choose decrement delay over u-value for room in roof?

[MarkyP]

I'm in the process of a loft conversion and extension (4 years into the process to be exact!). My loft conversion and side extenion was timber 1st fixed and re-roofed ages ago and am finally getting around to looking at insulation of the new upper floor. I cant recall the value off hand (and I will check with BC before I commit) but I think I've got to achieve at least a pitched roof u-value of 0.18 for renovation/extension regs. I've got 140mm deep rafters and really can't afford to go much thicker than 100mm under rafter before it will start to encroach too much into the room. I'm looking at comparisons of a couple of insulation build ups. Having read various posts about decrement delay, and having a large south/southwest facing roof in an exposed spot I am worried about overheating the upper floor which is all "room in the roof".

 

build up option 1

15mm fire grade PB/skim

100mm dense wood fibre under rafter (0.038, 110kgm3)

140mm flex wood fibre between rafters @ 400cc (0.036 Steico Flex, 60kgm3)

OSB sarking

counter batten/breather, batten, slate tile

u-value around 0.17, decrement delay around 11hrs (just scrapes regs I think but not a great u-value)

 

build up option 2

15mm fire grade PB/skim

100mm PIR under rafter (0.023, 30kgm3)

140mm flex wood fibre between rafters @400cc (0.036 Steico Flex 60kgm3)

OSB sarking

counter batten/breather, batten, slate tile

u-value around 0.13, decrement delay around 8hrs (better u-value but at cost of reduced decrement delay)

 

taking the conventional view that a lower u-value is better, I'd choose the option with PIR under the rafter over dense wood fibre. With regard to decrement delay I think I've grapsed the summer overheating bit but still puzzling over which build up would perform better in winter. My conventially influenced mind is pulled to the PIR under rafter option. I used the concrete centre thermal calculator but not 100% sure I've captured the bridging effect of the rafters, perhaps at least the decrement values are comparable relatively if not in absolute. 

 

any views on which is the better choice? 

 

 

 

[ProDave]

Try re running the figures with Frametherm 35 in between the rafters. A lot easier to fit than the other options.

 

[Cpd]

Don’t forget your vcl layer 

[tonyshouse]

I would choose the u value first then work out what to do to achieve it.  For me 0.1 or below would be the aim

 

 

[MarkyP]

that's fine for new build Tony but I'm converting/renovating so somewhat constrained (note I've re-roofed and so I'm limited by rafter depth and what I can reasonably accomodate under rafter without compromising the room space). 0.1 isnt realistic here.

 

the point of my question is less about the absolutes and really the relative merits of the two build ups. I've got a relatively high mass, good decrement delay vs lower mass slightly better u-value decision.

 

If I had a magic want I'd lift the roof off and replace with with 350mm i-beams at 600cc fully filled with celllulose and enjoy the u-value and decrement delay, but sadly depite a few waves the wand isn't working ?

 

[MarkyP]

well,  I could get a u-value of around 0.12 with 100mm PIR between rafter ( now leaving ventilated void of 40mm from my 140mm rafter) and 100mm PIR under. But dec delay would be around 5hrs. I could put more PIR under rafter and get closer to 0.1 but the room space is getting tight. 

 

Is u-value really king?

 

I have build ups above with very different mass profiles, and much longer dec delays. 

[Gone West]

In our area in the summer when the sun shines, in the afternoon there is always a breeze. This helps to prevent a heat build up on the surface of the roof, therefore decrement delay is less of a factor. Unless your property is well sheltered I would be more concerned about heat loss. Presumably the room has good ventilation.

[Cpd]

@PeterStarck that’s a really good point ^ and one I have not taken into consideration, I live within a few hundred meters of the sea facing the oncoming winds, in one of the windiest spots around as I have hills on either side of the property that funnel the smallest of winds straight at my various buildings. In my shed roof build up I put 70mm of PIR under the rafters and 150mm of rockwool bats between, there is then a vented void.  my thinking was that the rockwool though not a great insulator in these thicknesses would act as a buffer to stop wind wash over the PIR. recently i put my hand between the wool and the PIR and found it nice and warm ! (Probably lots of heat loss as only 70mm PUR !) So it is definitely  doing something to help keep the heat in and slow wind wash. Not sure if any of this is relevant but could be a consideration in some circumstances. 

[Jeremy Harris]

30 minutes ago, PeterStarck said:

In our area in the summer when the sun shines, in the afternoon there is always a breeze. This helps to prevent a heat build up on the surface of the roof, therefore decrement delay is less of a factor. Unless your property is well sheltered I would be more concerned about heat loss. Presumably the room has good ventilation.

 

 

Which raises a very good point about knowing the conditions that prevail at your location, before making decisions on design and construction.  I failed to do this.  I assumed that the weather data from a station about 10 miles or so away would be close to the conditions at our site.  I was wrong, but only began to realise this when we were doing the groundworks, and found that we had temperatures close to 40°C towards the excavated rear of the plot.  The only thing I changed in light of this was the MVHR, switching from an ordinary passive unit to an active one that included an air-to-air heat pump.  That wasn't enough.  What I should have done was change the glazing for something like Sage glass as well.  Hindsight is a wonderful thing . . .

 

I've been recording the temperature outside every 6 minutes for several years now.  It's consistently between 1.5°C and 3°C warmer in summer than the temperature recorded at the nearby weather station.  I'm pretty sure this is just due to being in a well sheltered position, with very little wind most of the time.

[Declan52]

What about cross 50mm battens underneath the rafters to give you a depth of 240mm and get one of the spray foam type insulations. Will completely fill the voids and make it super airtight. Just another option for you to price up. 

[ProDave]

Here is another angle to look at it:

 

Are you having any solar PV?

 

If so then best absolute U value will serve you well to minimise winter heating, knowing that you will have free electricity when it's hot to run active cooling for free, making decrement delay less important.

[Jeremy Harris]

Solar also reduces the heat on the underlying roof by about 16% to 18%, because of the energy it extracts from the sun.