Boarding over loft insulation

fox1977

Hi there,

I am currently a third of the way through insulating my loft properly with the insulation from npower. I am just busy replacing the insulation on two thirds of the loft. The middle third is currently boarded over with some old insulation.

Once i have finished the two thirds i plan to take up the boards and replace the insulation. There is only space for about 170mm under the boards. I was tempted to put two layers of 100mm in and squash the boards down. I am thinking of putting down 170mm and then covering the insulation with this thermawrap general purpose foil before putting the boards back down:

http://thermawrap.co.uk/general.html

Anyone any thoughts if this is a good idea? I need to keep the boards down as I need to store a lot of stuff in the loft. I also need to come up with some way of insulating the loft hatch.

thanks

jennifernil

Rather than squashing the insulation, you could increase the height of the joists to accommodate it. Just screw some battens of the thickness you need onto the top edge of the joists.

For the hatch......secure the insulation in place by covering it with some fabric. Rather like covering a chair seat. Or you could fix framing to the hatch first, then fabric stretched across it once the insulation is inserted.

fox1977

The battens are likely to work out expensive given that i need to raised them by about 100-150mm!

Thanks

WestonDave

Have a google for "spaceboard" - its a solid insulation that you lay across the joists and then put boards on the top. Its strong enough to walk on provided the weight is spread by having the boards on top. It is more expensive that normal insulation but you only need do this in the areas which are worth boarding (i.e. not in the bits where the roof level is so low as to make it impossible to store things). Focus sometimes have it on BOGOF - not sure if they have it this year.

adaze

compressing the insulation won't really do any good, if you want to keep the boarding why not cut one of the sheets down (can be done with a hand saw and a straight edge), assuming you are only taking 30mm off (200-170=30)?

Petrarch74

Agree with adaze here, compressing the insulation down severely reduces its thermal efficiency. The spaceboard sounds like a good idea but if you dont cut properly you will end up with gaps which will reduce the effectiveness of the insulation. Best bet is to raise the battens if at all possible.

andrew-b

I'm still not totally decided how i'm going to handle the same problem.
To get the full amount of insulation underneath would require 7"x2" timbers on top of the existing 4"x2" ceiling joints..6"x2" is a compromise getting you 250mm space for insulation. Whilst at first look that sounds simple, think again as how are you going to attach them across the existing joists? Also think about the weight these sizes of timber become really heavy ..not to mention the costings! I really wouldn't be going beyond 4"x2" additional timbers and only then in small areas.

At the moment i'm favouring 2"x2" timbers on top of the existing 4"x2" ceiling joists to give about 150mm of space for insulation...i.e. 170mm insulation compressed slightly...or two lots of 100mm compressed much more (choice depending on which way i run the additional timbers - though i'm favouring screwing on top of the joist parallel with them and 170mm insulation). This has to be better than 100mm...though will be worse than non-compressed 170mm insulation but still a long way off the full recommended amount. There's no real harm in compressing the insulation...just it won't do it's job quite as effectively.

I hadn't considered the thermawrap-type insulation ..it's relatively cheap for the area covered (currently £10 a roll at B&Q) so not going to break the bank. However if i were to use it i would roll it out loose going between the ceiling joists before your rockwool going under pipes and cables where necessary (with cut outs where needed) - that way cables and pipes are still accessible (once you've unscrewed the boards obviously!). Maybe i'll add some under my own boarded area.

I've considered the spaceboard stuff which appears ideal but it's pricey. Even at half price (B&Q have 2 inch type at about £18 a board...though they have it at £5 that looks like an inch thick?)

Worth noting that chipboard loft boards themselves plus any junk on top will also provide a degree of insulation. But i think to board any area you have to make a compromise somewhere along the line. If you keep the boarded areas to a a minmum then it probably won't make a huge difference in energy saving .

jennifernil

Yea, we have 200mm plus boards and a load of junk.......the loft is freezing!! Our joists were much thicker to start with, so not so much needed to build them up (only 65mm).

Are there some types of stuff that insulate better than others? We used 2 x 90mm of Kingspan in a (new) ceiling where we were limited on depth, and I wondered how that compares with the Rockwool batts that we have elsewhere (thickness for thickness).

Petrarch74

Hi, still dont think compressing is the best option, would say use less insulation rather than use more and compress. Something else you could do is use a granulate top up to fill to the top of boards avoiding the need for compression. Something like our Rockwool Rockprime would be an option or Knauf option. Kingspan boards (as jennifernil mentions) provide better thermal in comparison to thickness, but the cutting problems would reduce that down significantly if doing yourself, also the cost issue.

Inactive

As far as the loft hatch is concerned, I filled two very strong plastic sacks with insulation and stapled them to the inner side of the hatch.

benjus

The whole point of the insulation is that it is not compressed - the pockets of trapped air help to give it its thermal insulation properties.

I would favour adding a 2nd layer of 4x2 "joists" over the existing ones, but at right angles - not only is this a more stable structure, but if you attach them in parallel with the joists you'll get "cold bridging" where the heat escapes around the edge of the insulation where it meets the joist.

Fixing them to the existing joists is easy - just hammer in nails diagonally from both sides. Once you've attached the boards over the top it'll be completely stable. OK, the timber does cost a bit, but it's still a fair bit cheaper than the insulating boards discussed above.