Advice on airtightness and insulation

Lochlannach

We have an extension on the back of the house which is approx 10-15 years old. This room is intended to become our kitchen in the coming months so it's time to deal with it's main issue - it's a cold room. 

External walls are constructed of 9" cavity block (vertical cavities). These have been pebble dashed on the outside (to match the rest of the house built in 2000).

As far as I can tell at this point the wall has been capped with a flat block under the wall plate so the cavities are sealed.

Internally the walls are lined - 2" battens with 1" EPS between against the block. Finally there's standard plasterboard which has been skimmed. This obviously leaves a cavity between the plasterboard and insulation.

There is a vaulted ceiling - based on what I can see from where the original and extension rooves meet there is likely 4" fibreglass insulation up there below the felt. 

I have 2 problems here from what I can see. Primarily there are airtightness issues and secondly there is insufficient insulation. I want to tackle and resolve both. 

Air is making it's way into the cavity behind the plasterboard. This would appear to be coming in via the soffit vents. There is nothing preventing that air from getting into the cavity between the plasterboard and insulation. The skirting is pretty well fitted so not a huge amount of leakage there but removing sockets results in a strong breeze (particularly on a windy day). The room is fairly ok to heat up given it's volume and the stove/radiators being slightly undersized for same. However it loses any heat quite quickly once heating stops. While not perfect, and there are things to address later, the main house is noticeably warmer all the time.

As there is some internal rearranging work happening in this space approximately 1/3 of the wall and ceiling lining will need to be replaced in any case. At this point I'm planning on pulling the lot out and starting from bare walls and rafters inside. 

I'm thinking along the following lines: 

100mm PIR between rafters (leaving 50mm air gap)

Airtightness membrane over same, taped at junction with wall)

80mm warm board under rafters

Tape windows and doors

25mm battens on wall (service gap for wiring) 

80mm warm board on wall

Open to thoughts and advice from those who are more familiar with this type of work.

10-10-20

Hi, you're mentioning adding in 100mm of PIR between the rafters but also mentioned that there might be around 4" of insulation at the joist level, is that correct? If that's so then I would not insulate at the rafter level at all, I'd overlap the existing insulation with new glasswool and stay away from insulating the rafters (maybe that's what you meant).

I'd also suggest thinking about lifting the lower few tiles all along and fitting a plastic eves protector and also fit a rafter tray to maintain the 50mm gap. When fitting these use the access provided to fix insulation (glue/foam) around the wall plate and as far down as you can go (yes, I like giving myself a headache 😁 ).

Lochlannach

There's not a low level ceiling in play here - it's full height with the ceiling board on the under side of the rafters.

I was thinking that some fibreglass rammed in there isn't going to be as good as replacing it with pir. I forgot to add that I was considering another layer below that too.

Hmmm... I'll have to think about the tiles part. I know I have to leave a gap alright but want planning roof work to achieve it. It would give access alright.

MicktheMan

@Lochlannach

Yoy say (which is probably accurate based on what you write):

I have 2 problems here from what I can see. Primarily there are airtightness issues and secondly there is insufficient insulation. I want to tackle and resolve both. 

Yet your solution is predominantly insulation related with little air tightness changes to the one element which probably is a major leak air leak … your walls!

Lochlannach

Given they're capped on top and rendered outside? This is what I'm here for - to try and figure out if there's anything I've missed and should consider.

Blocks are porous...a cavity block with thin walls more so…that l know. However from what I've been reading the main problem with this type of wall seems to be where the air can get into the cavities and out through the inner leaf bypassing the blocking provided by the render. These appear to be capped though I'll know more in due course when I start removing plasterboard. If they're not then that's an issue to be addressed but something I can't be sure of atm. I've not seen much to indicate a big issue with a rendered wall letting through much air? Maybe it is the case?

In my case the ventilation for the roof has direct access to the back of my room skin between what little insulation is there and the plasterboard - it's bypassing the block wall when causing the problem. I'm sure the doors in the ceiling aren't doing me any favours either but the plan has surface mounting for lights to avoid that one. My thoughts were to bring the insulation right down to the wall, leaving a gap between it and the felt for ventilation, with the membrane taped to the wall to seal - maybe the membrane should continue to floor level?

Warm board fitted to the wall, tightly butted up to the roof insulation & membrane, with a sealant, with all plasterboard taped and skimmed would form a continuous layerfrom floor to floor each side?

Insulation is still needed in any case - make the structure completely airtight and you still need heating and insulation for comfort.

SC024

https://www.boards.ie/discussion/2058395829/advice-on-airtightness-and-insulation

by doing the walls this way your putting a hole in your insulation every place you have a socket / switch etc.

You'd be better to fix insulation directly to the walls & then foam & tape all joints in the insulation to seal them & tape to floor & ceiling. (Ideally allow insulation from walls continue up to meet PIR insulation in roof & tape these joints also. Then fix batten through the insulation with long concrete screws to the wall behind to crate your service cavity inside of the insulation & plasterboard & skim over.

Lochlannach

@MicktheMan what would your suggested approach be?

MicktheMan

For the walls and given the fact you will be stripping them back to blockwork;

My preferred method would be to insulate externally (ewi) and air tighten internally (sand/cement plaster the walls inside).

MicktheMan

If ewi not feasible the iwi is only option.

I would be battening with insulation between, then counter battening with 2nd insulation layer, then air tight membrane, then counter batten for service cavity, the pb/skim finish. All insulation to be breathable as with the membrane.

Lochlannach

https://www.boards.ie/discussion/comment/123160660#Comment_123160660

I did have a concern about sockets to be fair but figured people were working on the basis that the holes as a necessary evil.

I had also come across the idea of battens on the inside but wasn't sure how well that would work to carry the plasterboard. What side battens are needed to allow for sockets?

SC024

https://www.boards.ie/discussion/comment/123160752#Comment_123160752

2 x1 will be deep enough for the majority of sockets & switches. Shaver Sockets, Shower switches, Some cooker switches are usually deeper. Ensure your electrician uses drywall boxes instead of puncturing the insulation with screws or cutting metal boxes out of it

Lochlannach

https://www.boards.ie/discussion/comment/123160724#Comment_123160724

Cheers…another option to consider…

Lochlannach

https://www.boards.ie/discussion/comment/123160836#Comment_123160836

There will be a cooker switch to watch for...cheers

SC024

https://www.boards.ie/discussion/comment/123160893#Comment_123160893

Majority of cooker switches arnt an issue, the shaver sockets normally catch ya

10-10-20

https://www.boards.ie/discussion/comment/123160893#Comment_123160893

If mounted flush with tiles then you'll get 1cm of relief!

Lochlannach

Hmmm. No shaver sockets in the kitchen so no issue there! No tile relief where the sockets will be either given the current favoured design. Quickly looking at screwfix suggests that cooker sockets seem to need 47mm backboxes though that's probably more of a recommendation from the manufacturer I'll talk to the electrician.

SC024

https://www.boards.ie/discussion/comment/123163616#Comment_123163616

Exactly, I've done walls with cooker switches this way & had no problems

Lochlannach

Another one on this. How would wiring for ceiling lights be managed in the airtight insulated ceiling? Is there a need to batten out and include a service cavity behind the plasterboard there too or is there another way? Given the height I have to work with it's doable, but then again it's only going to be wires to 4 points as we'll be using track lighting to avoid puncturing the envelope.

SC024

https://www.boards.ie/discussion/comment/123166309#Comment_123166309

Ideally a service cavity inside the airtightness line if you have the height.

Lochlannach

Thanks all for the advice.

Plan is to create a full wrap, insulation|airtight barrier|service cavity|plasterboard, all round walls & ceiling - it will be a massive improvement over the current situation.

Anyone recommend any suppliers in the south Kildare area worth talking to? I'll be pricing with the local providers anyway but want to check with a few places to get the best deal.