Bubblewrap Insulation Under Roof

FrankGrimes

I've started insulating the roof of my extension which has a warm roof as the the internal ceiling will run with the line of the rafters (i.e. rafter slabbed internally).

Based on advice from a builder I know, I've started insulating between the rafters with 100m rigid kingspan foam. As VikingHouse pointed out in another thread, this is a bit of a painful job and you often end up with gaps at some edges, which I plan to fill with fibreglass or rockwool. I'm leaving a gap between the slates and the foam for ventilation

My friend also advised that I lay 'bubblewrap' on the internal side of the rafters before slabbing. I'll have to dig it out from storage but I'm pretty sure this is foil backed bubblewrap.

Is the bubblewrap really necessary? What's the best way to fit it if so (staple to rafters, but overlap it at edges or butt it up together and tape the joints)?

Thanks

dixiefly

Would you not be better off with a Thermal Plasterboard with the insulation already attached to the board?

Mellor

FrankGrimes wrote:

I've started insulating the roof of my extension which has a warm roof as the the internal ceiling will run with the line of the rafters (i.e. rafter slabbed internally).

I've started insulating between the rafters with 100m rigid kingspan foam...... I'm leaving a gap between the slates and the foam for ventilation.

You said warm roof buy what you described is actually a cold roof structure.
Is this bubble wrap a low-emissitivity insulation. These sheets reflect heat as opposed to stopping it from passing easily like normal insulation. Most types need a good seal between laps as well as some space (suchy as in a vent space), they wont work to well if they are sandwiched between slabs and insul boards.
You would of been better using mineral wool insulation. Quilt ir the semi-rigid kind that you can compress slightly and get a very good fit between rafters.

FrankGrimes

Mmm, you sure its not a warm roof? Not an expert on it by any means but I'm sure the description I read had mine down as warm, and it kinda makes sense that an insulated roof is a warm one, whereas an uninsulated roof, which would have an insulated attic floor below and no rafter insulation, would be a cold one, but interesting to see if it's actually the complete opposite that's true.

A thermal slab with foam on the back means I'd be losing space on the inside of the room and I wonder how say a 50mm foam backed slab would compare to 100mm rafter insulation....could imagine quite well due to the lack of gaps like with the rafter insulation.

yep, mineral wool does sound like it would have been ideal, will know for next time I suppose! will wait for my friend to get back off his honeymoon this week and ask him where he planned to use the bubblewrap and why.

Mellor

Oh, sorry.
When you said extension i assumed it was a flat roof, warm and cold roofs are used alot to describe flat roofs. A flat roof with the insulation between joists is a cold roof.
a pitched roof is warm and cold in the situations you described. Sorry, I dunno why I assumed in was going to be flat.
But my other comments with regards mineral fibre insulation is even more relevant to a pitch roof.

FrankGrimes

thanks.

I think my friend suggested rigid foam to prevent the insulation from sagging - how could I prevent against this with mineral fibre?

Would mineral fibre give a much better level of insulation in this scenario? How much would it cost of about 35 sq metres of it? I'd consider just scrapping the foam and replacing it with mineral fibre if it's cheap, but over half the foam is in already so it would need to be pretty drastic to justify it.

tribesman

Don't scrap the foam. 100mm of polyiso (kingspan) is more effective that 100 mm of mineral fibre. Granted, it is more difficult to fit without gaps but it's a better job if done carefully.

I don't know anything about bubblewrap but adding an insulated plasterboard underneath would give you a continuous layer of insulation with no gaps (cold bridges) at the rafters. If you already have 100mm in between the rafters then you could use the thinest insulated slab you could find. AFAIK this would be about 34.5 mm (25mm for the insulation and a 9.5mm slab) This wouldn't take much space. These are about 28-30 euro each.

Viking House

FrankGrimes wrote:

thanks.

I think my friend suggested rigid foam to prevent the insulation from sagging - how could I prevent against this with mineral fibre?

Would mineral fibre give a much better level of insulation in this scenario? How much would it cost of about 35 sq metres of it? I'd consider just scrapping the foam and replacing it with mineral fibre if it's cheap, but over half the foam is in already so it would need to be pretty drastic to justify it.

If you insulated two similar roofs to the same U-value with Rockwool and Kingspan. The house with the Kingspan roof will have a 15% higher heating cost. This has to do with the Decrement delay and the specific heat capacity of the insulation material.
They rarely use Polyeurethene in roofs in Scandinavia because it gives bad Thermal Imaging results at the rafter joints.

JohnBoy

it's hoorish stuff to put up, i'm doing it at the moment....

with a bit of care you can get fairly decent seals around it too, I plan on running expanding foam into any gaps to seal em. i'm also considering using an thinly insulated slab under the kingspan to offer double protection against gaps and reduce thermal bridging.

how does the thickness->u value ratio of rockwool compare to kingspan? because the reason i was given for the kingspan was that it was rigid but also had a high insulation to thickness.

i'm putting in 100mm kingspan (well it's quinn-roof, but similar)

also frank i think the term is warm room as opposed to warm roof

secman

To Viking House :

You seem to be very knowledgable on insulation matters, so I would welcome your thoughts on insulating a dormer bungalow with slate roofed sunroom, as in which style of material to use in each area.

1. Roof area - between rafters
2. would you insulate above upstairs ceiling
3. Upstair partition walls
4. between upstair floor/ downstair ceiling
5. Inside of exterior walls.
6. Downstair interior walls - necessary ?

We were thinking of drylining the inside of all exterior walls. ??

Thanking you in anticipation, reading all the various sales litrature is melting my head, THEY are all the BEST !

FrankGrimes

the kingspan is all in now, and funnily enough I was also thinking of filling the gaps with expandable foam. I'm not going with downlighters now so don't have to worry about the foam catching fire.

Will consider putting up thin insulated slabs to reduce thermal bridging, though I wonder if the bubblewrap would do the same job, maybe less effectively but could be reasonably effective? I bought the bubblewrap ages ago when starting the renovations so would have hassle bringing it back and I think it's expensive enough so I'd like to make use of it if it will do the job.

Mellor

Have a look at the products website, they will have the correct details up. if its done wrong its useless really.

Viking House

FrankGrimes wrote:

the kingspan is all in now, and funnily enough I was also thinking of filling the gaps with expandable foam. I'm not going with downlighters now so don't have to worry about the foam catching fire.

Will consider putting up thin insulated slabs to reduce thermal bridging, though I wonder if the bubblewrap would do the same job, maybe less effectively but could be reasonably effective? I bought the bubblewrap ages ago when starting the renovations so would have hassle bringing it back and I think it's expensive enough so I'd like to make use of it if it will do the job.

Hi Frank

Use the bubblewrap as a vapour barrier, tape the joints and tape it to the wall with special tape and leave a cavity between it and the plasterboard.

Viking House

secman wrote:

To Viking House :

You seem to be very knowledgable on insulation matters, so I would welcome your thoughts on insulating a dormer bungalow with slate roofed sunroom, as in which style of material to use in each area.

1. Roof area - between rafters
2. would you insulate above upstairs ceiling
3. Upstair partition walls
4. between upstair floor/ downstair ceiling
5. Inside of exterior walls.
6. Downstair interior walls - necessary ?

We were thinking of drylining the inside of all exterior walls. ??

Thanking you in anticipation, reading all the various sales litrature is melting my head, THEY are all the BEST !

Hi Secman

Is this a newbuild or a refurb?

1. Roof area - between rafters
You will get the best payback by insulation your attic, consider 400mm of rockwool. Try to eliminate the cold bridging of the rafters by insulating over them. A dense 30-50mm windstop softboard works well when used with rockwool as it allows your roof to be ventilated properly without the risk of the heated air being blown out of the rockwool. 1mm of softboard gives the same insulation effect as 10mm of Rockwool.
I am not a fan of putting Kingspan between rafters or joists because of fire risk, offgasing and attic overheating.

2. would you insulate above upstairs ceiling
Yes

3. Upstair partition walls
Mostly for noise

4. between upstair floor/ downstair ceiling
Yes, I would only heat my living area below and put no rads upstairs.
I normally put Devi mats in the bathroom.

5. Inside of exterior walls.
Insulation outside the exterior walls works much better as it eliminates cold bridging.

6. Downstair interior walls - necessary ?
We always do it anyhow to maintain the heat in the zones and for noise.

There is plenty of info on insulation and links to some of the best insulation companies in Scandinavia on my website. www.viking-house.net

anon1

What is softboard?

Viking House

http://www.viking-house.net/softboard
There are some links on this page to some suppliers.
We normally buy it in Poland for floors and roofs.

ircoha

Viking House wrote:

If you insulated two similar roofs to the same U-value with Rockwool and Kingspan. The house with the Kingspan roof will have a 15% higher heating cost. This has to do with the Decrement delay and the specific heat capacity of the insulation material.

VH: very interesting observation indeed.

As the focus at the consumer level is at the heating cost, how should we adjust the U-value to reflect the different costs of heating 2 houses, both with the same U-value but using kingspan and rockwool.

As I understand it the current Irish building code, TGD Part L, if I am not mistaken, requires builders/buildings to meet specific U-values.
If I am correct then it needs to reflect the import of your observation.