To double insulate a loft room when plasterboarding or not?

Further to my previous pleas for help, I’m wondering what people think about going to town with the insulation in a loft room?

My plan is to board the ceiling and angled eave sections with insulated plasterboard. I’m wondering if I should also fill the rafter voids with wool insulation too? I’d like the room to be comfy in summertime and if this extra layer of wool will help that it’d be worth it.

Also, if anyone has any advice on what order to do things in I’d appreciate it! (e.g. board from ceiling down, whether to board in the wee awkward corners first or last).

I plan to leave the vertical walls alone and just tape over the join to hide the wee step that would otherwise be visible. Doing this will save a lot of awkward wee corners and joins around cupboards, the radiator etc. Then I’ll probably apply lining paper all round and paint it.

This is all a venture into the unknown for me and any tips would be great!

Yes. More insulation is always better. Though you do get diminishing returns. You might need to consider condensation management.

If you fill that void with rock wool you risk rotting timber in years to come. I’d not do it.

You won’t like the cost but if your doing it wingspan is your friend but be sure to leave an air gap!

We used king span or similar, 100mm(?) between rafters and then a layer of 50mm(?) over that. Then plasterboard. That was for a conversion so I think was the minimum legal requirement. If you can afford to do solid insulation it’ll be a lot easier and less itchy to do.

I put battens inside the rafters and used 50mm kingspan in that – gives air flow under the roof but still insulates. then insulated plasterboard over the top.

So would filling something like half of the inter-rafter void depth with kingspan be a good compromise, before then putting up insulated board? They’re 120-125mm deep. I don’t want to start deepening the rafters to take BS standard levels of insulation!

I’d echo what the others say – foam insulation is easy to fit and ensures you have an air gap. When I did similar building control insisted I fit loads of roof vents to ensure air circulation in the voids which actually has led to drafts. I have added insulation to many parts of my attic flat over a long period of time and no evidence anywhere of condensation.

So would filling something like half of the inter-rafter void depth with kingspan be a good compromise

exactly.

I used the battens to m,ake sure the kingspan stayed flush with the bottom of the rafters and no air gap netween it and the insulated plasterboard.

50 mm between the timbers and then insulated board over the top would work well I think – anythings got to be better than nowt.

Your roof is in better nick than mine.

This is what I planing for mine. Not suggesting you do the same just throwing it in the mix.

Remove the old cardboard stuff that is there already 🙁
Use Tyvek house wrap ( or knauf breather membrane ).
Re batten over the breather membrane
Insulate
Board with Plywood ( takes knocks much better than plasterboard ) more suited to what I plan to use the loft for.

Edit. To answer your question, dont double insulate let the roof breath, but shut out the drafts on your side with membrane.

I used the battens to m,ake sure the kingspan stayed flush with the bottom of the rafters and no air gap netween it and the insulated plasterboard.

I assume you’re talking of using very thin battens that bend slightly to stick in place?

righog – Member
Your roof is in better nick than mine.

This is what I planing for mine. Not suggesting you do the same just throwing it in the mix.

Remove the old cardboard stuff that is there already
Use Tyvek house wrap ( or knauf breather membrane ).
Re batten over the breather membrane
Insulate
Board with Plywood ( takes knocks much better than plasterboard ) more suited to what I plan to use the loft for.

Yep, that hardboard stuff is awful, flimsy and all round crap! Are you attaching that membrane to the roof timbers between the rafters? I’m drawn to insulated board more than plywood as I suppose it’ll be better insulated! But I do like the appeal of good thick solid plywood instead of crumbly plaster.

Board with Plywood ( takes knocks much better than plasterboard ) more suited to what I plan to use the loft for.

*raises eyebrow*

I assume you’re talking of using very thin battens that bend slightly to stick in place

I just screwed them down the length of the rafters rather than across between them. As long as you trim the board acurately it presses against the battens and is held in place when the plasterboard is added.

This sort of thing (although I used 25mm battens to save money and just spaced them appropriately.

The insulated board is good and would be easier to use ( depending on how big your access is of coarse ) but is expensive.

I plan to just staple the membrane to the rafters stretching across the entire roof making sure I leave no gaps. This will leave the original air gap between, in my case the rafters and tile ( you have a wood layer there ) then the battens insulation and ply on top of this.

*raises eyebrow*

Why please elaborate

Aah wwaswas I thought you meant battens BELOW the insulation foam! Ok, how do you hold the foam sheets in place as you fit the boards to the rafters? I just can’t picture these elements so it’s all got to be asked! I guess if you try and cut them 5mm wider than the gaps they can just be pushed in and hold themselves

If you want it to keep cool in the summer wood fibre insulation will help more than the nasty mineral wool stuff. Will also help with avoiding condensation

Just jam the slabs in between the joists and board over them with insulated plasterboard. We did something similar in my son’s bedroom, which is in an attic above the garage and outside the thermal body of the house so was freezing cold. Now he just plays GTA and the heat from his computer is enough to keep it toasty in there.

I just cut the insualtion boards with a knife so they were a very snug fit – measure the top and bottom (they’re never the same!) and use a straight edge and a bread knife to cut the board to size then bang in with the flat of your hand.

My house was victorian so non-standard spacing so I just bought 8×4 sheets and cut to size as needed.

Raises eyebrow

Plasterboard isn’t just for getting a smooth finish, it’s fire protection as well.

20 quid a board for 37mm board with closed cell backing then a 50mm cellotex between rafters. Going for a 75mm cellotex between rafters and pb straight over timber may be cheaper. Time is a big factor when cutting cellotex. May be just easier to go with a 50mm backed board. You need to do the maths…

it’s fire protection as well.

and sound.

You really don’t need fire protection at that point of a house.

If you’re not short of height, why bother putting insulation between those joists, just put all the thickness you were planning to use below them. Much less cutting, jamming in, filling gaps with expanding foam, taping with foil tape, sweeping up foam bead debris from cuts. Also much fewer joined edges, so you have a lot less chance of messing it up and leaving gaps and cold spots. Job will be done in half the time too.

you just need the right tools 🙂

As above Id partial fill with polyurethane/foil batts.

If you want to boost the u-value even more, you can staple Silver TLX or equivalent across the lot before boarding.
http://www.tlxinsulation.co.uk/products/tlx-silver/

Like this
(the rockwool you can see comes up from the partial fill of the block leaves, it’s xtratherm in most of the pitch)

A good tip for putting up plasterboard is to always put the plasterboard perpendicular to the joists so that the cut ends are on the joists and if you can , put a full board up in the middle of the ceiling. That way you have a 90 degree angle to work with and you can measure into each corner from the fixed plasterboard . Hope that makes sense.
You also don’t have to use thermal plasterboards. It’s easier to put kingspan between the joists and another thinner sheet of kingspan on top of the joists. Then you can just use normal board’s or duplex boards ( foil back ) using longer drywall screws

If you’re not short of height, why bother putting insulation between those joists, just put all the thickness you were planning to use below them.

I’m not sure I’ll have space at the window for that as the top of the window’s close to the ceiling.

Whatever you do leave min 50mm min air gap above the insulation and ensure that your have cross ventilation to this void.

I’ve priced this up and looks like £450 for the 70mm insulation and insulated 27mm plasterboards plus a bit for screws and tape. I’ve got some joinery to do first now as I’ve removed some old cupboards and need to reinstate the walls. The jobs list doesn’t half grow arms and legs!

Cap off water supply to sink before removing sink and unit
Get an electrician to wire and place new lights and sockets
Joinery, sort area of missing floorboards and wall
Remove skirtings
Strip wallpaper
Fit insulation between rafters and tape
Remove nails from floor
Fit plasterboard
Tape and fill joints
Build partition wall for walk in storage
Fit new skirtings
Apply corner tape to edges of hardboard in certain areas
Paint skirtings
Apply lining paper and paint all ceilings and walls
Fit electrical components to walls/ceiling
Carpet fit in loft and stairway (full wallpapering and paint required through stairway too).

I’m starting to see this being a 2018 completion date job! Free days are in very short supply

Wee question for anyone that might know about plasterboarding… I’ve discovered I can’t get a sheet of 8×4 up the stairs, so it looks like I’ll be doing my cutting downstairs

Or go down a size – will 1800 x 900 fit up the stairs?

Stairs must be tight then. Time to get your maths Headon to make the offcuts work.

Bare in mind you will need to provide ventilation to the new roof cavity at the eaves externally and possibly add proprietary ventilators to the pitched roof dependent on the area you are insulating.

I’d typed a load more there that hasn’t appeared. With the cutting there will be a combination of tapered edge joints and straight joints now. Will this work ok if it’s all being covered with lining paper?

I’ll probably be cutting up the boards into 8×3, as that should fit up the stairs.

I’ve always rasped the edges down to make all tapered to allow a decent taped joint

I wouldn’t recommend going straight to lining paper without taping the joints it will look carp.

if your going to do 70% of the job just finish it please. I do realise all the skill/cost is in the last 30% but it makes all the difference to the quality of finish.

Hadn’t thought of rasping down the joints. Yes that sounds like a plan. Rather annoyed at not being able to do all the cutting in the loft, it’ll be constant trips up and down the stairs! It’s only because the halfway landing on the stairs is under the eave section of roof and therefore has a lowish ceiling. The stair width is quite normal otherwise.

could you take the board up with two of you . – one stands on the landing the other stands on the stairs and flip it over the bannister rather than trying to take it round the landing ?

Nope, a board that size mathematically will not fit up these stairs. Got a lot of up and down to do!

If only they made rigid insulation bendy.