Baffled by roof insulation - best between rafters solution?

viki83viki

Hi,

We removed a stud wall in our loft, which then led on to us pulling all the internal boarding off (it was only hardboard that had a tiny amount of skim and then wallpaper). We then discovered our dormers had ZERO insulation and the timbers were rotten. There was 175mm isowool between the pitched roof timbers. We have removed this as assumed we could get better. Now the rot is sorted, we need to insulate, and want to do it well, but can't afford to lose much space within the room itself.

The rafters are 175mm in the pitched part, and then we have dormers front and back with 125mm rafters to the top, and 905mm timber frames on the cheeks.

I was all set to use 140mm Actor Hybris between the main rafters and then use their H Control vapour barrier over, 38mm over batten and then board. With a slimmer hybris for the dormer but the same overall setup.

But looking online in forums apparently Actis only works really well (0.16 u they claim) used in conjunction with other insulation types. Actis don't seem to make this clear. So I'm having doubts and wondering if some kind of rigid insulation would be better? We're not reroofing so can't add anything over rafters. Also apparently it's poor acoustically.

I'd be grateful for any pointers or recommendations on where to go from here!

Many thanks.

tacpot12

I doubt the internet forum is correct because the Actis system is designed to be a stand-alone system and not used with other insulation types. Can you provide a link to the forum?

Have you had the U vale calculation from Actis? If not, I would suggest you get this. I'm also slightly surprised that you have removed 175 mm of insulation and not installed 170mm of Hybris in it's place. Was the recommendation to use 140mm Hybris from Actis?

It might be poor acoustically, but does this matter to you? It might, if you live in Houndslow under the Heathrow flight path, or near a motorway, but if you are installing roof insulation to get a quieter house, Actis is almost certainly not the best product. Overboarding the Actis with an acoustic plasterboard might be a good solution to improve the soundproofing. You should talk to Actis if you want to know if this route would be a good option for you.

viki83viki

Hi, thanks for your reply.

My gut instinct would say fill to the depth of the rafters - but on reading the online technical information from Actis we do need a 25mm air gap between the hybris and the H Control. The H Control is stapled to the front of the rafters, so we can't fully fill between them. Hope that makes sense?

We live near a busy road, not on it. So acoustic insulation is a consideration but not a priority. You're right about sound boarding over. That may be a good option.

I am awaiting a reply from Actis on their thoughts on the installation and hopefully a u value. I'll post back with the forum link, but I basically googled 'Actis Hybris reviews' and it came up.

I'm guessing that the sound and thermal insulation we had was pretty rock bottom before - we were sleeping under a dormer with the cheeks constructed of only wallpapered hardboard, a sheet of ply, with the felt stuck on over. (Now rectified!). So perhaps anything would be an improvement?!

viki83viki

It won't let me post a link to the forum I found. :-(

tacpot12

The point about needing a gap between the Hybris and Control H does suggest you would be better off with the 140mm Hybris, but you could check this with Actis. Given you have a fixed gap to fix, they should be able to confirm what configuration will give you the most insulation.

Generally anything that provides thermal insulation will provide some insulation against noise, but Hybris is not dense material, so its sound deadening properties are not as good as specialist acoustic insulation. Adding a dense plasterboard will change the U value calculation though, so better ask Actis to include an acoustic plasterboard you can afford. I notice that Actis claim Hybris is a good noise insulator, I've not looked into their claims but I would not be surprised if they don't have some data to back up their claim.

I had a play with their U value simulator and it suggested a calculated U value of 0.15 W/m2K based on 140mm of Hybris + 45mm of H Control Hybrid. This exceeds the building regulations requirement slightly. Getting an official figure from Actis would be better as you can use it to show compliance with Building Regs. If this figure can include an acoustic plasterboard, so much the better as the acoustic plasterboard will tend to provide less thermal insulation that standard plasterboard.

Celotex or equivalent is more efficient for thermal insulation than Hybris, but it should be more expensive than Hybris. So if you can achieve the U value and sound insulation you want with Hybris/HControl why not use it.

I found comments on forum.buildhub.org.uk. regarding "multifoils". I'm not sure if this was what you read but it sounded very much like it. This basically said that Multifoils were "snake oil, pure and simple". However, when you read the comments, the source of the data is the BBA Certificate for H Control Reflex+. This is a completely different product to H Control Hybrid. Reflex+ is 8.5mm thick and has a R value of 0.25 m2K/W, Reflex Hybrid is 45mm thick and has an R value of 2.8m2K/W (when used in roofs). Reflex+ will also have minimal sound insulating properties.

The BBA Certificate for Reflex+ does indeed say that it needs to be used with other insulation to achieve an acceptable U value. However the comments you have read omit any discussion of Actis Hybris as that other insulator. 170mm of Hybris has more or less the same thermal insulation as 100mm of Celotex. You can use Hybris or Celotex as the other insulation, providing you use enough of it.

I'm not a fan of Celotex for refurbishment projects as it needs foam to fill the gaps. This is nasty stuff and the less we use of it the better.

Hope this helps