u values of flat roof

[michae1]

Hello everyone, I am new here and hope someone can help.
I wanted to refelt a flat roof kitchen extension but I now know I canot just do that!! I have been told that if I can prove that I will not recoup the cost of all the extra insulation required to bring it up to building regs ( inc cost of fitting) over 15 years then I will be allowed to just refelt it. I don't think I will recoup this cost of around £500 but my question is , how do i find the present u values of my old roof and the new values of the proposed new roof so I can compare them , and then how would I work out the savings in heating costs of this room only?
Talked to building reg. guys who say they cannot (or will not) tell me how to do it. They suggest an architect, but this will cost even more money and I am trying to save it!! Any advice gratefully accepted. Mike

 

[freddiemercurystwin]

It is not within Building Control's remit to tell you how to do it and I doubt they know anyway. Frankly unless you've got pretty good insulation already (very unlikely) I'd be surprised if you wouldn't get the benefits back over 15 years, £500 over 15 years is £33 per year, sounds possible to me.

You could work it our yourself, you need to start with the construction as you propose it, then find an online U value checker to confirm the U value. Then work out how much it costs to heat the room with your existing heating system for the next 15 years (may need to post in the gas forum someone may be willing to help on there) then do the same for a roof with the upgraded U value and work out the difference. Of course whether Building control would except your fag packet workings out or not I don't know!

Anyhoo a SAP engineer would probably be able to work it out very easily and for not much either. Might be worth a ring round. No point approaching an Architect.

Or you could just ignore it and stick two fingers up at Building Control, plenty do although you risk it coming up on a search when you come to sell.

 

[ajrobb]

Look for something like BR_443_(2006_Edition).pdf "Conventions for U-value calculations"

It is unlikely that you cannot come up with some insulation with a payback time of less than 15 years; thinner insulation will pay for itself quicker.

You also have the clause about being technically feasible.

You will also need to work to BR_PDF_ADL1B_2010.pdf "Conservation of fuel and power in existing dwellings"

 

[jeds]

There are plenty of free to use uvalue calculators around. Try Celotex.

However, you can't just calculate uvalues. You need comparable SAP calculations showing with and without insulation. Also, you can't just fit no insulation at all. You are still obliged to upgrade insulation to a level that will be recovered in 15 years.

 

[michae1]

Thanks for your thoughts on the matter. I appreciate that I may need to do some calculations. I was hoping to avoid the need to put any additional or alternative insulation in the flat roof. The roof is not leaking and I merely wish to avoid the possibility of a leak occurring in the future as the felt and roof are approx 14/15 years old and the felt will obviously not last for ever. The decking is assumed to be good and it seems a shame to take up the decking ( with the possibility of damaging the plaster ceiling below when nailing the new boards back )and replace the existing 100mm Rockwool insulation with something like 100mm of celotex. (I still wish to retain a 50 mm air gap over the insulation between the 150mm joist as I have a cold roof and the soffets and facia presently have ventillation between the joists).

Doing some very rough calculations, my annual heating costs amount to £466 and the room in question which has the flat roof is less than 15% of the overall size of the building. I am not sure that I would save £33 p/annum over 15 years ( the cost of new decking and insulation inc additional installation costs) when the proportionate cost to heat the room at present only amounts to approx £70pa and some of the total cost of heating the room would presumably relate to the heat lost through the walls and concrete floor.

I appreciate that the celotex would have a better insulation value than the rockwool and if the boards/decking were damaged then I wouldn't hesitate to upgrade the insulation.

I must confess to not personally having read the Bdg Regs Part L referring to Flat roofs and was not aware that there was a qualification referring to technical feasibility. I thought that if I could show that it was not economic to upgrade the insulation ( over a 15 year period, that the Building control people would agree that I could just refelt the roof.

Any further thoughts or comments would be much appreciated.

Mike

 

[ajrobb]

With warm-roof insulation I think you MUST lose all the existing ventilation. The timbers are warm so do not need ventilation to keep them dry.

If you fill the gap between the 150mm joists with mineral wool, that will give you an R-value of 3. You still need to find another 2.5 R-value to get above the 5.56 target (U-value 0.18 W/m²/K). You could use Celotex TD4000 (90 mm + 5.5 mm), which seems to be the thinnest with an R-value of 4.1. This gives a total R value of over 7 or a U-value under 0.14 W/m²/K. I'd want to seal the ceiling below to avoid condensation in the mineral wool. With full-fill mineral wool between the joists, I doubt it is so essential to fully seal the cavity.

Another alternative is to leave the gap between the joists empty but fully sealed from draughts. You would then need to go to 120mm + 5.5mm TD4000 to get the required U-value of 0.18.

 

[michae1]

Thanks ajrobb. I was hoping not to disturb the exterior ply to replace the insulation.I was hoping to justify to Building Control that the est cost of £500+ to replace the boards (Exterior ply) inc cost of new insulation such as Celotex would not be economic although I appreciate would be technically feasible.

Using an excel calculator provided by Celotex, the replacement of Rockwool (0.46W/m2K) with 100mm Celotex GA4100 as recommended ( 0.31W/m2K) would only achieve a 7% saving in energy when also taking into account the U - values of the walls and floor which also absorb heat. Not a great deal of saving when the annual cost to heat this room amounts to approx £77 a year. The saving in energy amounts to less than £6 a year. Over a 15 year period I would save £90 but it would cost me £500+ in works to achieve this. Does anyone think I will have a problem in convincing Building Control with these calcs ? Mike

 

[ajrobb]

Small point, the Celotex includes 5.5mm exterior ply bonded to the insulation.

 

[freddiemercurystwin]

As mentioned you need to demonstrate to building control how you ascertained your figures. Try writing it in a clear and logical manner and submitting it to BC and see what they say. If it is clear it may be hard for Building Control to dismiss it but then again they may think you've just made it up and will expect it on some headed paper ie calculated by someone qualified to do so?

What about using this chappie here: http://www.plumbcenter.co.uk/plumb/heatloss/new_look.html

 

[michae1]

Thanks Freddy for the link. I will give the calculator a try. I am not sure if I can just submit my case to Building control including calculations and drawings or whether I also need to submit them with an application plus the relevant fee. Does anyone know ?
mike

 

[freddiemercurystwin]

My Local Authority charges £96 if you can't find it on their website you'll have to give them a ring. You'll need to fill in a form which should be available online too.

Out of interest who have you actually been speaking to at your Building Control? You gotta be speaking directly with the actual inspector who'll be dealing with your application, nobody else but him/her.

 

[michae1]

Thanks Freddy and ajrobb for your info. When speaking to Building Control, I was under the impression I was speaking to an inspector. Have had 2 conversations with them. Not overly helpful and got the impression that the person I spoke to last thought I was scoring points over him. I tried to be diplomatic and humble but I guess that the "economic" aspect mentioned in my enquiry & stated in the regs/guidelines was not he wanted to hear.

mike

 

[freddiemercurystwin]

Inspectors are area specific ie there will likely only be one inspector who will be allocated your application therefore it is that individual you need to get up a dialogue with. Something like this is, to a certain extent, down to the individual inspector's interpretation of the regs with regards to what evidence he wants to see. When you next speak to Building Control ask to speak to the inspector who deals with your postcode.

 

[michae1]

Thanks for the advice Freddy, I will make sure I only speak to the inspector for my postcode when I next speak to them.

Mike