Should I ask for more insulation?

[JP_]

Having a loft conversion done. I have queried the amount of insulation in the dormer roof with my builder and he said it is enough, because there is more on top plus another 25mm to go on the ceiling. I looked up some building regs / U value info for wooden dormers, and it suggests a minimum if 120mm, so if there is 50mm on top, this will be a total of 125mm, and "enough". (I guess the roof is a bit like this: http://www.acutecarpentry.co.uk/our-services/grp-roofing/ - I asked for fibre glass instead of felt).

But, there seems to be plenty of room for more, and still leave a space for 50mm air gap - see photo. The board there is 50mm, and the space from the bottom of the board to the top is 170mm, so if I had 100mm in there, there is still lots of space for ventilation. This will give 175mm insulation in total (assuming there is 50mm above that I cannot see).

Should I ask to have more - I will have to pay for the extra (he said only if building regs request a change, it will be free), but I am happy to pay for a few more boards, or thicker boards, if it means a warmer future with lower heating bills. It just seems a bit silly to not put in as much insulation as possible. But, am I being silly to think that?

 

[tony1851]

Is this intended to be a cold roof, a warm roof, or a luke-warm roof?

Seems an odd set-up.

 

[JP_]

Ignore: The plans refer to a "warm deck roof". That's all I know!

Edit - it's a cold roof, plans changed.

 

[tony1851]

Should be OK as long as there is a vapour control layer under the upper insulation (ie the insulation immediately under the finish) and the void between the joists is not vented.

 

[JP_]

Hi Tony, I am sure it would be OK - it is all being building inspected etc. But, won't a little more always be better in the long-term? If building regs update in 5 years to say flat roof dormers should have 5cm more insulation than I have, I will be laying there in bed kicking myself, while burning more fuel....

 

[catlad]

If that is supposed to be a warm roof then how can you be leaving a gap between the insulation.

 

[tony1851]

As long as there is no ventilation of the space between the joists and the outside, it is OK to split the insulation - to a point.

OP can add more insulation, but it is generally advised that the bulk of the total insulation thickness is on the cold side of the deck. Personally, I should be inclined to add no more than 25mm insulation below the joists, if only to prevent pattern-staining.

 

[JohnD]

"a total of 125mm"

Do you mean all Celotex, or partly foam and partly mineral wool? In what proportions? The foam is a much better insulator than the wool.

 

[cjard]

If building regs update in 5 years to say flat roof dormers should have 5cm more insulation than I have, I will be laying there in bed kicking myself, while burning more fuel....

Changing building regulations will have no bearing on the amount of fuel you burn. You're not compelled to upgrade your house when he legislation changes. You're free to use whatever you want that meets current regs, and thereafter until renovating.

The better question for you is, realistically how much will it save? What are you thinking of putting in there, what will it's insulative value be, how does it affect the U value of the roof and hence what overall effect wil it have. You might find that it will cost you an extra 300 quid, and save you £20 a year on your heating bill. That's a 15 year payback

 

[JP_]

"a total of 125mm"

Do you mean all Celotex, or partly foam and partly mineral wool? In what proportions? The foam is a much better insulator than the wool.

I believe (did not see it installed) 50mm foam based within the fibreglass flat roof, and then 50mm between the joists (as seen in photo), and then a further 25mm will be added under the joists (that covers the whole ceiling), before the plasterboard is added.

cjard - what I really mean is, if I am not having extra insulation simply because building regs currently says I have enough, but if extra will make a better room, maybe I should.

I had a single story extension with vaulted ceiling built a couple of years ago and in that I am pretty sure it was 75mm Celutex plus an equal thickness of rock wall in the sloping sides, and then the same, plus an additional layer of solid insulation on the flat part on top, somewhere under the felt (possibly 200mm total on the flat roof).

It just seems that the loft conversion is getting almost half the insulation the new lounge has. So long as it all stays warms, I really don't mind. But it is the sort of thing I hope is right from the start!

 

[cjard]

You can do the calcs if you want.. Thermalcalconline.com has a calculator that will let you work out the U value for your other buildup and for this one. If they are comparable then the two rooms should have equivalent heat losses via the elements you calculated. Whether it translates to feeling the same warmth is a question of all the other effects of the rest of the room (like glazing)

Provided there are no air gaps that will make the hollow area above the pictured 50mm a wind tunnel, then 125mm of polyurethane board will outperform 75mm board+75mm wool for certain.. But you're right that the time is now if you're thinking to fill that void with insulation. Do the u values calc, then you'll know if you're hitting regs or passing them significantly. You'll also be able to work out approximately how much more money you'll save on your heating (someone here can help with that calc once you have the numbers) by adding the extra. You might find the payback is looooong

 

[theprinceofdarkness]

" You might find that it will cost you an extra 300 quid, and save you £20 a year on your heating bill. That's a 15 year payback ". That is definitely half true. In the 70s the same argument went for putting in 2" of glass fibre in lofts. Its a matter of taste, I would put in as much as possible as upgrading it is 10 years time would cost an arm and a leg.
FWIW My daughter has had a very large dormer conversion done on her bungalow complete with £900 worth of Celotex installed (150mm?). It has led to the upstairs bedrooms getting too hot, because they have gone open plan including the stair case, so all the downstairs heat ends up in the upper bedrooms.
Frank

 

[JP_]

Well, spoke to my builder today and he said that two lots of 50mm are going in there, plus the 25mm underneath, and described the roof as a cold roof. So that is all very well. But, I had a look through the last opening just now, and it looks like only half is with the extra 50mm board.

I looked at the U value website and totally confused by that!

The one you can see at the end is 120cm from the dormer wall, so one width of Quinn insulation board. Looks like it has 125mm insulation immediately next to the dormer wall, but only 75mm in the rest of the room. Still seems odd to me! Will mention it tomorrow....

 

[JP_]

"
FWIW My daughter has had a very large dormer conversion done on her bungalow complete with £900 worth of Celotex installed (150mm?). It has led to the upstairs bedrooms getting too hot, because they have gone open plan including the stair case, so all the downstairs heat ends up in the upper bedrooms.
Frank

Is it freezing downstairs then? Sounds a nightmare. I have never liked open plan, at least not in northern Europe!

 

[tony1851]

Does your builder know the difference between a warm roof and a cold roof?
If he has put 50 kingspan above the deck and under directly under the fibreglass, and
is then putting more in the void below, it suggests to me he doesn't - though I may have misunderstood your
setup.

If it's going to be a cold roof, make sure he puts a vapour barrier immediately above the plasterboard.
Don't be fobbed off by him saying 'it doesn't need one because the foil on the Kingspan acts as a vapour barrier' -
it doesn't work like that. Experience taught me some time ago that you do need the additional barrier.