Roof to keep first floor extension cool - wood fibre in a cold roof?

[Adsibob]

I have what i think is known as a mansard roof over our first floor extension. It is flat at the top and then has a pitched perimeter along the three sides (on the fourth side there is no roof as that is where it adjoins our house).

We had planned on a warm roof, but it's looking like we don't have space due to some design changes that we need to make.

The rafters are 175mm.

Area of room, which is a rectangle is about 4.3m long by 3.75m wide. Half of that floor space (half of the 16.125 m2) is in the extension and the other half is in the first floor of the original house, with a second floor above it. So although the majority of the external walls are brand new cavity walls, there is a 2.1m bit of wall that is just solid wall with 40mm of K5 insulation on the outside.

The flat part of the roof has a large skylight 900mm by 1800mm which has solar control double glazing SN70/35 (g value 35%, U value 1.0). 

The only other window in the room (about 2.2m by 1.4m) is made from the same glass (g value 35%, U value 1.0)

We were hoping to use wood fibre to increase decrement delay of the roof so that overheating is minimised. My architect wasn't entirely sure this was necessary as although the outlook of the room is west facing, until about 11am, possibly later, the majority of the roof will be shaded by the large loft conversion on the floor above and the neighbouring house. But with afternoon sun, that might still make it uncomfortable in summer. 

But with switching to a cold roof, with insulation between the rafters, I only have 175mm of space for insulation.

My architect is checking to see if 100mm of wood fibre plus 75mm of PIR will meet regs. If not, he says we just build up the inside with additional PIR or insulated plasterboard. But I know regs only care about keeping the heat in, not keeping the heat out. Anyone know how much wood fibre, when used with PIR, is needed to have a decent impact on preventing (or delaying, to use the decrement delay terminology) heat from coming in from this type of situation? I know I should do a proper model, but don't know where to start with that.

 

Alternatively, is there a wood fibre product that is rigid enough to be used on top of the rafters and double up as a structural board for the roof covering? I don't have a lot of space, but could maybe squeeze something in.

[Adsibob]

Now managed to have a play around with a calculator and figured out that if we fill the rafters fully with soft wood fibre wool, and then add PIR below, we will meet regs for heat loss and manage a decrement delay of 9.15h, although the solid wood rafters probably won't keep the heat out as much as the soft wood fibre wool in the voids in between, so not sure how accurate that 9.15h measurement is. Assuming it is more or less accurate, is that enough to be comfortable for the type of bedroom I describe above, or should I add an additional layer of wood fibre below the PIR to try and hit 11h or even 12h decrement delay? This would compromise head height, but it might be worth it. Was hoping @SteamyTea might share his thoughts on this as I think he's done quite a bit of research and experimentation on this.

[SteamyTea]

3 hours ago, Adsibob said:

Was hoping @SteamyTea might share his thoughts on this as I think he's done quite a bit of research and experimentation on this.

Were you now.

Not followed your build too closely, but as a general rule, the more insulation you put in, the lower the temperature change will be, it is what insulation does after all.

Wood fibre, like timber, has strong hydrogen bonds in it, this is good for storing thermal energy, and if it is stored, it is not transmitting though.  So the more wood fibre you can get in, the better.

As it is a roof, can you reduce the amount of direct sunlight that is hitting it.  PV modules are good for this, and you can use that excess energy to run an AC unit, or just a large fan blowing in air from the North (though this will only drop the interior temperature to that of the outside).

A reflective roof covering would also work as well.

[Adsibob]

1 hour ago, SteamyTea said:

As it is a roof, can you reduce the amount of direct sunlight that is hitting it.  PV modules are good for this,

That was my plan, but in this thread you had a look at my roof and didn't think PV would work well.

 

We are using a heat reflective membrane/felt within the roof build up, something made by Tyvec, which is meant to be quite good, but I'm not sure it will actually work.

I hadn't considered a reflective coating though.

[SteamyTea]

Right, yes, I remember now.

Can you fit mirrors, plastic ones. They will work.

 

[Adsibob]

2 hours ago, SteamyTea said:

Right, yes, I remember now.

Can you fit mirrors, plastic ones. They will work.

 

Are you serious? On the pitched roof, I think the neighbours would complain to the council and I would get in trouble. But on the flat roof on top of the loft conversion, that is probably fine, as long as it doesn’t affect the performance of the roof covering, which I think will be GRP - assuming that is the standard covering for a loft conversion’s flat roof???

[SteamyTea]

6 hours ago, Adsibob said:

Are you serious

Semi serious.  was more thinking on the flat area.  If you go for GRP it can be done in a lighter colour than the normal lavender grey that seems to be usually used.

A lighter colour, in itself, does not make a huge difference as the surface soon looses its sheen.

 

There is always aerogel insulation, though it is very expensive.