Insulating between roof rafters

[Dave]

Hi all.

I've just moved into a 1930s-built detached house, and I'd like to insulate between the rafters in the roof so that the loft space isn't quite so cold (I'm not looking to use the loft space for habitation

- only storage). I've read a bit on the web about leaving ventilation space above the insulation, but as far as I can tell, this is only needed when roofing felt is present beneath the tiles. In my case, there is no roofing felt - just tiles, horizontal battens and then the 100mm rafters themselves.

Also, my copy of "Reader's Digest Complete DIY Manual" (1999 reprint) suggests putting up builder's paper between the rafters "if there is no felt on the underside of the roof".

I guess I'm just looking for some pointers/hints:

a) Given that I have no felt on the underside of the roof, do I still need the 50mm gap between the battens and the builder's paper? (or does the absence of felt remove that requirement?)

b) Does the combination of builder's paper and standard 100mm thick loft roll sound like a suitable solution? I've also seen Wickes General Purpose insulation slabs - while they're more "sturdy", they're also 3 times as expensive (per square metre). Any there other recommendations?

Any pointers gratefully received... :-)

Dave

[.]

will those compressible polystyrene blocks fit between your rafters ?

[Dave]

Hmmm. I've just spoken to Knauf's "Technical Advisory Centre" - their "advisor" strongly recommended against me insulating between the rafters at all! (He said that I'd get problems with condensation!) Now I'm even more confused...

[.]

eh ? but .. but .. isn't that what it's designed for !

yuo and me both mate ! anyone else know if the knauff bloke is right ?

[Dave]

I've just found a useful document "If a non-breathable roofing membrane is used, Building Regulations require a 50 mm ventilated air space above the insulation to remove any moisture vapour and avoid the risk of condensation formation. If a breathable roofing membrane e.g. Kingspan nilvent is used, the need for roof space ventilation is negated."

Well, I guess that having _no_ roofing membrane is classed as "breathable", so I don't need that roof space ventilation...

Shame the "advisor" at Knauff didn't seem to know what he was talking about :-( (Disclaimer: I could, of course, be completely wrong!)

[mcbrien410]

I posted this recently...............

Just thought I'd share this info with everyone. I've just added a Dormer to a new upstairs room. Basically 4" x 2" (95mm x 45mm) stud walls. I've 'stuffed' it with the following insulation called Rocksilk by Knauf. The spec. sheet gives the following details...

UNIVERSAL SLAB RS45 Thickness: 100mm (I have also used

75mm) Length: 1200mm Width: 600mm Area per pack (m^2): 3.6 Lamda (W/mK): 0.035 R Value (m^2K/W): 2.85 Fire ClassSlabs per Pack: A1 Slabs per Pallet: 5 Product Code: 2361392

The price for one pack from Sheffield Insulations in Aberdeen was =A332. So that's nearly =A310 per square metre. Quite expensive I thought.

Chris.

[.]

that's what I thought. as long as there's a decent air gap it's ok.

there's a lot of it about.

IMO, you're not and neither am I :-)

[Dave]

Interesting: Wickes' "ordinary" 100mm loft roll currently works out at £1 (one pound) per square metre - granted, it's R Value is only 2.25 m^2K/W (that's 21% "worse" than the Knauf slabs) but it is only one tenth the price...

[Phil L]

The knauff bloke is right, if it was used in Dave's situation it would cause condensation, this is because there is a hell of a draught blowing under his tiles, if he blocks this draught he will either have to make another hole in the roof (a roof vent) or install facia vents outside, to allow cross flow ventilation to the roof timbers.

[Phil L]

The roof is supposed to be draghty and cold, if you get rid of the draughts, the roof timbers rot away. You can cut down on a lot of the draught by doing what you first intended, either using paper to hold up the insulation or as is preferred in the trade, nylon netting, the drawbacks with this will become evident if you do not create another form of ventilation in there, either by roof vents (expensive) or the much easier method (providing you have gables*) brick air vents in each side, approximately 9 X 6 and you will need about four each side to create cross-flow ventilation.

• gable: it is hipped all round, you will require roof vents, hipped:

[.]

if he has fascias, if the insulation does /not/ allow for x mm of free air circulation ... ridge vents .. lots of reasons to use poylstyrene, not many reasons not to, given the info so far ? need more info !

[Rob Morley]

A breathable membrane would keep moisture away from the rafters, your roof doesn't do this but relies on ventilation to stop it being a problem.

[marble]

25mm thickness of celotex has an r value of over 7 and costs a lot less than that.

[Dave]

Errr, I get different figures, according to Celotex's website:

25mm thick product only has a R-value of 1.05. I assume it's measured in the standard units (m^2K/W) even though their brochure doesn't explicitly say.

Out of interest, where did you find the "r value of over 7"?

[marble]

My appologies I got from here:

look under Isocyanurate these are values for 1inch thick ie ~25mm :

almost certainly use imperial units ft farenheit and BTU's, being American.

[.]

100mm rafters - 25mm insulation = 75mm air gap. the OP won't have any problems using the concertina polystyrene, it's what it's designed for.

[Phil L]

You misunderstand - the rafters would have plenty of ventilation, the ceiling timbers wouldn't, and neither would the perlings and other beams within the loft.

[manatbandq]

No you misunderstand. Insulating between the rafters creates a warm loft so you don't have to worry about anything other than the space between the insulation and the roof. That's taken care of with soffit and ridge vents.

How many loft conversions do you see with air bricks in the gables?

MBQ

[.]

all he's doing is making a *warm roof*

example here

where only a 20mm ventilation gap is left.

here's the same thing done with sheeps wool

right there's no problem.

[dave]

Been following this interesting thread and ok I now realise I know even less about this than the little bit I thought I did.

I (naively) thought it was always ok to fill between the rafter with insulation provided air gaps "around the edges" weren't blocked.

Also can't see how such insulation creates a "warm loft". I thought, as heat rises, that such insulation will reduce the heat rising into the loft - thus making the loft space colder. No? (feel free to laugh and point - I don't mind as long as I learn something).