Thinner loft insulation?

Nebbit

My son has a recently bought a small house dating from the 1930s. It has a standard cold loft, with some loft insulation, but needs topping up before winter. In future years we may want to board the loft for storage. Can any recommend a type of insulation which is equally effective as the traditional mineral wool, but thinner, so that it can remain in place when boards are fixed onto the joists?  Thank you

FreeBear

In terms of performance, ease of installation, and cost, fibreglass & mineral wool are the most effective choice, but you need around 300mm to hit current thermal performance figure. PIR/PUR (Celotex/Kingspan) is twice as effective, so you only need 150mm to get the same level of performance. But cutting it so that the sheets are a snug fit between the joists is difficult, and you also have to get inflexible sheets up through a very small loft hatch. Furthermore, you don't want to be laying the stuff over the top of the joists where it can trap moisture. Just don't go for the multifoil quilts such as YBS - Whilst it will reflect back some radiant heat, it does little for conductive heat. The latter is what you need insulation for in a loft.

Stick with fibreglass/mineral wool and use loft legs to give a bit of lift to clear the insulation - There are two standard lengths on the market (170mm and 300mm). I used the 300mm ones to board out my late 1920s loft which gives me a bit of room to go for more than the recommended thickness of insulation.

Chickereeeee

FreeBear said:

In terms of performance, ease of installation, and cost, fibreglass & mineral wool are the most effective choice, but you need around 300mm to hit current thermal performance figure. PIR/PUR (Celotex/Kingspan) is twice as effective, so you only need 150mm to get the same level of performance. But cutting it so that the sheets are a snug fit between the joists is difficult, and you also have to get inflexible sheets up through a very small loft hatch. Furthermore, you don't want to be laying the stuff over the top of the joists where it can trap moisture. Just don't go for the multifoil quilts such as YBS - Whilst it will reflect back some radiant heat, it does little for conductive heat. The latter is what you need insulation for in a loft.

Stick with fibreglass/mineral wool and use loft legs to give a bit of lift to clear the insulation - There are two standard lengths on the market (170mm and 300mm). I used the 300mm ones to board out my late 1920s loft which gives me a bit of room to go for more than the recommended thickness of insulation.

Or loft storage stilts, which are 200mm height.  

takay9

FreeBear said:

In terms of performance, ease of installation, and cost, fibreglass & mineral wool are the most effective choice, but you need around 300mm to hit current thermal performance figure. PIR/PUR (Celotex/Kingspan) is twice as effective, so you only need 150mm to get the same level of performance. But cutting it so that the sheets are a snug fit between the joists is difficult, and you also have to get inflexible sheets up through a very small loft hatch. Furthermore, you don't want to be laying the stuff over the top of the joists where it can trap moisture. Just don't go for the multifoil quilts such as YBS - Whilst it will reflect back some radiant heat, it does little for conductive heat. The latter is what you need insulation for in a loft.

Stick with fibreglass/mineral wool and use loft legs to give a bit of lift to clear the insulation - There are two standard lengths on the market (170mm and 300mm). I used the 300mm ones to board out my late 1920s loft which gives me a bit of room to go for more than the recommended thickness of insulation.

Interested to see your thoughts on foil insulation. We have a room in roof and the eaves are a bit cramped in places and the insulation around the room (fibreglass fixed to stud walls) is old and possibly not very effective. I was considering foil insulation to put under the rafters - would you not recommend?

FreeBear

takay9 said: Interested to see your thoughts on foil insulation. We have a room in roof and the eaves are a bit cramped in places and the insulation around the room (fibreglass fixed to stud walls) is old and possibly not very effective. I was considering foil insulation to put under the rafters - would you not recommend?

A while back, Kevin McCloud was raving about this wonderful material used by NASA and now it could be used to insulate a home - Out in the cold hard vacuum of space, there is very, very little heat lost by conduction. Radiant heat loss is the major problem, and multifoil insulation excels in this situation... A few years later, after a couple of Grand Designs had used this new fangled material, Mr McCloud's opinion had shifted considerably.

If you go looking for thermal conductivity measurements for multifoil, you will have yo look very hard - All I can find is thermal resistance, which makes it darned difficult to compare with other insulation materials... But YBS do give some clues... 100mm of Celotex is rated at 4.545 m²K/W (pretty good), and SuperQuilt at 1.52 m²K/W (fairly poor). Interesting that YBS recommend using Celotex/Kingspan type boards in conjunction with their products to achieve Building Regs compliance - For the cost of the stuff, I'd just whack another layer of Celotex in and be done with it. You need a 25mm air gap either side of the SuperQuilt, so may as well fill the gap completely with a proven insulation material - If you then use some grab adhesive between the Celotex and plasterboard, you'll end up with a very solid wall.

takay9

Thanks Freebear. If I was to use celotex or kingspan - could this be fixed under the rafters or would it have to go in between? I thought I could get one that is vapour permeable/breathable then it would be ok - rather than having to cut them to fit in between?

Doozergirl

takay9 said:

Thanks Freebear. If I was to use celotex or kingspan - could this be fixed under the rafters or would it have to go in between? I thought I could get one that is vapour permeable/breathable then it would be ok - rather than having to cut them to fit in between?

You can fix them over.  Most people fix them between as they're more commonly used on loft conversions where maintaining headheight is important.  

FreeBear

takay9 said:

Thanks Freebear. If I was to use celotex or kingspan - could this be fixed under the rafters or would it have to go in between? I thought I could get one that is vapour permeable/breathable then it would be ok - rather than having to cut them to fit in between?

You need to leave space between the insulation and the roof tiles & felt - Typically, 50mm to allow air flow which will minimise the build up of condesation within the void. Cut the sheets (tip - Use a sharp serrated kitchen knife rather than a saw) so that they are a snug fit, and use expanding foam (with a gun) to fill any gaps. Tape any joints with aluminium tape. If you need another layer of Celotex in order to hit the target U-value, place it on top, covering the rafters (mark their position as you go), and again, foam & tape the joints. On the final layer, fix some 25x50mm battens at 600mm spacing, screwing through previous layer(s) of Celotex in to the rafters. Fill the space with yet more Celotex (and yet more foam & tape). Using a combination of screws and grab adhesive, fix the plasterboard (you did mark where the battens are, didn't you....). Then call in the plasterer unless you are confident of skimming yourself.

Celotex/Kingspan type boards are foil faced, so once the joints are taped, the multiple layers should act as a very good vapour control membrane - The foam insulation is pretty impervious to moisture anyway...

If you have any electrics to install, fit the back boxes to good solid timbers as you go, and don't forget to put in conduit at the same time - A little bit of planning beforehand goes a long way. Over windows, use 12mm plywood in place of plasterboard (extending a minimum of 150mm either side) so that you have a solid backing to support curtain rails. If you can, wrap a 25mm thick layer of insulation round window reveals to kill any cold spots - With my house, I have fairly thick window frames, and with 25mm of Celotex and 12mm of plywood in the reveals, the frames look pretty slim.

Nebbit

Many thanks for these replies. We certainly could not use any rigid insulation as the hatch is tiny. I was wondering about the quilted rolls but in view of the comments above I will stick to the traditional material 

moneysaver1978

This thread might help - https://forums.moneysavingexpert.com/discussion/comment/80207637#Comment_80207637

We didn't go ahead with PIRs because we already have the Knauf rolls and they were far too costly for us. Plus we were not sure if the boards would go through the loft hatch!