Barn conversion - best way to insulate roof.......?

[Simon Hawthorne]

Hi all

I know I've been here before and caused some healthy debate on spray solutions.... but now I've decided to actually remove the roof and refit - I need to insulate it (which of course I didn't with the foam).

I've posted a small photo on my site

- but there are others with the roof in profile.

The rafters are about 3.5" deep. My roofer will (when he calls me back!) confirm he is using breathable membrane. I've emailed 'seconds and co' who say you must leave a 50mm gap unless you use a breathable membrane - which I am - so can I assume I can use the stuff sold by 'seconds and co' to butt up against the membrane?

I would then like to use as thin plasterboard as possible, again between the rafters - I really want to retain them as a feature.

So, my questions really are:

1 Can I insulate right up to the membrane 2 How would I fix the insulation in place 3 How would I fix the plasterboard in place. 4 Anything I have missed 5 Ooops - other than how thick do I need to go with the insulation board?

Thanks all in advance

Simon

[Rick]

Here goes, my architect has designed me this

I put battons onto the sides of the rarters, to hold the insulation & platerboard. 50mm insulation. On the outside this give me a level surface. I then add 50mm more insulation over the hole outside, fising down with battons on top of the rafters screwd through the insulation. Then the membrane, and battons going side to site to fix the slates to.

The U value will keep IMM happy, but the insulation will cost a bomb.

What I may do is create rebates in the insulation between the rafters so as not to have a gap between the insulation and the plasterboard.

I will use celotex / kingspan insulation.

Rick

[Christian McArdle]

Yes.

Cut 1mm oversize. Wedge it between the joists.

Screws.

Do you have a target uValue?

3.5" is about 90mm.

Read

want either

"Unventilated fully fitted insulation between and under rafters - no sarking board" (figure 1a)

or

"Unventilated fully fitted insulation between rafters - no sarking board" (figure 3a)

The second method is not as good, but easier to achieve, as it makes fixing the plasterboard easier, as it butts against the rafters.

For uValues, the first method gives (taking the 75mm between and 25mm below)

0.25. If you can find 90mm board, then interpolation gives 0.23. The figure can be improved by placing more than 25mm below the rafters, but only up to the thickness of the board used between rafters (probably 75mm). My rough calculation is that 75mm between and 75mm below would give 0.15, which is very good.

The second method gives 0.36 (75mm) or ~0.32 (90mm). Again, 90mm might be difficult to source. You can't improve upon the figure.

It is likely that you will be required to use the first method to produce a good enough uValue. Also, the first method is superior because it will reduce cold bridging through the rafters that can lead to increased moisture and mould on the plasterboard where the rafters meet.

Christian.

[Simon Hawthorne]

Hi Christian

I don't want to lose the rafters though - so even with the plasterboard fitted, I'd want there to be enough mm of rafter left to distinguish it from the rest of the ceiling.

Simon

[Doctor Evil]

Simon, keeping the rafters exposed will mean more work.

- From inside, 25mm x 25mm batons on the inside of the rafters,

- Plasterboard between the rafters, so they are exposed.

- Platerboard fixed to the batons.

- A vapour barrier behind the plasterboard

- Fill with high insulation value Celotex or Kingsspan between the rafters up to the outer edge of the rafters.

- Cover all the rafters with sheets of Celotex to prevent cold bridging.

- Over the outer Cellotex fix vertical batons that run with the rafters, secured through the Cellotex and into the rafters.

- Cover with Tyvek, secured to the batons.

- Fix batons 90 degrees to the batons on the rafters - horizontal.

- Fix tiles on the horizontal batons.

- Have vented ridge tiles, so air enter behind the tile, taking any water vapour away via the ridge tiles. Check with Tyvek, on this. I have see many fixed without the ridge tiles, as the water vapour will escape via the gaps in the tiles. The vapour barrier should prevent most of the vapour enter the roof.

Make sure you have the highest possible insulation values in the Cellotex, as you don't have much thickness. You have to use high performing foam to get decent insulation values.

Then a roof spce that is not vented, looks the part and well insulated.

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[jacob]

You can get some useful brochures from Celotex, Kingspan, Tyvek etc with roofing variations. Re-roofing I'd board the whole lot with ply. Or 7x3/4 inch sawn redwood boards if the roof isnt level enough for ply. I'd hold the insulation up with thin battens along the rafters say

10x18mm. Its very difficult to cut the insulation exactly as there will be variations, bendy bits etc all over the place, and the battens will also close the gaps. Then plasterboard (foil backed) to the battens but skew nailing into the rafters as the battens a bit small on their own to hold the plaster board. You would then have an 18mm air gap of free insulation. You don't actually need any air gaps above the insulation with breathable felts. Least thats what they say - I wonder how they will perform 30 years on. Not a lot of insulation here, we are planning on 300mm kingspan in our chapel conversion - costa bomb but will pay for itself quite quickly in saved fuel and make a big space habitable.

cheers

Jacob PS to cut insulation board I have a saw bench with sliding table but you could knock one up with a cheapo circ saw mounted in middle (or off centre) of big sheet 8x4ft ply, with boards for fences held on with g clamps. Dusty so ventilation needed or good vac. Really tedious doing it by hand.

[Roger]

The message from "Doctor Evil" contains these words:

The devil is in the detail.

What real purpose is served by having vented ridge tiles on a Warm Roof?

A vented ridge is normally required where the upper section of each rafter is above the insulation and the void between the upper surface of the insulation and the underside of the roofing felt is potentially a condensation trap.

[Mike]

Yes

Brute force. Though some long Spax screws provide a backup

Glue it to the insulation

How much of the rafter do you want showing ? But if this is a brand new conversion of a barn you'll need about 125mm to meet the building regs so you won't see them anyway.

Instead why not place the insulation on top of the rafters and then membrane and tile on top of that. The Seconds and Co website shows how to do this. The roof is raised by the height of the insulation this way but you can happily put in 10" of insulation and keep the rafters fully visible.

[Mike]

This is as opposed to low insulation value Celotex or Kingspan perhaps ? ;-)

[Mike]

Not with a good hand saw and enough practice. Not worth the effort of automating.

[Doctor Evil]

is exactly how I described how to do it, except it has no insulation between the rafters. It also has the Tyvek with gap over and under. It is not directly on the insulation.

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[Doctor Evil]

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[Doctor Evil]

Tom, erm.erm..." taking any water vapour away via the ridge tiles"

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[Owain]

"Simon Hawthorne" wrote | I would then like to use as thin plasterboard as possible, again | between the rafters - I really want to retain them as a feature.

My first thought was, if you can get PP to raise the ridge height somewhat, Structural Insulating Panels laid on the outside of the rafters to form a new roof. Takes care of providing sarking boards on the outside and you may not need plasterboard on the inside, giving you the full depth of the rafters.

Owain

[Roger]

The message from "Doctor Evil" contains these words:

DrivEl it is a well known fact to everyone but you that tiles or slates do not provide an airtight seal so the void beneath remains adequately ventilated without a ventilated ridge.

Well a moron like you wouldn't consider a vented ridge necessary in the circumstances you describe as "not good stuff at all" but luckily for the world in general you are in a minority of one.

[Doctor Evil]

Mr Foolery, you clearly haven't a clue about this topic. Please re-read all my posts on this 8 times.

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[Doctor Evil]

For once a good idea. The problem may be that the rafters may not take the weight. It may be worth taking off the rafters, install a SIP roof and fix the old rafters to the inside of the SIP panels. None structural, only for aesthetics.

Also with the current roof and just installing Celotex, there may also be a weight problem. The new tiles should be as light as possible, or use the existing tiles supplemented by similar new tiles.

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[Christian McArdle]

Then you have a choice of breaking the law, or deepening the rafters.

Other alternatives, involve a complete reroof. If you are doing so anyway, you can insulate between and over the rafters, or even just above the rafters. However, this will raise the roofline, which may require planning permission.

50mm between and 50 mm above will give you 89-50-12.5 = 26.5mm of rafter showing. Using paper backed insulation saves you the plasterboard thickness, giving 39mm. Insulation entirely above gives you the full 89mm to play with, minus the plasterboard width.

Christian.

[Mike]

And spending a fortune heating the place which probably hurts more

Just had a look at our barn. It depends on the roofing material used and how the ridge is made but if it is the old course slate like ours it looks like all the slates could be removed, 100mm of insulation put in lower down then 80, then 60, 40, 20 near the top and nothing for the last slate as it goes under the ridge. The angle of the roof will change slightly but I'm pretty sure all the 20mm changes in height would be unnoticable compared with the natural undulation of the roof currently. It would take some clever roof-laying as well of course.

[John Rumm]

after which you would have even less clue....