Installing suspended timber floor with PIR

[tommyleestaples]

I have just had an extension done and before it can be signed off by building control they want to see the whole ground floor which is suspended timber floor has been insulated. 

 

In order to achieve the U-Value required they have told me it will need to be 130mm of PIR. However, my joists are only 100mm so I would appreciate any advice if people do this for a living or have had to do it on their own property. 

 

I was planning to friction fit, seal up any gaps with expanding foam then tape joins to create a vapour barrier. I was also thinking of using roof battens underneath to give me the extra depth I need or using light duty brackets fixed to the joists. The only problem I think I will come across is in some places there is pipework just under the joists and then you have the sleeper walls. Would you advise cutting section out of the PIR insulation for the sleeper walls or just putting a piece in that is less depth such as 100mm?

 

Thanks

 

 

[TheMitchells]

Have a look at gapotape for a really good fit of the insulation - several YouTube video's have shown how hard it is to get a good fit just by cutting or expanding foam.   This video gives a good demo.  Costs more than foam but should give a perfect fit and no air leaks ☺️

 

 

[Redbeard]

Are you doing this from the top or from below, with the boards in situ? I assume from the top as you refer to taping as VCL (on the warm side). Why not carefully cut joist-width slivers of 25mm PIR and glue to underside of joists, then when you slide the 125 PIR down it should meet at the bottom of that 'fillet'. Yes, the joint won't be tight as a duck's whatever, but an extra piece of softwood won't be very insulative either.

[Iceverge]

58 minutes ago, TheMitchells said:

Have a look at gapotape for a really good fit of the insulation - several YouTube video's have shown how hard it is to get a good fit just by cutting or expanding foam.   This video gives a good demo.  Costs more than foam but should give a perfect fit and no air leaks ☺️

 

 

 

Or you could save some money and just insulate the floor with gold leaf. 

 

1 hour ago, tommyleestaples said:

I have just had an extension done and before it can be signed off by building control they want to see the whole ground floor which is suspended timber floor has been insulated. 

 

In order to achieve the U-Value required they have told me it will need to be 130mm of PIR. However, my joists are only 100mm so I would appreciate any advice if people do this for a living or have had to do it on their own property. 

 

I was planning to friction fit, seal up any gaps with expanding foam then tap joins to create a vapour barrier. I was also thinking of using roof battens underneath to give me the extra depth I need or using light duty brackets fixed to the joists. The only problem I think I will come across is in some places there is pipework just under the joists and then you have the sleeper walls. Would you advise cutting section out of the PIR insulation for the sleeper walls or just putting a piece in that is less depth such as 100mm?

 

Thanks

 

 

 

What U value do you need? 

 

PIR between timbers isn't a good idea in my book this detail from ecological building systems is much more robust.

 

[Mr Punter]

1 hour ago, TheMitchells said:

Have a look at gapotape for a really good fit of the insulation - several YouTube video's have shown how hard it is to get a good fit just by cutting or expanding foam.   This video gives a good demo.  Costs more than foam but should give a perfect fit and no air leaks ☺️

 

 

 

The difference between Robin Clevett's version and what typically gets done on site is vast.  The solution needs to be much simpler to allow widespread adoption.  The average site worker is not going to cut insulation with a Festool to a precise compound angle.  Just not achievable or affordable.

[tommyleestaples]

Thanks for your comments. 

 

I need a u value of 0.18 W/m²K for sign off. 

 

If I could do it this way then I would, it would be much cheaper but in order to get the Uvalue needed I would need to have loads of it under there and also I have read somewhere that mice/rats love it 

[Iceverge]

You've had a new extension done with 100 mm joists? 

 

This is unusual. They must be very closely spaced and regularly supported by sleeper walls ?

 

In any case 130mm of PIR between the timbers alone wont meet a U value of 0.18. It would be closer to 0.3 by the time you take the bridging effect of the joists into account. 

 

Do you have space above the floor to put more insulation there below your finished floor height?

 

Pics of the build as it stands would be useful. 

 

 

 

 

 

[tommyleestaples]

The builders done suspended timber floor to match existing floor. 

 

Please see below a photo I believe the spacings are 400mm

 

When I have run U Value calculators it shows I need 130mm PIR between the joists. I have included a picture of build and calculation. 

 

 

 

Edited Wednesday at 16:33 by tommyleestaples

[Mr Punter]

Just do it with 130mm and cut it carefully round the sleeper wall plates.  It can rest on them.  Foam and a layer of polythene as a vapour barrier.  The sticky foil does not work well with the dust from cutting.

[tommyleestaples]

Thanks for the advice Mr Punter.

[Russell griffiths]

I believe you only need 100mm if under a screed in an extension if the insulation in other areas is more than adequate 

talk to building control and ask them if 100 will do. 
airtightnes in this floor will be just as important as the insulation levels. 
you will need to be meticulous with this. 

Edited Thursday at 07:25 by Russell griffiths