# Flat Roof Insulation - Kingspan or Rockwool?



## teddymac (8 Mar 2011)

hi, 
I am getting a flat roof in an extension replaced and upgrading the insulation which is probably non-existent. I've had 2 builders look at it. They are both suggesting a PVC roof but their insulation methods are different.

One proposes using 2 layers of 60mm Kingspan foil faced insulation boards - the 2 layers laid in opposite directions to minimise gaps. The boards go above the joists (I think) and then there is a sheet of plywood followed by PVC.

The other proposes laying 200mm of rockwool on the existing ceiling and then plywood/PVC layers similar to above. He says the rockwool is better than the rigid boards as it can be pushed into every area.

How would these 2 options compare in terms of effectiveness of insulation and cost - I don't have prices from them yet. The man proposing Kingspan says the combination of the 2 60mm layers would have a U value of 0.18.


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## Lex Foutish (8 Mar 2011)

teddymac said:


> hi,
> I am getting a flat roof in an extension replaced and upgrading the insulation which is probably non-existent. I've had 2 builders look at it. They are both suggesting a PVC roof but their insulation methods are different.
> 
> One proposes using 2 layers of 60mm Kingspan foil faced insulation boards - the 2 layers laid in opposite directions to minimise gaps. The boards go above the joists (I think) and then there is a sheet of plywood followed by PVC.
> ...


 


In the long run, the Kingspan would be a better option, I reckon. Its insulating qualities will last longer. The Rockwool certainly would be easier to get into all the space but it shouldn't be compressed. The Kingspan would probably take a bit longer to put in but it'd be worth it. 

If it's going between the joists, you could use [broken link removed]. It fits into place in concertina fashion. Kingspan do a version of it as well and it's slightly easier to squeeze.

For the record, Rockwool has a U value of 0.44 so 200mm of it will give a U value of 0.22. If you do decide to go the fibreglass route, don't use Rockwool. Use [broken link removed] instead. It has a U value of 0.34 so 200mm will give a U value of 0.17. Much better! A 7 square metre roll 100mm thick will cost you about €60.


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## threebedsemi (9 Mar 2011)

You have here two proposed options, one builder is proposing a 'warm roof' (the kingspan option with the insulation over the rafters) and the other is proposing a traditional 'cold roof' (insulation laying between rafters).
The warm roof is a MUCH preferable option as you avoid the need to provide ventilation over the insulation. If you lay insulation between the rafters, you need to allow a min. 50mm deep cavity between the insulation and the plywood above, with this cavity fully ventilated to allow airflow to pull off the condensation which will appear and avoid moisture build-up and subsequent deterioration of the roofing timbers.

I would not touch a builder who did not at least raise this issue, and who said that he was going to 'stuff in' rockwool everywhere. It shows a fundamental lack of understanding of building physics.


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## Roamer808 (10 Mar 2011)

Stuffing anything in will not work. Any insulation has to be 'fitted'. That means installed according to the makers wishes. I insulated my entire house after I gutted it (back to bare walls, top to bottom). Where ever possible I used foil faced Kingspan or Xtratherm. And I used as much as the situation would allow.  Foil faced has way higher values for the thickness used and does not sag. It is also moisture resistant, which any wool product is not.


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## Lex Foutish (12 Mar 2011)

Roamer808 said:


> Stuffing anything in will not work. Any insulation has to be 'fitted'. That means installed according to the makers wishes. I insulated my entire house after I gutted it (back to bare walls, top to bottom). Where ever possible I used foil faced Kingspan or Xtratherm. *And I used as much as the situation would allow.* Foil faced has way higher values for the thickness used and does not sag. It is also moisture resistant, which any wool product is not.


 
Good point. I cannot understand why people use a bare amount when insulating. You'll never have too much of it.


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## haZ (16 Mar 2011)

*Rockwool vs Kinspan*

Hi guys,
Allow me to join in: Rockwool vs Kinspan.

1. The properties of these materials are different. Rockwool is flexible  and allows for slight movements in the building. Kingspan is rigid and  doesn't allow any shifts (due to seasonal changes, temperature day/night  shifts). Kingspan is fitted with insulation foam, which will flake (due  to these movements) and even 1mm gap may increase U value by 500% (not a  good thing).

2. Breathable/Non-breathable. Rockwool would allow air movements and  escape of moisure, whereas with Kingspan will trap condensation, which  will rot the joists.

3. U value = lambda/m. 
Golden standard of U value in residential buildings is 0.16. (even  thought SEAI recommends U value = 0.23 for atticks with Rockwool).  That's what we were taught in FETAC Thermal Insulation course.
a) 3 layers of Rockwool (300mm), thickness 0.30m. U=0.044/0.30=0.146. Rockwool will go above the joists. 
b) 2 layers of Kingspan (60mm), thickness 0.012m U=0.022/0.012=1.83. Mmm... Really bad U-value!!!

4. My favourite. Rockwool consists of fibres, so just be careful.  Kingspan is board with phenolic resins - really bad for the health.  Rockwool meant for inside, Kingspan for outside (and mainly industrial  buildings)

5. Finally, Kingspan lost NSAI Agrement certification and only Weber is  approved to be fitted inside the residentials. Ring NSAI, if in doubt.

Hope this helps to fire builder #1 (cowboy) and get the job done with  the most appropriate material, as recommended by builder #2 (good  reliable guy).


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## Lex Foutish (16 Mar 2011)

Very interesting post, haZ. What surprises me, generally, when people are talking about fibreglass insulation is that Rockwool is the one generally referred to. Moy Metac is a vastly superior product and not much more expensive if you cost it per square metre. Rockwool has a U value of 0.44. Metac has a U value of 0.34, so 200mm will give a U value of 0.17. I've used a lot of it myself and found it great. 

The FETAC Thermal Insulation Course sounds a very interesting one. Did they refer to products like Metac or Knauf Rafter Roll, which has a U value of 0.32? I would have used Rafter Roll myself but I found it almost impossible to source and it was much more expensive than Metac.


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## onq (17 Mar 2011)

haZ said:


> Hi guys,
> Allow me to join in: Rockwool vs Kinspan.
> 
> 1. The properties of these materials are different. Rockwool is flexible  and allows for slight movements in the building. Kingspan is rigid and  doesn't allow any shifts (due to seasonal changes, temperature day/night  shifts). Kingspan is fitted with insulation foam, which will flake (due  to these movements) and even 1mm gap may increase U value by 500% (not a  good thing).
> ...



Haz, some of what you say is justified, but not your insulation recommendation.
They say a little knowledge is a dangerous thing.

How exactly are you to move around an attic to deal with servicing issues with 900mm quilted insulation in your way?
How can you provide the 50mm air gap from the eaves to the open area of the attic?
You cannot - you will cause condensation within the attic space that's left.

Condensation on the quilted insulation will reduce its effectiveness and may affect the timbers over time.
We had threads about this on boards.ie and aam in the cold snaps this past Christmas and the Christmas before.

If you super-insulate internally it will tend to cause problems with the masonry as well as it will not dry out well and the surface will spall.
Up until the rpesent age of super insulation heat passing through the structure promoted evaporation, dispelling moisture.
Moisture remaining in cold walls at ambient temperatures on cold days means flaking due to frost action can occur.
If you're relying on the surface for weathering you have to detail it to deal with such harsh conditions.

You have to apply super-insulation externally where it can do most good and benefit the structure.
If there is a problem, its outside the structural envelope and can be got at.
This approach is based on retrofitting existing masonry structures.

Construct Ireland regularly features [broken link removed] on such systems.
A lot of thought is required regarding servicing and details.
A competent archtiect should design these details.

Whoever is teaching that course needs a good rattle to wake them up.
You need a competent architect advising you on the placement of insulation from the start of the project.
I strongly advise people to use people competent and experienced in the design of buildings to advise them on insulation
You cannot just employ somone who seems to have read the so-called "approved details" on the Dept. of the Environment website from back to front.
Design of the built work starts with the plans, but moves directly to the section for detailing purposes, before resolving everything in the elevations to create the "Look" required.

Finally Haz, seeing as this is your first post and you seem very knowledgeable, can you confirm you relationship, if any to the building/insulation industries and any of the companies mentioned?

ONQ.

  [broken link removed]

  All advice on AAM is remote from the situation and cannot be relied    upon              as a defence or support - in and of itself - should    legal      action    be      taken.
  Competent legal and building professionals should be asked to advise   in               Real Life with rights to inspect and issue reports on   the       matters    at      hand.


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## Fiskar (17 Mar 2011)

b) 2 layers of Kingspan (60mm), thickness 0.012m U=0.022/0.012=1.83. Mmm... Really bad U-value!!!


You have a miscalculation in any event, U value should be 0.183, 
 2 layers of Kingspan (60mm), thickness 0.12m U=0.022/0.12=0.183.


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## onq (17 Mar 2011)

Thanks Fiskar.

I didn't check the poster's calculations.

Mine was a basic building physics amd usability review only.

ONQ.

  [broken link removed]

  All advice on AAM is remote from the situation and cannot be relied     upon              as a defence or support - in and of itself - should     legal      action    be      taken.
  Competent legal and building professionals should be asked to advise    in               Real Life with rights to inspect and issue reports on    the       matters    at      hand.


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## Lex Foutish (18 Mar 2011)

Does anyone want to take up my point about Rockwool Vs Moy Metac............?


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