Insulating a vaulted ceiling?

anon1

What is the best method of insulating a vaulted ceiling?

YAPP

Hi,

Depending On The Rafter Depth, Easy Method Is
To Fit Ridgid Kingspan (or Similar Approved)
Insulation (140mm Preferably)
Inbetween Rafters.

If A Breather Membrane Is Fitted In Lieu Of Felt
And The Rafter Is 225mm Min, Then You Can Fit
Fibreglass Quilt.

If The Surface Area Needing Insulation Is
Not Large, Then I Would Recommend The
Solid 'rigid' Board Insulation For Ease Of
Fitting.

S

CJhaughey

You could blow in cellulose insulation.

rooferPete

Hi anon1,

For a new build I would go with the system Yapp posted, it is important to adhere to the Building Regs 50mm clear air space between the insulation and the roof covering as vaulted roof spaces can get very warm due to the fact that heat rises.

For a retro fit I would go with CJ's method.

.

anon1

The felt used is protect which is breathable. The rafter depth is 6 inches.
Should I put a 100mm quilt in between the rafters leaving an air gap above and some sort of rigid insulation under the rafters.
I want to use a vapour barrier underneath that is breathable. What can I use for under rafter insulation in this case?

anon1

Currently, warm roofs can be built in two ways: any type of insulation with any underlay and a ventilated airspace between the underlay and the insulation, or the use of a high vapour resistant insulation board (or vapour barrier) and vapour permeable underlay, with no ventilated airspace between the two.
However, a space open to the atmosphere is still required above the underlay and below the roof covering in these situations to prevent excessive condensation in a roof or roof void above an insulated ceiling.
In warm pitched roofs this can be achieved by ensuring a 50mm clear airspace from low to high level. This option is available with any insulation and underlay type.
The alternative of not ventilating the space between a high vapour resistance insulation board and a vapour permeable underlay, provided an independently certified vapour permeable underlay with a resistance of not more than 0.25MNs/g is used and that counter battens and ventilation are provided above the underlay.
My question is if the roofer puts Protect VP400 vapour permeable underlay over the rafters and then puts tiling counter battens directly over the underlay is there enough of an airspace between the underlay and the slates to satisfy standards etc?

anon1

http://www.roofconsult.co.uk/articles/partL.htm

anon1

See attached file:
Do I need counter battens ?
In general, counter battens are not required if the underlay is draped between rafters, ie a
nominal 10 mm. If however it is not draped but pulled taut, counter battens are applied.

rooferPete

Hi anon1,

I think you will find my colleague in Roof Consult is referring to the UK building regulations which are also subject to Local Authority Building Bye Laws.

The counter batton method has been in existance for at least 150 years, it was originally used over close boarded roofs, I expect it was used more in England and Ireland than Scotland because of the high winds experienced in Scotland they use a sarking felt and nail straight to the close boarding.

I can assure you and this is from my experience with Roofing Consultants I have worked with in the USA that it is not a good idea to fill the rafter depth with insulation that makes contact with a breather membrane.

I expect Protec will tell you that insulation touching their product is counter productive to using a breather membrane, in fact any underlay supplier who is familiar with the Dew Point in insulation should tell you the same thing, in fairness I would not expect them to be taking account of freeze / thaw cycles in Ireland.

The purpose of a vapour barrier at ceiling level is that it should not breathe because it may allow moisture into the insulation which will render the insulation practically useless over time and cause your rafters to rot because the moisture will be absorbed by the timber.

There are two ways of insulating your roof to a high standard while staying within the building regulations (IRL) and good building practice :

Use 100 mm insulation board fitted tight between the rafters, fitting light guage polythene under the rafters and then slabbing the ceiling.

As above without polythene but use foil back plaster boards and depend on the foil with each joint being properly taped and sealed, you can then skim same, however this method will get you a vapour check as opposed to a vapour barrier.

The jury is still out regarding use of the polythene as a full vapour barrier because of the fact that the fixings for the plaster board will be puncturing the sheet.

To the best of my knowledge even if you do use a counter batton the chances are a knowledgable Architect will not approve the installation if it does not have an eaves vent that provides a through flow of air under the breather membrane.

Roof or ridge ventilation will also be required to ensure the air has a through flow and is not expected to find it's way through your underlay because it is defeating the roof ventilation.

The above is of even more impotance if you are slating the roof as concrete tiles do have more spaces between them acting as ventilation by accident.

I hope the above is of some help.

.

anon1

What do you think of this vapour check at ceiling level?
http://www.ecologicalbuildingsystems.com/products/intello/

rooferPete

Hi Anon1,

First thoughts are not much difference between that product and the Protec that you are using on the roof, it might be a better option to use the foil back plaster boards because both will provide a vapour check but not a vapour barrier.

.

toshy321

Bringing up this post again just a quick question with regards to roofer petes post the light gauge polythene that you put underneath the 4inch insulation this wudnt be breathable would it as it's plastic.

rooferPete wrote: »

Hi Anon1,

First thoughts are not much difference between that product and the Protec that you are using on the roof, it might be a better option to use the foil back plaster boards because both will provide a vapour check but not a vapour barrier.

.