass would be the cheapest option but if you want more info call your local council Building Control and ask them.

ThisIsANiceCage · 08/02/2011 20:57

Note, if you use a rigid material like Celotex between joists, you'll have to go round stuffing all the cracks where it hasn't fitted exactly, otherwise air will just flow around it and you'll lose much of the benefit.

Thandeka · 08/02/2011 20:57

I think we are going to struggle to build the joists up to get much more than 150mm of fibreglass underneath the boarding out bit of loft which isnt enough insulation really..... Hmm but building regs issues maybe avoid the multifoil option then.....

am scared of building control but should bite the bullet.....

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Thandeka · 08/02/2011 20:58

and thanks so much for replying. :)

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DaisySteiner · 08/02/2011 21:00

I don't see why it would be a building regs issue as you're not doing anything that requires building control approval are you??

ILikeToMoveItMoveIt · 08/02/2011 21:01

lol, don't be scared of building control. They quite like the sound of their own voices so will be more than happy to give you advice.

Thandeka · 08/02/2011 21:02

pondering some more- are there any hyper efficient fibreglasses that give you better insulation for thinner thicknesses? (am guessing this is probably a not really since air is one of the best insulators around!

Shall I just get a few 100 metres of bubble wrap and silver foil and make my own. ;)

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Thandeka · 08/02/2011 21:03

we are doing other stuff in the house that probably does (installing an upstairs loo) so I need to contact them though the builder assures me he knows all the regs and is making sure it meets them.... but because the work is happening very fast we havent done anything about building control yet (is that really bad?)

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ILikeToMoveItMoveIt · 08/02/2011 21:04

Building control will give you free, impartial and professional advice. They aren't just there for BC approval.

ILikeToMoveItMoveIt · 08/02/2011 21:09

My last comment was to Daisy - just incase you are wondering what I am whittering about.

Sorry I haven't got anymore building advice as dh has sodded off out.

Ask building control about hyper efficient thin fiberglass. I'm sure there probably is, but you have to pay for it

If you're scared (!) of building control, just mention it is work you are thinking about doing, rather than in the middle of.

ThisIsANiceCage · 08/02/2011 21:14

I've never tried these, but there are also a few types of rigid insulating board designed to be laid instead of chipboard.

Some here, tho they don't like high insulation performance.
Also here, look better.

Thandeka · 08/02/2011 21:34

thanks all very helpful.
will ponder some more and then make DH make the final decision so I can blame him

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jeanjeannie · 09/02/2011 08:03

You could do something similar to us - DH is a builder (and he nodded in agreement with everything Iliketomoveit's DH said!) and we built up to the joist with insulation and then laid thick insulation boards over the top...then laid the floor over that so creating a floating floor. Howver DH did witter on about something to do with the electrics and this method....so you may need to check. It's bought our floor right up and there are no cold spots.

Definately call building control as one of the councils round here are a bit sniffy about the Tri iso stuff.

Apparently in Berks (Maidenhead) If you rip up your existing floor (which isn't insulated) and then lay insulation it's a matter for the LA to sign off Me thinks many folk don't bother telling them

jeanjeannie · 09/02/2011 08:04

Just seen thisisaNiceCage's post...yep, something similar to that but a bit more £$

ThisIsANiceCage · 09/02/2011 11:07

Yeah, electrics a problem. The rating for electric cables changes when they are surrounded by thermal insulation, as obviously they can no longer lose heat as expected. So damage or a kink causing a hot spot could become a problem.

Iirc, household cables should be OK if attached to joists with cable clips, or even left lying on the ceiling, so that the cable is not completely surrounded by thermal insulation and can cool if necessary.

I AM NOT AN ELECTRICIAN! If in doubt, seek professional advice.

Thandeka · 10/02/2011 08:03

Our electrician says cables okay- I kept nagging him about that as came up on survey.

In the end the floor couldnt be boarded out without bring ceiling down- nightmare! But managed to get a refund on the tri iso stuff we had just brought and the builder is reinforcing the beams in places and we can have a strip of boarding up there for storage of xmas decks and not too heavy stuff but nowt else. It was that or pay around £5k for what essentially is the first stage of a loft conversion and its just not worth it on this house (wont get the return and we defo wont be converting the loft).

Thankyou so much for all your help guys!

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