Internal Wall Insulation

[Rudie]

Hi,

I currently have a solid wall (no cavity) double brick house where all the wall are dry lined with 12mm plasterboard which are dabbed onto the bricks. I am thinking of replacing all the external walls with 50mm celotex dual foil backed insulation and fixing/screwing these to the walls after removing the old plasterboard. I will then apply 12mm normal plasterboard on top of this and skim. I remember when i had the walls dry lined, there were visible air bricks above the floor boards. Are these ok to cover over with the insulation? To re-fit the rads I intend to use wooden batons and screw the rad brackets onto this to be more secure. I plan to leave all the internal walls as they are.

I'm after some advice on the above to see if thats ok to do. Also, im going with 50mm insulation as i dont want to go with 40mm only to find that it is too thin.

Thanks in advance.

 

[Doggit]

The first problem you need to consider, is the air bricks. You normally get air bricks below the floorboards so that moist air doesn't get trapped under the floor, and rot the joists. Air bricks above the floor are a bit unusual, and imply the house needs damn good ventilation, and so shouldn't be covered over. Is there a dpc in the wall, as a lack of one would suggest the need for air bricks. Is there good insulation in the room that would allow you to take out the air bricks, and put a standard brick in it's place.

As to the insulation; celotex themselves would recommend you put 2x1 treated wood battens on the wall first, and then fix the celotex to these. This then allows you to screw the plasterboard to the wooden battens using long plasterboard screws. If you work out where the radiator brackets go, then you can fit slightly oversized wood blocks to the wall that will come up the the level of the celotex, and so sit just under the plasterboard. I'd use easydrive flat countersunk concrete screws to fix the battens to the wall, and that'll keep them nice and secure, but you may want to start the battens from the floor to give them extra support. Use the aluminium foil to cover the joints in the celotex, and there's an argument for using adhesive foam to fill any gaps first.

I assume you're using celotex and plasterboard because it's cheaper than the insulation backed plasterboard, but the kingspan backed plasterboard can be dot and dabbed straingt to the brick wall, just not sure if you can do it onto the existing plasterboard. You should be using about 65/70mm of celotex to bring the wall up to current building regs rather than 50, but the 50 will still help.

 

[Rudie]

Hi, thanks for your reply. Yes there is a slate dpc and the air brick is split feeding above the floorboards and below. As you say I aim to keep the air flow below.

Yes the reason I'm going with the 50mm celotex is that I don't have much space to play with in terms of internal space due to wardrobes upstairs and sofas downstairs. Total space I'll be losing is around 65mm (50mm insualtion + 12mm plasterboard and a few mm of skim).

I've read posts where people have put on batons and then cut and fixed insulation into the sections so you see a timber frame filled with celotex but decided to ask about my method in this post.

I'm just after a lot more Heat retention as when the heating turns off it gets cold really quickly.

Thanks.

 

[Doggit]

I've never come across that setup with the airbricks split above and below the flooring, and I can't think of any reason for it. They'd normally be below the floorboards, and therefore, below the dpc. I'd be inclined to resite them so that they are completely below the floorboards.

2" battens with insulation between the battens is going to give you cold bridging where the wood is, and you may end up seeing where the battens are on cold days. Can you take the old plasterboard off, and then put the 2x1 battens on the wall first; or can you afford to use insulated plasterboard D&D on to the bare bricks to give you a tight fitting. If you really haven't got the space, then ring celotex technical support, and ask them whether 2x1 battens and 40mm insulation, or 50mm infill would be best.

Have you considered external wall insulation instead. Yes, a lot more expensive, but a much better job, as you insulate the whole house. Putting up the insulation is a DIYable job, then you need a plasterer for the render, then you can do the topcoat on yourself again.

 

[Rudie]

Hi, yes the airbricks are below th DPC.
I may check how much more cost the insulated plasterboards are - but another reason for me wanting to go for the celotex is that they are foil backed on both sides - is that the same with insulated plasterboard?
I have considered external wall insulation, even had an offer to have it done at cost price from a friend but dont want to lose the brickwork effect on the outside - sounds odd but i live in a 1930s home and like the look from outside.

 

[Doggit]

One thing I've never yet manage to work out on cellotex or kingspan, is how they can put foil on either side, and still call it breathable. From what I've managed to work out, the foils are supposed to add anther level of warmth ???? to it. The insulated plasterboard is more expensive, but requires less work as it's an all in one product. I don't think you can D&D the foil backed stuff though, so it'll depend on which way you decide to jump. Again, talk to their tech support guys.

As to the EWI, you can use brick slips for the top covering (okay, it'll cost more), to replicate you're current look. Personally I'd have a serious chat with your friend, because it'll stop the house getting cold, and is far more efficient than internal insulation, and will improve the value and saleability of the house. You'll be able to put pictures etc up on the inside walls - and save yourself a lot of hard wok and hassles.

 

[Rudie]

Thanks for your reply Doggit. I've seen some of the brick work effect external insulation and Im not too keen...more to the point neither is the Mrs - which is what really counts

Luckily the only things hanging from the external walls is curtain rails and rads.

The few things I may have problems with external my mate said was the front bay section and a passage way between house and garage which will reduce that quite a bit.

I'll speak with tech support and get their advice as I'm not sure which way to go now. If I go with internal I will take your advice and get plasterboard insulation rather than seperate insulation and plaster board.

 

[Rudie]

One thing I may try is blocking part of airbrick above the floor (behind the plasterboard) and obviously leave open the section that feeds under section of the house. Will see if this makes a noticable difference, as there is an air cavity between the plasterboard and the wall. Just seeing if there is a non-messy solution before i start. I'll give celotex a call today too.

 

[Doggit]

You've brought up an interesting conundrum on the airgap issue. if you leave the ventilation holes open, it'll allow insects etc in, and you won't know what's going on behind the plasterboard until it's too late, but if there are any moisture issues coming through the wall, then the ventilation will allow it to escape. I suspect celotex would tell you to seal the gap, and treat it in the same manner as a double glazed window. If you allow the outside air into the space behind the plasterboard, then you're cooling that airspace down, and reducing your thermal efficiency.

 

[Rudie]

I have one of the inspection cameras. I'll take a peek through the airbrick to see whats going on behind the plasterboard. I cant get through to Celotex so will try Monday and update on here.

 

[Rudie]

Hi,

I have a reply back from Celotex.

Thank you for your enquiry.
Providing the wall is free from moisture ingress and in good condition then a number of insulation options can be considered.
Please find attached 2 of the most common solutions which outline the process of either using standard insulation boards or insulated plasterboard.

1st option is:

Installation Guidelines

Celotex insulation boards should not be installed when the temperature is at or below 4°C and falling.

• Ensure that existing walls are permeable. Strip any gloss paint or vinyl wallpaper.
• Install a breather membrane where brickwork is only 103mm.
• Use the Celotex Insulation Saw to cut the 1200mm x 2400mm Celotex GA4000 boards to fit the floor-to-ceiling height of the room.
• For optimum thermal performance, the unprinted foil surface should face the batten cavity.
• Fix horizontal battens over the insulation at the top and bottom of the wall to hold the boards in place.
• Seal all board joints to create a vapour control layer (VCL) using the Celotex Insulation Tape.
• Add further vertical battens over the insulation at appropriate spacing to provide adequate fixings for the selected lining system.

Ensure that a batten coincides with each lining board joint.

Internal Solid Masonry Wall Application

Wall Insulation

• Apply an appropriate sealant around the perimeter of the insulation to provide a vapour seal.
• Additional battens are required around all the openings to provide fixings for linings and grounds for skirtings etc. Battens should be minimum 50mm x 25mm treated softwood.
• The batten depth must be increased to take account of the conduit, if cabling is to be located within the cavity created by the battens.
• Use independent horizontal battens for heavy wall mounted components. Longer fixings may be necessary to fit heavy items to the masonry, independently of the battens.
• Line window and door reveals with thinner Celotex TB4000 boards to reduce the risk of thermal bridging. Fix a batten around the edge of the opening and scribe the board to fit the reveal. Cut the dry lining to suit and mechanically fix into the masonry reveal using proprietary fixings. Finish using an angle fillet at the frame and an angle bead or scrim tape at external corners.

2nd Option:

Installation Guidelines

Celotex insulation boards should not be installed when the temperature is at or below 4°C and falling.

Installation guidelines for internal lining systems using dot and dab

• Ensure that existing walls are permeable. Strip any gloss paint or vinyl wallpaper.
• Use the Celotex Insulation Saw to cut the 1200mm x 2400mm Celotex PL4000 boards to fit the floor-to-ceiling height of the room.
• Ensure a continuous seal at skirting, ceiling level and at openings by applying a continuous band of gypsum adhesive. Gypsum adhesive at perimeter edges can be replaced with thin timber battens.
• Apply further dabs of gypsum adhesive. This should be in accordance with the adhesive manufacturer’s instructions.
• Align sheets against the dabs and secure into correct position.
• Once the dabs are set, it is recommended that additional secondary fixings be applied to the Celotex PL4000. Exact fixing details should be in accordance with the recommendations of the fixing manufacturer.
• Joints between the boards must be tightly butted, taped and jointed using appropriate tape and jointing material to create the vapour control layer (VCL).
• Line window and door reveals with thinner Celotex TB4000 boards to reduce the risk of thermal bridging. Fix a batten around the edge of the opening and scribe the board to fit the reveal. Cut the dry lining to suit and mechanically fix into the masonry reveal using proprietary fixings. Finish using an angle fillet at the frame and an angle bead or scrim tape at external corners.
• Please note that to avoid the load being directly applied to the Celotex PL4000, suitable mechanical

fixings should be used for other internal fittings. Advice on suitable fixings should be sought directly from the fixing manufacturer.

Direct fix

Fig. 2

Timber battens

Metal

Upgrading Solid Walls (Internally)

Wall Insulation

Installation guidelines for internal lining systems using mechanical fixings

• Ensure that existing walls are permeable. Strip any gloss paint or vinyl wallpaper.
• Use the Celotex Insulation Saw to cut the 1200mm x 2400mm Celotex PL4000 boards to fit the floor-to-ceiling height of the room.
• Secure Celotex PL4000 with suitable mechanical fixings. Fixing details should be in accordance with the fixing manufacturer’s instructions.
• Joints between the boards must be tightly butted, taped and jointed using appropriate tape and jointing material to create the vapour control layer (VCL).
• Line window and door reveals with thinner Celotex TB4000 boards to reduce the risk of thermal bridging. Fix a batten around the edge of the opening and scribe the board to fit the reveal. Cut the dry lining to suit and mechanically fix into the masonry reveal using proprietary fixings. Finish using an angle fillet at the frame and an angle bead or scrim tape at external corners.

Installation guidelines for internal lining systems using mechanical fixings to timber battens

• Ensure that existing walls are permeable. Strip any gloss paint or vinyl wallpaper.
• Fix treated softwood timber battens to the masonry. They should be set out a maximum of 600mm vertical centres to coincide with the edges of the boards. As a minimum requirement, horizontal battens should be used to support the top and bottom of the board edges.
• Galvanised clout nails or timber drywall screws should then be used to fix the boards to the battens. Specific advice on suitable fixings should be sourced directly from the fixings manufacturer.
• Joints between the boards should be tightly butted and finished by taping and jointing using appropriate tape and jointing material to create the VCL.

Installation guidelines for internal lining systems using mechanical fixings to metal lining systems

• Celotex PL4000 boards can be fixed to a number of proprietary metal frame lining systems. The system should be fixed in accordance with the manufacturer’s instructions.

Dot and dab seems my best option minimise floorspace loss.

 

[Rudie]

Also, regarding the airbricks they sit on the DPC, I have not yet had time to inspect with a camera. I still plan to fill the section behind the internal plasterboard with expanding foam to see if that helps this winter.

 

[Doggit]

How deep is the gap behind the plasterboard. If it's been D&D, then there's no real gap, and trying to get any foam behind it will be a waste of time and effort. And if the gap is an appreciable one, then the board will get pushed off by the foam as it expands.

As I've suggested, Celotex have said you need to seal the airbricks. If you've got a space issue, then the only real option, is to strip off the existing plasterboard, and either use the PL4000, or batten the walls, then use ordinary celotex, and then fix plasterboard through the celotex in to the wood battens

 

[Rudie]

Yes mate, plasterboard is dot dabbed but im sure the airbricks are right behind the skirtings.

 

[Doggit]

As the existing plasterboard is D&D, then there's no point in trying to put expanding foam behind it, so it's now down to you to decide exactly how well insulated you want the rooms to be, and how much work you want to put into it.

So it's now a case of deciding which way to jump. Do you want strip back to bare brick, and get the maximum amount of insulation, or do you go straight on to the existing plasterboard, and work from there.