Room in roof insulation buildup

[pauldoc]

I have read through a number of posts both here and on ebuild.

I just want to check I am heading in the right direction with my proposal.

Like quite a few of you, my upstairs is in the vaulted roof space. The vertical element is already pre insulated by the frame company with 120mm pir, plus an additional 40mm finished with a service batten. The roof comes straight of the wall. The rafters are 195mm and pretty sure are 600mm centres.

Now the architect has specified the following to insulate the roof-

160mm kingspan Kooltherm K7 between rafters, vcl, then plasterboard.

 

My main concerns are price and if this is the best solution for the job.

I have quite a large area to insulate. A quick look for prices and this K7 seems incredibly expensive, a 120mm 1200x2400 is roughly £72 ex vat

Also im sure  I read on here there are concerns about the real thermal and sound insulation properties this type of insulation has.

As its a room in the roof I am particularly concerned about the sound insulation qualities.

 

On a previous topic, Knauf frametherm was suggested as a better alternative.

 My guess is that I would go with 2 layers of Frametherm 90mm 35 between the rafters, with a pir as thick as I can afford across/under the rafters-prob 30/40mm, finished with a service batten and plasterboard.

Would this be a better solution both u value wise, heat decrement wise and sound insulation wise?

And would it work out cheaper?

thanks in advance

[Declan52]

I have a room in the roof build and I went the knauf route. Much easier to cut and while it mightn't be as good on paper u value wise by the time you cut the pir up and go round foaming and tapping they will be not far apart. I have 140mm high density between my rafters then 50mm pir underneath. Pir has all the joints taped up then I have an airtightness membrane and then the plasterboard. Didn't need a service cavity as I made sure all the plugs etc where on the inside block walls. 

[pauldoc]

10 minutes ago, Declan52 said:

I have a room in the roof build and I went the knauf route. Much easier to cut and while it mightn't be as good on paper u value wise by the time you cut the pir up and go round foaming and tapping they will be not far apart. I have 140mm high density between my rafters then 50mm pir underneath. Pir has all the joints taped up then I have an airtightness membrane and then the plasterboard. Didn't need a service cavity as I made sure all the plugs etc where on the inside block walls. 

Is the 140mm high density frametherm?

[pauldoc]

and what u value did this achieve do you know?

[ProDave]

My roof is (starting from the outside) Tiles, battens, counter battens, breathable membrane, 100mm wood fibre board (used as sarking) 195mm rafters filled with two layers of Frametherm 35, 9mm OSB, 25mm battened service void, plasterboard.

 

Originally I was going to use blown in insulation to fill the void, until I found the price.  Frametherm 35 gives the same U value as the blown in insulation would have, at a much lower cost, plus it's a bit more of the job I can DIY.

 

The frametherm is nice(er) stuff to work with and is stiff enough to stay put when you press it in between the rafers before boarding (I have a test piece that has been in place for 6 months now, not boarded, and it hasn't fallen out or slumped)

 

In my case the wood fibre on the outside is mitigating any cold bridging and adding to the insulation, rather than having extra on the inside.

 

I think the overall U value is about 1.3
 

[Declan52]

Approx 0.15.  I only had 150mm rafters so was at my limit. If you can go to 180mm and put a min of 50mm underneath then your looking at 0.13. the thicker pir underneath the better it will be. It wasn't called frametherm when I done mine, was just rafter roll 32 as from then knauf has done a rebranding but it is more or less the same stuff. Can get in   thermal conductivity of 0.040, 0.035 and 0.032 .It is dense enough to stand up straight between truss with out sinking. 

[PeterW]

43 minutes ago, Declan52 said:

 

I have a room in the roof build and I went the knauf route. Much easier to cut and while it mightn't be as good on paper u value wise by the time you cut the pir up and go round foaming and tapping they will be not far apart. I have 140mm high density between my rafters then 50mm pir underneath. Pir has all the joints taped up then I have an airtightness membrane and then the plasterboard. Didn't need a service cavity as I made sure all the plugs etc where on the inside block walls. 

 

 

Pretty much the same build up I have specified apart from I will get 180mm between the rafters. 

 

Airtight membrane is belt and braces on case the adhesive fails on the tape I would expect. 

[pauldoc]

thanks guys, so just to be clear, you both think the frametherm stuff will be better for sound proofing and insulation purposes than a pir type

It will keep the heat out in the summer aswell?

sorry with all the questions

depending on what my funds can stretch to, would you recommend 140mm frametherm with 50mm pir, or 180 mm frametherm with 30mm oir?

 

[pauldoc]

Also any pointers  where I might get the best price? I read Prodave mentioned SIG, I gave my local branch a call earlier and he had never heard of framtherm!! asked me to spell it too Doesn't fill you with much confidence! Anyway he said his branch didn't have it and to call Bedford or Liverpool branch (im in Kent)

Didn't offer to say they could get it in or give me a price

[Declan52]

Just price all the options up and compare them with whatever the u values will be and make your choice. Just remember the thicker the pir the longer the fixings you will need,

Ask your building control how much he will let you fill with the frametherm. If you use breathable felt and cross batten the roof he should let you fully fill it but check with him before you proceed.

Not sure why that quote has appeared here maybe the mods can remove it!!!

[Stones]

Our initial spec was 195mm full fill PIR between rafters and 25mm across rafters.  In the end I changed this to 180mm frametherm 35 between rafters and 50mm PIR across rafters.  Three reasons, first was cost (cheaper using the frametherm and at that level of insulation the payback was something like 50 years for the extra), secondly, I thought it would give me a better balance of insulation properties (i.e. decrement delay) so a more comfortable house, and thirdly, I thought the frametherm would be far more forgiving in terms of installation - my concern was that a couple of badly cuts pieces of board insulation between rafters and the performance would suffer.

[pauldoc]

Morning guys, thank you all for your info.

For those now living in the house, how have you found the sound insulation properties? are the bedroom quiet or can you hear outside more in the upstairs?

I ask because in our previous house our bedroom was in the roof above the garage and not as sound insulated as the rest of the bedrooms. It wasn't an old house but I suspect not insulated by the developer very well

[PeterW]

32 minutes ago, Sensus said:

 

This can be one of the downsides of PIR (Celotex/Kingspan) insulation: it's becoming the 'standard' solution in room-in-roof situations, because it's about twice as efficient as mineral wool in terms of heat insulation (and so eats into your headroom less for a given ride height), but it's nothing like as effective at sound attenuation.

 

Friends with a new house reported a "drumming" noise when the wind blew with their new house - builder assured them it was not an issue (!!) and it go so bad they took part of the ceiling down in a built in wardrobe. PIR between the rafters had been cut very cleanly into panels about 25mm narrower than the rafter spacing - the drumming was the ends of the insulation panels moving in the breeze blowing up through the roofspace !!  Cue the builder having to replace a lot of ceilings....

 

PIR is fine when its fitted properly - mineral wool has lower thermal efficiency but better noise attenuation but tends to fit better if the cutting and rafter spacing is correct.

[tonyshouse]

The insulation will be do virtually nothing and it sounds to be way too thin too.

 

can you get any photos please.

[ProDave]

Our present house has 100mm PIR in the sloping part of the ceiling, and 300mm rockwool above the flat ceiling. I know we are on a quiet road, but when a car does go past, we hear it via the window, not through the walls or roof.