New 1st floor extension - improving air tightness help?

[rh2205]

Morning,

 

Probably a bit late in the day to be posting this but our architect didn’t really have much knowledge of the area & our builder just does things the old way (much more common in the refurb area than new build I know)!

 

The new roof & 1st floor is a warm roof formed of 100mm PIR between the rafters with 50mm air gap & roof membrane & then 50mm phenolic insulated plasterboard below rafters. For the walls it is 70mm PIR between the studs with membrane, then 70mm PIR externally with another membrane on top before the cladding. Outside walls & roof etc done but insulation still exposed on the inside/not finished as pretty big area for them to cut between. Anything we can do internally at weekends at this late stage to improve airtightness?

 

We have ended up going with a air source heat pump as no gas but are now concerned we should be doing more on airtightness as nothing else other than the description above put on the drawing so I presume it won’t be particularly air tight hoping others have some suggestions as really struggled with information on what we can do to reduce leaks?

 

FYI downstairs is a refurb with 90mm EWI & 50mm EPS cavity fill we are having velfac windows set out using lugs into the insulation but doors/patios are another leaky conundrum and have to be set on the bricks (not even in line with cavity due to some old steel lintels we hadn’t considered & render not being able to go over these). We know this is a cold bridge issue but window orders are done so cannot bring insulation into the reveals now & velfac sashes have no fixed frame it all moves so cannot overlap at all externally. Would some Xtherm 20mm insulated plasterboard on the door internal reveals make this situation any better or is this just throwing more money after bad? 

 

These are all things we requested to be added on insulation wise with architect but really we are not experienced in this & have just been trying to make sure we could get it built quickly given we are renting so our ideas may be somewhat of a halfway house.

 

Basically we think other than the downstairs door screw ups we had some kind of insulation plan just execution might now not be right as I get the impression from reading it’s actually the details that matter especially to stop air leaks? 

[Russell griffiths]

So you don’t have an airtight membrane at all then, boards between studs or rafters will not form an airtight seal. 

 

So you either turn what you have into a membrane or install  a separate membrane over the top of what you have. 

 

Version 1 all boards need fitting tightly to the studs rafters, squirty foam everything, then silver aluminium tape every join, join each board to each stud then the next until you have an entire room that is silver, there won’t be a bit of timber on show. 

 

A bloke on here recently did a very good job zoothorn look up his extension questions it’s in there somewhere, it will basically look like a cannabis grow room. 

 

Version 2. buy a roll of air tight bembrane and tape it over the face of all walls, ceilings, taping all joins and taping walls to ceiling and walls to floor including void under the first floor ceiling. 

 

Look on youtube, loads of product videos showing procedure

 

poinless having lots of insulation if it’s air leaky.   

[Onoff]

I've only refurbed one room here and even being on this forum I didn't quite grasp the importance of air tightness otherwise I'd have designed elements differently. It'll certainly influence what I do on future rooms. A refurb is always imo harder than starting with a clean slate of course as you have to constantly work around other people's screw ups! It also took a visit to a highly insulated, low energy house and an understanding of it's construction to make me see (as I now do), the light.

 

Basically I renewed the ceiling joists:

 

 

I infilled with pir:

 

 

Foil taped the joints:

 

 

The went over the lot with a membrane:

 

 

That shows the membrane concept. At that point it was pretty toasty and airtight.  (Still cold bridges at the joists). All this is a bit moot as I have bfo cutouts for downlights, speakers and a ginormous, ceiling mounted body dryer. I've always had a plan to reinstate the insulation and membrane at these points but it's more work, a lot of which could have been avoided (not so much the body dryer mind). I maybe SHOULD have counter battened and used low profile LEDs.

 

Lowering the ceiling a tad with counter battens would too have helped with tiling and decreased the room volume a bit meaning less to heat. 

 

The walls followed with battens infilled with pir then a taped vcl.

 

 

 

Ceiling vcl joins the wall vcl joins the floor vcl:

 

 

The only saving grace to all this is I hope to eventually have a warm roof and ewi effectively wrapping all my mistakes in a thermal envelope.

Edited November 29, 2020 by Onoff

[rh2205]

@Russell griffiths @Onoff? roof rafters will all be overboarded with some pretty expensive phenolic insulated plasterboard, can we make this layer into the airtight membrane or is this impossible? The insulation cut and installed between the roof joists has not been inserted with squirty foam gah! I don’t see any internal membranes or tape on the roof internally..

 

The entire outside walls including studs (but not the roof ?) have been wrapped in big boards of PIR, they used a membrane to wrap the house before the external PIR went on and then a Kingspan one after - this was the insulation brand . I didn’t see any tape but it all overlaps, surely all these membranes are for airtightness or manufacturers wouldn’t sell it?!? They also both looked more like a breathable membrane than a plastic looking one compared to your images. There is also PIR cut between the wall studs but the studs are very thick so insulation not flush and again pretty sure no foam used.. the walls will just be plasterboarded no fancy insulation plasterboard...

 

Pretty worried now that our expensive insulation is about to go to waste but also at least the wall build up sounds a little different from the usual approach or am I wrong on that too?

 

Why aren’t there better guides for this and why don’t architects understand the principles pretty sure they are working exactly to as drawings described ?

[rh2205]

@Russell griffiths I enclose some images of the most complicated roof structure we could have chosen (or rather to appease planners). As you can see the roof detailing for airtightness has not been considered properly in hindsight & I am not sure I grasped what a warm roof was but it turns out that’s not what we’ll have rather a cold roof oops! Considering this is all new roof we could have done better but oh well. I see that I need to tell the builder to not stuff little bits of insulation in gaps like that as they haven’t finished & maybe tape it ourselves as not specified but this might be pointless as no squirty foam used, there is then some insulated plasterboard covering over it all. Is it ok to use an airtight membrane inbetween the rafters and the insulated plasterboard or will this create damp? I also don’t know how we easily tape a membrane over such a high complex vaulted ceiling my YouTube searches have been fairly unsuccessful in figuring out how we do the second option if it’s pointless to attempt option 1 as foam hasn’t been used?

 

The new external walls (unfortunately only to 3 sides as asymmetrical dormer pitch) don’t suffer quite this same problem as I guess they are effectively warm walls if you can call them that, but obviously the junctions between the cold roof and warm walls have not been considered at all so still a screw up as anything we do now to improve can be internally only with difficulty.. we do have some downstairs EWI going on existing masonry which can maybe improve the junctions slightly but probably not totally.

 

The guides I’ve found don’t seem to help me especially for the cold roof as we also have old joining onto new and some sensible new warm wall arrangement with a less sensible new cold roof coming together! Thoughts on last minute additions for upstairs before it’s all boarded in plasterboard !!?

Edited November 29, 2020 by rh2205

[rh2205]

Sorry for the multiple posts but I am reading different things on different websites regarding what is a warm roof.. maybe we do have a warm roof but what we are missing is the vapour control layer no.6 in diagram attached otherwise our roof structure is identical to this image with the addition of insulated plasterboard. However I am slightly confused as we are not using just plasterboard rather insulated plasterboard do we still need this air tight barrier before the insulated plasterboard goes on or can it just be taped like normal for plastering as a 3mm skim will be going over it all?

[Big Jimbo]

Do you have insulation just inbetween the roof joists, or do you also have it above the roof joists ? .....If you dont have insulation above the roof joists you do not have a warm roof.

[Russell griffiths]

You can fix this easily 

ditch the insulated plasterboard, it’s very expensive and only does half a job. 

Instead change it into two different products doing the same and a better job

fit pir board on the inside taped and joined to each other to make a complete flat silver surface, then add a batten to form a gap to run services through and so no lights or other things penetrate this silver vapour control layer, then add plasterboard. 

 

You need in theory to make a fully sealed up tinfoil box, any water moisture will find any gaps and go inside the structure, it then turns into condensation as it hits a cold surface, if you stop it getting there by keeping it inside your box then that’s what you want. 

[Russell griffiths]

this is a picture of mine, ignore the junk we where trying to plan our kitchen. 

You can just see the ceiling. 

Thats 70mm of pir board on the underside of my joists, then a batten, plasterboard is fitted to this batten

if you can see the boards are taped together and sealed to the wall with an air sealing paste, you will just tape it to your pir in the walls. 

 

I noticed you have a few cables running along the sides of joists, is it to late to move them inside the thermal envelope of the building, every penetration through the vapour layer will need sealing. 

Edited November 29, 2020 by Russell griffiths

[rh2205]

@Russell griffithsIt’s too late, unless we want to pay for the electrician again ?. Is this the only alternative for resolving? I think I spoke to the builder last week. It is not easy getting hold of materials at the moment, he said there wasn’t other cheaper options other than the insulated plasterboard as he didn’t want to use it as bloody expensive and obviously in contract with him I think the order went in when we spoke. Basically covid and Brexit we are going to really suck at this rate.. plus that’s really what made us hurry up, didn’t feel we were in good conditions to be organising the perfect design. What can we do now that we’re probably already stuck with the insulated plasterboard design?

Edited November 29, 2020 by rh2205

[rh2205]

If it transpires we can get PIR boards without great difficulty and builder hasn’t ordered the insulated plasterboard then will the cables improve impossible to work with without resulting in damp patches? Obviously this may not be able to be undone in terms of ordered materials but they definitely haven’t arrived on site yet. Also not sure what thickness we will need as the Insulated plasterboard was 42.5mm thick k118 stuff. Always on the back foot of everything!

 

I am also certain none of the external PIR wall boards were taped?‍♀️. But this layer is a lot less unbroken anyway and there’s two membranes so I am kinda hoping for the best as it’s a slightly better insulation arrangement.

[Russell griffiths]

There are very few materials shortages, just people using it as an excuse. 

 

Just use 50mm pir, then tape it then plasterboard, it will be cheaper, that insulated plasterboard is very expensive. 

 

 

[rh2205]

@Russell griffiths Thanks your input has been invaluable. It does mention a surge of PIR orders on insulation hub website so as much as maybe orders can be fulfilled in some way but not with the same product so I do believe what he is saying is true and probably depends on what merchant he is speaking to, we do not have an architect so effectively you could say he is also the PM for it and therefore it would be very time intensive for him to shop around considering he does also do physical work on the site and manages a few multi skilled employees under his set up also along with the subcontractors. Seems he can maybe say that extra man hours cost to cut double the stuff and man hours are expensive in this part of the country so I am still not sure whether it will be cheaper in our case.

 

Is there any effective solution to work around the cables as we definitely don't want to undo this part as the wiring that has already been done over a pretty large roof? Not looking for perfection but something than goes a bit better than what is currently being done especially if it does transpire the insulated plasterboard order has gone in. Unfortunately this is pretty much an all in contract so it gets a bit messy when you try and change things unless you are the one to physically do the changes which is what we were hoping for in this case - for instance us going in and fixing on a VCL on the internal roof or taping joints before plasterboard goes up would be quite a clean way of rectifying the matter if that makes sense. 

[rh2205]

£400 to cancel the order from the merchants as already loaded (it was quite a big order) any saving in materials worked out about the same in labour. Turns out VCL was specified in one of the cross sections I’d not looked at so now that will be going on albeit after all the insulation. Our light scheme is a basic single pendant arrangement. Any recommendations for sealant around the cables given we aren’t going to redo the electrics now?

 

Just got to go in at the weekend and tape everything for peace of mind that this sleepless night over some insulation was worth it!

[Dave Jones]

On 29/11/2020 at 09:20, Onoff said:

I've only refurbed one room here and even being on this forum I didn't quite grasp the importance of air tightness otherwise I'd have designed elements differently. It'll certainly influence what I do on future rooms. A refurb is always imo harder than starting with a clean slate of course as you have to constantly work around other people's screw ups! It also took a visit to a highly insulated, low energy house and an understanding of it's construction to make me see (as I now do), the light.

 

Basically I renewed the ceiling joists:

 

 

I infilled with pir:

 

 

Foil taped the joints:

 

 

The went over the lot with a membrane:

 

 

That shows the membrane concept. At that point it was pretty toasty and airtight.  (Still cold bridges at the joists). All this is a bit moot as I have bfo cutouts for downlights, speakers and a ginormous, ceiling mounted body dryer. I've always had a plan to reinstate the insulation and membrane at these points but it's more work, a lot of which could have been avoided (not so much the body dryer mind). I maybe SHOULD have counter battened and used low profile LEDs.

 

Lowering the ceiling a tad with counter battens would too have helped with tiling and decreased the room volume a bit meaning less to heat. 

 

The walls followed with battens infilled with pir then a taped vcl.

 

 

 

Ceiling vcl joins the wall vcl joins the floor vcl:

 

 

The only saving grace to all this is I hope to eventually have a warm roof and ewi effectively wrapping all my mistakes in a thermal envelope.

 

lights ?

[Onoff]

7 minutes ago, Dave Jones said:

 

lights ?

 

Big holes in the vcl and insulation! ?