Thermal qualities of insulation plasterboard vs plasterboard

[servlan]

Dear all

Can you help me please? Im currently having a replacement flat roof installed in my utility room and re-configuring the layout so that half of the space will become my office. As this is attached to the side of the property it gets very cold in the winter and I wish to make this area thermally efficient.

I was considering using Expanded Polystrene beads (EPS) in the cavity walls, but as the new roof is now laid this will be very difficult and I have heard that this does not insulate properly. Alternatively I have noticed that you can get insulated plasterboard and wondered if this was better just to attach to the inside wall?

I have a few questions on this please:

1) Are Kinspan insulated plasterboards better than the others - if so, whats the minimum depth used?
2) Would it be cheaper to purchase insulation boards and then fix plasterboards over the top?

I am very sorry for all these questions but Im a complete novice when it comes to this and want to make sure I don't get hyperthermia

Dimensions of the space (just in case you are interested)

Wall 1 - Main 450cm L x 230cm H
Wall 2 - Window 215cm L x 230cm H
Wall 3 - Door 226cm L x 230cm H
Wall 4 - Garage 120cm L x 230cm H

Best regards
Paul.

 

[urbanite]

The insulated plasterboards are generally the same spec between different manufacturers, some have PIR type insulation and some have PU insulation. the ones with PIR are marginally better so you can use a thinner beard to give you the same result as a PU insulated board. We are talking about 5mm difference. although when your tight on space it matters. Kingspan are quite pricey, you can shop around on-line but you'll be surprised they keep their prices similar. Supply and demand and all the price rises lately do not help the situation.

I can't see it being cheaper to insulate with kingspan or celotex board then plasterboard over, these boards are pricey, about £40 per 8x4ft sheet. The plasterboard adds about a fiver to it, if your fixing yourself you have to look at a way to fix through the insulation board into the wall or battens. The insulated plasterboard does work out cheaper. You can stick it to the wall using drylining adhesive, you may have to remove the plaster to do this.

Go to the celotex u value calculator (google it) select walls and give them your build up from the choices, brick, cavity, block, plaster, etc. then choose to a u-value close to 0.25w/m²K or lower. this will give you an idea of the thicknes you need for keeping to the current regs. you may have to register to use it but you'll find it quite handy to check out what options are available

You will see that the list of products you can choose from have differing values, this is because the have different resistances (K value or R value)

Personally i believe they are worth it. i would try to aim for a 50mm insulation on a 12.5mm board, you will really notice the difference. don't forget you will need to get inside the window and door reveals, at a product thickness of 62.5mm you will struggle so you may need 2 different thickness's. The reveals are the cold spots so don't assume you'll be ok,

There are also alternatives which are foil based layer insulation, such as tri-iso, people like these as they are very thin although they are expensive and need some fiddly installation details. Often used in attic spaces due to head room and weight.

 

[servlan]

Thank you so much Urbanite - this is really useful.

I only mentioned about using a combination of insulation + plasterboard as BSO.co.uk have a Celotex GA4000 board for £21.96 Inc Vat and thought this might be a cheaper option?

Can you possibly recommend anything for the roofing area as this is a flat roof. My issue is that the rafters are only 100mm deep but government regs for a loft is 270mm. Would you recommend loft roll of 100mm and then apply 170mm insulated plasterboard - is their such a thing?

Again thanks for your help.

Cheers
Paul

 

[tony1851]

Whatever you do, don't pack the space between the joists with mineral wool as you will probably get condensation in the roof space which will ultimately cause the timbers to rot.
The best you could probably do is to put 1" or 2" Kingspan uner the joists, and finish with foil-back plasterboard.

 

[urbanite]

If it's a flat roof without ventilation then rigid foam (PIR) between joists and under draw the joists with a 25mm+vapour check plasterboard, that should be adequate, if you have the headroom under-draw with a thicker PIR, may as well whilst your at it. You are fixing through to the joists so no need to get pre-bonded insulated plasterboard.
The GA4000 is a good all rounder board I specify this for under screed insulation although in a ceiling void it may be a little overkill as you don't need the compression strength, a thick PIR board from Xtratherm or Quintherm with foil should suit you.

If you have ventilation within the roof void you may need to maintain a 50mm cross flow, so you will have to use 50mm between the joists.

Again, consider the NASA designed foil insulation, i've heard good things about it used in ventilated roof voids, you are better to back this off under the joists with the insulated plasterboard. Building regs usually require a minimum of 25mm insulation to avoid condensation risk to the underside of joists, which actually makes sense when you think about it.

Also check out some insulated wall paper linings, such as wall rock thermal liner. This helps you heat a room faster and is breathable so avoids any potential mould build up.

You should also think about trickle ventilation if you are having an office. The b'regs requirement is a fresh air supply of 10ltrs/sec per person. If you are extracting the air out then you need to let in the new. You can get heat recovery units that are quite small. This is a consideration as it can get cold during winter and if you have a small heater in your office then most of the heat will be wasted without one.

 

[tony1851]

If the extension is on the side of the house, and the roof joists are only 4" deep, it is probable that he will have no cross-ventilation. If this is the case, would putting any thickness of insulation board between the rafters be a good idea? , as then you are reducing the void, and possibly further reducing the chance of any vapour escaping
Its a shame that the roof has already been done, as the best idea for a flat roof is to have a 'warm' roof, in which the insulation is put on top of the deck.immediately below the outer finish. As its already done, I would have played safe; have a slightly lower standard of insulation by only putting some board under the joists, and finishing with foil-backed plasterboard, which helps as a vapour check.

 

[Richard C]

Before you go too far, you should be aware of Buiding Regulations that may govern what your doing;
http://www.planningportal.gov.uk/permission/commonprojects/roof#Worktoanexistingroof

 

[servlan]

Thanks guys,

From what I can hear then it might be worth ditching the loft 100mm insulation - keeping this as airspace and then using a insulating plasterboard for the ceiling?

In terms of regs - this is an existing dwelling so its just repairing the roof which was leaking.

Cheers
Paul

 

[Richard C]

In terms of regs - this is an existing dwelling so its just repairing the roof which was leaking.

Unfortunately that makes no difference. From the Planning Portal link I provided;

If you want to carry out repairs on or re-cover less than 25 per cent of the area of a pitch or flat roof, you will not normally need to submit a building regulations application. You will need approval, however, if:
• You carry out structural alterations
• The performance of the new covering will be significantly different to that of the existing covering in the event of a fire
• You are replacing/ repairing more than 25 per cent of the roof area, in which case, the roof thermal insulation would normally have to be improved.

IMO the scope of work your doing is notifyable & I would advise you speak to your LABC for guidance & clarification. You are at liberty to ignore it of course but you may you have a problem with a future sale if you cannot produce a compliance certificate & if LABC catch you out or a nosey neighbour “dobs” on you, you could be served with an enforcement notice!

 

[servlan]

Thanks Richard C,

I totally understand and from memory I think the total roof space is between 23-30% of flat roofing - yet only half is insulated as the other half is for the garage. So I guess at most this would only be 10-15% of the flat roof space being insulated.

To be honest this is more for me than building regs as I don't want to freeze my nuts off when working in the office. Thats why the flooring is going to be insulated as well (currently concrete flooring). Im looking at insulating the walls to 50mm insulated plasterboard so I know the place is going to be warm!

Cheers
Paul