Go Back   Askaboutmoney.com > Don't Askaboutmoney > Sites, planning, self-builds and extensions

Reply
 
Thread Tools
  #1  
Old 21-07-2011, 01:53 PM
Midgirl Midgirl is offline
New User
 
Posts: 9
Default Cold Roof or Warm Roof

HI

We are about to start work on our roof. We have a sunroom and a living room with vaulted ceilings. We are trying to decide on whether we should put a cold or a warm roof for both of these rooms. Budget etc.. is a big factor, but I am willing to scrimp on my finishes if we have to spend more on insulation etc..

My question is : is a room with a cold roof with insulation of 150mm kingspan between the rafters and insulated plaster board on the slope of 72.5 a colder room than the room with a warm roof insulated with 120mm and 80mm kingspan.

I have been told that the warm roof is a dearer construction, both labout and materials, so I really just want peoples opinions on why the warm roof would be much better than the cold roof. We need to decide on this by tomorrow.

Thanks for any advice.

MidGirl
Reply With Quote
  #2  
Old 22-07-2011, 08:47 AM
Sandals Sandals is offline
Frequent Poster
 
Posts: 1,054
Default

I have no idea what the difference is between cold or warm roof but I do have a sunroom with just fibreglass rolls between the vaulted ceiling and roof. Last year we fitted double glass doors with side panels and fanlights overhead between the sunroom and kitchen to hold the heat in the kitchen/rest of house as nighttime temp dropped considerable in the sunroom.

I would recommend spending every penny you can on making the rooms warm as invaluable later on (plus too messy a job to fix later as in our situation).
Reply With Quote
  #3  
Old 22-07-2011, 08:51 AM
Slim Slim is offline
Frequent Poster
 
Posts: 1,585
Default

In didn't know either...see this: http://www.delston.co.uk/coldvwarmroof.htm

Still not sure I get it. Slim
Reply With Quote
  #4  
Old 22-07-2011, 02:25 PM
onq onq is offline
Former user
 
Location: Dublin
Posts: 4,432
Default

Quote:
Originally Posted by Midgirl View Post
HI

We are about to start work on our roof. We have a sunroom and a living room with vaulted ceilings. We are trying to decide on whether we should put a cold or a warm roof for both of these rooms. Budget etc.. is a big factor, but I am willing to scrimp on my finishes if we have to spend more on insulation etc..

My question is : is a room with a cold roof with insulation of 150mm kingspan between the rafters and insulated plaster board on the slope of 72.5 a colder room than the room with a warm roof insulated with 120mm and 80mm kingspan.

I have been told that the warm roof is a dearer construction, both labout and materials, so I really just want peoples opinions on why the warm roof would be much better than the cold roof. We need to decide on this by tomorrow.

Thanks for any advice.

MidGirl
You are going to have to look deeper into this yourself and relate the depth of the insulation to its thermal resistance and cost it out.

The 72.5 sounds like 60mm insulation with 12.5 facing in plasterboard leaving 60 so its 60 + 150 = 210 insulation.
The other roof is 120 + 80 = 200 insulation is there is no plasterboard.
This is on a simple like-for-like depth comparison only.

You need to clarify both whether plasterboard is included in the latter specification AND the thermal resistance of the materials.

There are also hidden costs for both because typically
- the 150mm insulation between the rafters has to be cut while the 72.5mm has a finished plaster face.
- the 120mm + 80mm Kingspan will neither have a plaster face and you have to put up a supporting substrate in marine ply, WBP or Superply [depending on the detailer] and then plasterboarded ceiling under the supporting joists

The main issue with a cold roof is whether or not the space over the insulation can be vented properly and where this roof is pitched and therefore visible from the ground, what it will look like. Some proprietary finishes that are okay on a flat roof out of site looks like a dogs dinner on a pitched roof. You have to decide on the "look" but you also have to comply with your permission as both methods imply different materials for finished surface.

I will reiterate what I said only last week - you cannot cherry pick a specification like this and expect direct comparison cost.

Furthermore you cannot attempt to get direct comparison cost without the benefit of a BER assessor to tell you the insulation factor definitively and a quantity surveyor to assess the costs. A competent architect with rates agreed with the builder and the assessors software could give you a ball park subject to checking by the pros and the builders pricing of the job.

ONQ.

http://www.oneillquigley.eu

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.
Reply With Quote
  #5  
Old 22-07-2011, 02:44 PM
Midgirl Midgirl is offline
New User
 
Posts: 9
Default

thanks for all the replies.

I am still confused though with the warm and cold roof method

Our sunroom is small enough 11sqm, with 4 (850mm * 1600mm) windows and a double door, and a large velux in the roof, we shall have 150mm insulation in the floor, 150mm pumped bead insulation and quinlite blocks on the external walls.

The living room is larger 22sqm, with 2 (650*2250) and 2 (850*1500) windows and a triple door, and with 2 small velux in the roof. Insulation the same as the sunroom. we will have a stove in this room.

The insulation build up for both the cold and the warm roof both have the same uvalue of .12 or 0.14 (cant remember offhand). I suppose I just am trying to justify the extra work and larger soffits etc that goes with the warm roof, plus the cold roof is an easier roof to construct and insulated so that is why I would gear towards the cold roof, but would at a push go for the warm roof if I could get a good understanding of the real advantage of it. We shall also be making the roof airtight with an airtight membrame on the slopes and around the windows and velux (as as all the house).

Thanks again for your input, and if I manage to get more clarity on the situation I will post it here.

**ONQ , sorry forgot to say that the 72.5mm buildup includes the plasterboard, and the warm roof will also have a plasterboard applied under the insulation attached to the joists.
Reply With Quote
  #6  
Old 22-07-2011, 04:22 PM
onq onq is offline
Former user
 
Location: Dublin
Posts: 4,432
Default

Comments like -

"the cold roof is an easier roof to construct and insulated"

and

"would at a push go for the warm roof if I could get a good understanding of the real advantage of it".

- cause serious concern,

I don't think you understand any of the issues you raise.
If so, without a basic understanding of weathering detailing, structure, insulation and building physics, you may not be equipped with the right mental furniture to understand the answers.

It is not the function of AAM to teach technical details or comment in such detail on methods of construction to laypeople.
That could expose the poster to a huge level of liability should anything go wrong, especially as you seem to have neither an architect nor a competent contractor on board to guide you.

The bottom line for a client would normally the cost of the work and its performance in use and the architect would normally advise.
A client would not cherry pick the work undertaken to perform a certain detail, because clients have neither the training nor experience to do this.

For example, there is a great trend among builders to run metal or PVC bead up corners and apply external render up to them instead of battening out the corner and plastering first one elevation and then another.
The former method saves time in that you don't have to sequence the work, but the render typically cracks from the corner within a year. The latter method takes longer but the render should last a lifetime provided proper crack control joints are introduced.

Another example is the current vogue is to build in timber because its renewable - but its potentially far less durable than concrete and has a projected 60 year lifespan.
Where does that leave someone who has no clue about problems with time frame who grew up in a build-once-and-forget concrete building?

So you see, there are pros and cons to every detail - short term costs and long term savings - and the answers cannot be limited by on-topic responses to detailed questions.

The architect's preference for certain building practices rest on what he is happy to certify .
This preference is based on years of experience and knowledge of the working practices known to yield good results.

I try to sit clients down for four hours and answer all your questions and put some hard questions to you about where you are getting your ideas from and then dispel any myth you may have heard.
This should have been done at the start of your build by a competent building professional because otherwise your head can get filled with vague imaginings and concerns for detailing and costs that have little relevance to either the over all cost or the long term enjoyment of the house.

And no, that is not an invitation to PM me or appoint me, I am thankfully up to my eyes at the moment with work and registration research, but you do need to appoint someone competent to advise you.

ONQ.

http://www.oneillquigley.eu

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.
Reply With Quote
  #7  
Old 22-07-2011, 04:49 PM
onq onq is offline
Former user
 
Location: Dublin
Posts: 4,432
Default

Quote:
Originally Posted by Slim View Post
In didn't know either...see this: http://www.delston.co.uk/coldvwarmroof.htm

Still not sure I get it. Slim
Thanks for posting that link Slim.
That is the classic Cold and Warm roofs, both derivatives of uninsulated FLAT roof construction.
The warm roof you show protects the structure and the decking of the roof with insulation and needs no ventilation.

--------------------

I'm beginning to think that the OP is referring to a form of PITCHED roof where the insulation is installed between the joists without an air gap or with a small air gap and membrane.
It is a form of "the insulation follows the line of the roof" detailing but it is not a true warm roof and exposes the tops of the joists to extreme cold while keeping the lover face very warm.
I have serious concerns about doing that to any material but to do it to timber seems like a disaster waiting to happen. About twenty years weathering and building physics will see if I'm right

Here are a selection of confusing details, none of which are a true warm roof.
http://www.buildbase.co.uk/buildbase.../warmroof.html
http://www2.dupont.com/Tyvek_Constru...yvek_supro.pdf


On the other hand, here is the required level of ventilation in TGD Part F where a roof void exists.
http://www.environ.ie/en/Publication...ad,1647,en.pdf

See Diagram 11 Parge 28 TDG 2009

You can see that a 50mm gap is needed and some of the so-called warm roofs are merely "almost full-fill roofs" that do not comply.
Such details, as well as those promoted by "full fill insulation" roofs use a "breathable membrane" to allow water vapour to migrate away.
One problem is that in our humid Irish climate such gentle "breathing" is sometimes no enough and on a cold windless night problems can arise.
The other building standards and the latest revisions are here for those who might be interested in coming to terms with the technical issues arising.

ONQ.

http://www.oneillquigley.eu

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.
Reply With Quote
  #8  
Old 22-07-2011, 04:56 PM
Midgirl Midgirl is offline
New User
 
Posts: 9
Default

Thanks for the reply ONQ.

I do have both an architect and a BER assessor, and my contractor is good but old style, and we have a good roofer who can build either the warm or the cold roof but advice is generally that the traditional cold roof method with the amount of insulation we will be using would be thermally efficient, while they agree that the warm roof method is becoming more popular I am unable to get a definitive answer as to whether we should go with a warm roof. There would be no pipes in the warm roof, only possibly some HRV ducting.

I was just putting the question out there to see if any other self builder used a warm roof in their build and their experience with it, so I could get an understanding from that. I do not expect to understand the whole building physics etc.. as it is not my area of expertise.

thanks again.
Reply With Quote
  #9  
Old 22-07-2011, 05:06 PM
onq onq is offline
Former user
 
Location: Dublin
Posts: 4,432
Default

Midgirl,

You're very welcome.

This is actually helping me sort out my own detailing.
So in that sense I have a vested interest in answering your questions.

I have posted the link to TDG F in response to Slim's comments above on the definitions of roofs - please read it.
The "ceiling following the pitch of the roof" design on P. 28 Diagram 11 is simple to follow but can be interpreted in different ways.

The top thin line is generally interpreted to be the weathering membrane.
Traditional building required sarking felt underneath the slates to prevent water ingress.
The thicker graphic is the insulation and the space between is the ventilation void, 2" or 50mm deep from the top face of the insulation to the underside of the sarking felt.

However this may be interpreted differently by different building control officers and you should talk to your local building control officer for his view.
Insulation installed between the rafters must be cut to fit tightly or you will be riddled with cold bridges all of which will seem individually small but collectively will add up.

As to your question, the detail of it suggests you are not being comprehensively advised by your team.
More importantly talking to other self-builders may mislead you into false conclusions and worse, empirical evidence from a different micro-climate.
Where in doubt you should talk to your building control officer and read the building regulations as these are the law and the building control officer is the police in this area.

ONQ.

http://www.oneillquigley.eu

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.
Reply With Quote
  #10  
Old 23-07-2011, 08:58 AM
RKQ RKQ is offline
Frequent Poster
 
Location: Sunny south eas
Posts: 234
Default

Basically a warm roof does not need to be ventilated. It can be better insulated as there is no need for 50mm ventilation air gap. It should not be hugely more expensive that a cold roof. I do believe it is important to "counter batten" under slates, use a good quality breathable felt & install a vapour barrier to the warm side of the insulation.

I believe that a warm roof is superior to a cold roof. Put on a warm roof, you won't regret it. Good luck.
Reply With Quote
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Forum Jump


All times are GMT. The time now is 03:19 AM.