Insulated plasterboard or not?

[olivermarshall]

Hi all

I'm currently insulating my loft to make it in a den for me to escape whenever anyone calls my name.



I'm using foil coated insulation board of 50mm depth.

To save my sanity, could I instead use insulated plasterboard, and just screw it on top of the rafters?

Or perhaps both?

 

[foxhole]

Both is frequently used to achieve good thermal values.
Have 100mm celotex in my loft overboarded with 40 insulated plasterboard.

 

[Leofric]

Will you have a 50mm ventilated air space above the insulation ?

 

[wwwebber]

Will you have a 50mm ventilated air space above the insulation ?

Not needed - he has no membrane at all.

I will be doing exactly the same job, 50mm PIR with insulated plasterboard. I have no membrane either and I could go with 75mm PIR if I wanted but I am putting in a 25mm airgap anyway.

 

[John D v2.0]

Not needed - he has no membrane at all.

Even if he has a modern membrane it's non needed as they are now breathable. However leaving a 10mm gap to drape the membrane would prevent the need for counter battening which would otherwise be necessary to allow runoff.

 

[Leofric]

Even if he has a modern membrane it's non needed as they are now breathable. However leaving a 10mm gap to drape the membrane would prevent the need for counter battening which would otherwise be necessary to allow runoff.

Even with a breathable membrane you still need to leave an air gap over the insulation. Let's all have a look in Bldg. Regs Approved Documents and NHBC
Standards and see what they say .

 

[John D v2.0]

Even with a breathable membrane you still need to leave an air gap over the insulation. Let's all have a look in Bldg. Regs Approved Documents and NHBC
Standards and see what they say .

Ok, but i don't see your quote from them. And anyway that's not what the membrane manufacturer says.
See 7.4 of this for one example
http://www.dupont.co.uk/content/dam...o-Roof-underlay-Warm-non-ventilated-roofs.pdf

 

[Leofric]

Ok, but i don't see your quote from them.

I didn't say I was quoting from them, I said let's all have a look etc, but have you looked at BS5250 code re condensation in buildings ( free download from DLSCRIB ) and BRE Report BR262 thermal insulation avoiding the risks. There is also a school of thought that you should not rely solely on breathable membranes for ventilation, NHBC Standards Extra 48 asks for additional ventilation to breathable membranes. I could refer you to insulation manufacturers' technical data sheets where they specify a 25mm drape space above the insulation.
ps I am not against breathable membranes as ventilation, I must have been one of the first to propose breathable membranes as a way of ventilating roof spaces to the NHBC because at the time the local NHBC Building Control department had to get their relevant department at Milton Keynes head office to investigate the use of these membranes before they could give approval.
pps some say there can be a risk of breathable membranes losing their effectiveness in winter if moisture on the surface of the membrane freezes and blocks the pores.

 

[JP_]

I think 50mm insulation in the loft would be enough for an occasional den. Just plasterboard over, will be much easier to install, and give you more head room. A small heater + heat from below will keep it warm enough for a den. Unless you plan naked yoga I suppose....

 

[John D v2.0]

Thanks leofric yes just to address your specific points the drape space mentioned in the bre certificate is purely to give clearance in non counterbattened roofs to allow runoff of condensing water on top of the membrane, not for ventilation to outside air.
Regarding freezing that's a good point and I've seen that mentioned too. If we're going to suffer prolonged freezing conditions the structure may not dry out before suffering damage.
Otherwise i think youyou right the membrane isn't as breathable as free air, so you're right if your condensation risk analysis shows a buildup of moisture and it can't be resolved by VCLs and internal moisture control, then further action would be needed. Or in the case that the VCL is breached by faulty extractors etc. Whether that justifies leaving precautionary ventilation on the inside of a membrane in domestic circumstances is beyond my level of knowledge so thanks for the information!

 

[tony1851]

If you are putting Kingspan between the rafters, and then insulated plasterboard below, ensure that it is the Kingspan (yellow foam) insulated type and not just polystyrene-faced -plasterboard.
With the latter, you will get condensation forming within the rafter space as it does not have an integral vapour barrier, which the Kingspan-type does.

 

[wwwebber]

If you are putting Kingspan between the rafters, and then insulated plasterboard below, ensure that it is the Kingspan (yellow foam) insulated type and not just polystyrene-faced -plasterboard.
With the latter, you will get condensation forming within the rafter space as it does not have an integral vapour barrier, which the Kingspan-type does.

I intend on doing just that - use Gyproc Thermaline plasterboard on top of Kingspan PIR. The Gyproc stuff is already in place so I have no choice and I plan on temporarily removing it and put in Kingspan between the rafters. Are you suggesting that I need an additional vapour barrier ?

 

[tony1851]

Gyproc Thermaline has its own integral vapour barrier so would be OK.