Celotex / Kingspan

[OnOff]

Evening,

Probably one for a "builders" forum (anyone know a good one?) rather than here but hey ho! Wasn't sure whether to put this in the Pictures section either and there isn't a "can 'o worms" section! Anyway, finally making a start on my own main, downstairs bathroom in the bungalow. Everything is "wrong" on it. It was originally a flat roof extension to the original house comprising two added brick built cavity walls to make the new bathroom. Instead of putting in level roof joists and then adding firings to achieve the slope they actually sloped the the roof joists by about 6" one end. This meant the finished ceiling sloped. When the new roof was put on they KEPT intact the old flat felted roof. I've ripped down the old joists an uncovered all sorts of plumbing and electrical horrors - not a cable clip in sight! Not too worried as that's the upstairs ring albeit with some tap offs for sockets downstairs but that's all being being renewed



Am just about to put in new 6" joists and moisture resistant 1/2" plasterboard. Photo shows where I've put in new wall plates and shuttered and levelled the "connecting" wall - you can see the level issues. You'll see the old joists cut short which incidentally ran perpendicular to the the rest of the house. The new joists will run left to right in the photo:

 


My question relates to Celotex/Kingspan. The plan is to seriously insulate the new ceiling with this rather than the traditional "fluffy" stuff. Wondering whether to insulate over the top of the new 6" x 2"s thus leaving a service void or go between them? Thinking that leaving the void will give plenty of space for down lighters etc. Now, I've previously cut out Celotex and left damn great holes with fire hoods etc but this is my own place. I like the idea of keeping the "integrity" of the insulation. I've read recently that cutting out the insulation and fitting fire hooded voids breaks the foil moisture barrier & can cause a chimney effect where warm,moist air can be drawn up through the imperfectly sealed down lighter hole and cause it to corrode as well as unwanted heat loss. Just after any thoughts really as have a blank canvas. I also have a 9 something kW ceiling body drier to put in also and I'm considering how to maintain the insulation layer above that - thinking maybe ducting to it.

An "oddity" is that above the ground floor is a 24' x 24' dormer extension that sits on concrete pad stones. You can just the upstairs floor joists in the first photo. I may extend this upstairs floor into the loft space.

Thanks in advance!

 

[ProDave]

For a good building forum www.ebuild.co.uk. You might just see me there.

If you have the room go for the kingsplan over the top, BUT you have to do something around the edges, so a bit of kingspan between the joists for a little way in from the edges. Otherwise cold air will get in between the ceiling and the insulation and it will end up cold.

The details around the edges are vitally important to prevent cold air getting in. With the best will in the world you will never cut a slab of kingspan perfectly, so fill the gaps with expanding foam and tape all the joints between the sheets.

Attention to detail, and no (unfilled) gaps is as important as what you use and where you put it.

 

[OnOff]

For a good building forum www.ebuild.co.uk. You might just see me there.

If you have the room go for the kingsplan over the top, BUT you have to do something around the edges, so a bit of kingspan between the joists for a little way in from the edges. Otherwise cold air will get in between the ceiling and the insulation and it will end up cold.

The details around the edges are vitally important to prevent cold air getting in. With the best will in the world you will never cut a slab of kingspan perfectly, so fill the gaps with expanding foam and tape all the joints between the sheets.

Attention to detail, and no (unfilled) gaps is as important as what you use and where you put it.

Cheers. Daft question...........say the new Celotex sits atop the joists. I was thinking 150mm thickness. (I understand the comment ref insulating at the edges btw). To achieve the "seal" do I foil tape down the 150mm edge and onto the side of the joist? Ta.

EDIT: Just worked out how to add BIGGER photos from Flickr so have edited the OP. You might note some T&E clipped to the upstairs dormer floor joists - just visible behind the nasty pipework. I did that wiring, Then ripped up the old flat roof and found "spaghetti" white T&E & JBs you see just "hung" at the mo pending being ripped out.

 

[pewter]

I have done some work for a guy who owns a large fire stopping / insulating company. Some of his lads insulated his new build house. They used celotex/ kingspan  between joists and silver taped everything. They taped over the celotex onto the joists, in the loft all i could see up there was a sea of silver and no wood could be seen.

Job i am on at the moment, is a conversion of a commercial part of a building into domestic flat. It is under building control. There is an area where there is a flat roof. Building control made them drill holes at the top of the joists for airflow between joists, and have made them fit kingspan flush to the bottom with a gap above just under the flat roof. The holes were required for airflow, i have never seen it done like that before.

Its just what i saw, if its of any use.

 

[OnOff]

I have done some work for a guy who owns a large fire stopping / insulating company. Some of his lads insulated his new build house. They used celotex/ kingspan  between joists and silver taped everything. They taped over the celotex onto the joists, in the loft all i could see up there was a sea of silver and no wood could be seen.

Job i am on at the moment, is a conversion of a commercial part of a building into domestic flat. It is under building control. There is an area where there is a flat roof. Building control made them drill holes at the top of the joists for airflow between joists, and have made them fit kingspan flush to the bottom with a gap above just under the flat roof. The holes were required for airflow, i have never seen it done like that before.

Its just what i saw, if its of any use.

I have done some work for a guy who owns a large fire stopping / insulating company. Some of his lads insulated his new build house. They used celotex/ kingspan  between joists and silver taped everything. They taped over the celotex onto the joists, in the loft all i could see up there was a sea of silver and no wood could be seen.

Job i am on at the moment, is a conversion of a commercial part of a building into domestic flat. It is under building control. There is an area where there is a flat roof. Building control made them drill holes at the top of the joists for airflow between joists, and have made them fit kingspan flush to the bottom with a gap above just under the flat roof. The holes were required for airflow, i have never seen it done like that before.

Its just what i saw, if its of any use.

Ta, Yep I've been reading about the air flow. Seems you need 2" clear above the Celotex (according to their own bumpf)  for this to "wick" away any moisture.

I was debating filling in between the joists as you say and taping over the whole lot but have read this means you get different levels of insulation where the timber is? As I replace the wiring throughout I'm doing singles in galv tube anyway dud to a pathological fear of rats and squirrels! Was even debating taking the tube down thru the Celotex and feeding the very low profile surface mount down lighters you can get to avoid worrying about cutting bits out. Many ways to skin this cat I think.

Just joined too the forum suggested by ProDave and received a warm welcome.

 

[Evans Electric]

Last couple of jobs I've done the Celotex was fitted within the joists , builder said to clip my cables towards the top of the joist .

I think they cut it to slightly oversize , jammed  it between the joists then knocked a couple of nails in to hold it before plasterboarding .

 

[OnOff]

Last couple of jobs I've done the Celotex was fitted within the joists , builder said to clip my cables towards the top of the joist .

I think they cut it to slightly oversize , jammed  it between the joists then knocked a couple of nails in to hold it before plasterboarding .

Yes, I've seen this and took it as "the norm" and as I've said cut damn great holes and fitted fire hoods but going into it it seems to somewhat defeat the object. Getting some good feedback from the ebuild forum too.

 

[Manator]

As the government are introducing even tighter controls on building performances you will see this more and more. Buildings are now being designed that are so air-tight that BS7671 and building regulations like Part P are being compromised.

In the not so distant future heat retention and recovery systems will have to be deployed in all new builds. Many builders I have known do not install celotex accordingly with the instructions. Done correctly each joint is taped as Pewter said above.

There has to be an air flow, above and below to prevent moisture and the norm is for a breathable membrane to be deployed to assist this.

The question for electricians is, how can we install lighting in accordance with the manufacturer's instructions and still comply with other building regulations?

I am increasingly installing LED lighting, which are very efficient, however if driven, the drivers need a certain amount of air flow.The manufacturer instructions stipulate that they cannot be covered by insulation. So a compromise should be met. Or a different approach to lighting options needs to be addressed.

Just an option to debate!

 

[OnOff]

Down lighters are the least of my worries with the ceiling insulation / construction I'm considering:



Also looking to fit a ceiling mounted Triton body dryer which calls for the insulation to be removed for obvious reasons! Seems Triton DID a flat roof kit but I have no idea what that consisted of - guessing surface mount or some form of ducting?





Tbh it's an indulgence I picked up a while back from someone who'd bought it and then figured it too much trouble to fit. As said above this move to airtight / sealed ceilings is impractical with some features maybe. I mean how does this good practice not upset the whole idea of the warm roof or is it reliant on the lamp fitting making the "seal" and you just accept some loss from the ceiling:



I reckon I should keep the plasterboard off cut if/when I make the hole for it - it works OK on the bench now but if it goes wrong in the future I can see it coming out!

 

[ProDave]

As someone about to start building our new house which will be pretty much to passivehause standards, but not actually certified as such.

The key thing once you reach certain levels of insulation is ventilation heat losses start to become a major factor.  That's when you start sealing the house and making everything air tight, and ventilate the house with an MVHR unit (mechanical ventilation with heat recovery)  Only by doing that can you get to near zero heating requirements.

Services, both electrical and plumbing are simple.  It's normal to build the air tight building structure, then create a services void between the building shell and the internal plasterboard, 25mm is enough just for cables, but usually 50mm for walls that also need pipes in them.  then all your services can be laid in this void without compromising the air tightness of the building. and where you do have to penetrate the walls with a pipe or a cable, you seal that penetration to maintain air tightness.

Believe me, wiring a new build passive house with a services void is the easiest job you will ever have. nothing has to be compromised.

Regarding downlighters, quite apart from the fact I hate them, our new house will be built with all the insulation at rafter level, so the loft space will be inside the sealed and heated building envelope, so there will be no insulation at ceiling levels, so heaven forbid should I want downlghters they would not be compromising air tightness or insulation levels in any way.

building regulations have got tighter in recent years, but the house I am about to build will far exceed building regulation requirements in terms of insulation and air tightness.

It's a shame so few houses are built like this. It would make the job of all trades much easier.

I decided to go down this route for my new build when I was wiring one built like this near me.  It was the middle of winter, snow on the ground, certainly well below freezing, but the house was warm and cosy to work in.  What was heating it? it wasn't finished so the heating system wasn't running.  The answer was a single thermostat controlled plug in electric convector. Rated at 2KW but the average power would be a lot less than that as the thermostat clicked on and off.  So probably a heating load of 1KW to maintain a comfortable temperature throughout the whole house when it's well below zero outside.

Now in a normal house with stuff in use, people in the house, cooking etc, much of that heating requirement comes from just living in the house, hence they approach zero energy requirements.  It really was an eye opener what can be achieved.

 

[chris_k]

Im currently on a large hi spec extension. Celotex everywhere. The kitchen ceiling is vaulted and the client wanted lashings of downlights. I explained the problem with cutting holes in the cellotex to accommodate the lights so he redesigned the roof.

• 150mm of celotex in-between the rafters. tight fit with any voids spray foamed in.
• Multi-ply foil insulation stapled to underside of rafters and foil taped around all edges.
• 150mm rafters nailed on top of the foil then plasterboard.

This gave enough room for the downlights but must have cost an absolute fortune. But hey, he got the lighting he wanted.

 

[OnOff]

You've got me thinking now whether to insulate the main roof above and NOT the bathroom ceiling...........will knock up a sketch. Throwing this about on the ebuild forum too thanks to ProDave.