Need advise on roof Insulation with 7cm rafters

[GlanMenai]

Hi everyone! 
 

Could I get some advise on our pitched roof, and how to effectively insulate it? The space under the roof will be an office that will have heating. My main concern is keeping the space as cool as it can in summer, while keeping the heat in, in Winter.

 

It's a old Victorian house, that has currently has (listing from outside-in): slate, tile baton and then a Tyvek breather membrane that's nailed onto the rafters.

 

The issue lies with the fact that my rafters are only 7cm deep, which then sit on the purlin, which is 22cm deep (see attached pic).

 

What I'm thinking, is, from the Tyvek in;

 

• Tyvek breather membrane
• 50mm ventilating air gap 
• Multifoil Insulation (SF19 or SF40) - stapled on to the INNER side of the rafters, going into the recess to let it expand, and installed in continuous layer 
• Timber counter batten (air gap)
• 200mm PIR boards - installed as complete sheets, onto the counter battens
• Timber counter batten (air gap)
• Wood Wool board (25mm) (we're using lime for plastering so this is used throughout the house)
• Hemp lime render 

 

Does this sound ok? What I'm trying to deal with is the small rafters, and if I go about installing the multifoil in-between that space, then it leaves me free to install the PIR board as complete boards, and not having to cut anything down, which I imagine will help with insulating? The Multifoil I've read will help a lot with keeping the space cool in summer, as it will reflect the sun/heat from the slates, which sounds perfect to me.

 

I'm no too bothered about keeping the purlin visible, is we desire 'that look' when we come to decorating, we could always put a fake plant-on or something to get that 'feel'.

 

Does anyone have any advice on this? 

 

Many thanks!

 

[Redbeard]

200 PIR will give you 0.11 at worst. What is your target U value and how much extra will the M-F give you?

 

If the timber counter-batten is 25mm I would wonder about loading it up with the wight of the 'sandwich'. Are you going to use stainless screws to reduce thermal bridging? Even if the battens are only 25mm how much will x number of minimum 250mm S/S screws cost?

 

 

[Iceverge]

1 hour ago, GlanMenai said:

Multifoil Insulation (SF19 or SF40) - stapled on to the INNER side of the rafters, going into the recess to let it expand, and installed in continuous layer

 

Do some calculations is my advice. MF is largely oversold IMO. The insulant values are typically small. 

 

If concerned about heat protection I would sway towards denser insulations like woodfiver and hemp and even rockwool. Google "decrement delay" for an explanation. The ventilated cavity above the insulation will above an effect on the amount of radiated heat absorbed by the roof too. 

 

I echo @redbeards concerns about the weight of the build-up. Those rafters may be at the limit of their load capacity as is. 

 

I would look into making a bigger rafter by putting a 50*50 or 70x50mm battens below the purlin on line with the current rafters connecting it to the 2*3 battens with plywood gussets or plates essentially making an I beam. Then filling between the rafters with 250mm of insulation batts. 50+mm of. Insulation over the top. Wood wool and lime render to finish.

 

 

 

 

 

 

 

 

[saveasteading]

2 hours ago, GlanMenai said:

slate, tile baton and then a Tyvek breather membrane

This is your friend for the summer. Sun hits slate and makes it hot, air  flows through and under the slates and carries a lot of the heat away.

After that, your added insulation will do the rest, except for a few days a year.

 

I have little time for multi-foil. I even attended a building control presentation by one of the MF companies. (an excuse for free sandwiches). I kept quieter than usual, and my questions were polite, but there was a tendency towards...'this is approved by BC', 'the normal way of measuring insulation is not helpful'.

It absolutely depends on the air gaps on both sides, and a layer of dpm or bubble wrap would do at lest half as well, and better if reflective. 

 

Therefore I would save the cost of MF and instead buy PIR.  Read previous posts re gap filling.

Then line with more inside for the insulation and as a thermal break, before boarding.

 

RE the strength of the rafters.

The area at the eaves is pretty useless, so if you build a small wall down there, it will be tidier. But it wall also stiffen the rafters significantly.  Put down a wall plate first to spread the load, then fit a stud up into every rafter, with plenty of ring-shank nails or screws. then insulate and board.

Propping a rafter at 1/4 span pretty well doubles the strength.

Perhaps add some sockets.