Insulating loft?

paperclap

Hi all,

So we recently had someone insulate our loft. But in brief, they screwed it up. They ran it over the joists, rather than between the joists. So, I’m doing it myself.

To preface, I’m only insulating 100mm. I’m aware that The recommendation is 270mm. I want to see my joists, so I can walk on them. Plus, never really cold!

Now, when the guy did it, he used this stuff:
https://www.diy.com/departments/knauf-insulation-eko-roll-loft-insulation-roll-l-7-28m-w-1-14m-t-100mm/182146_BQ.prd

Good stuff in my opinion. But pre-cut to accommodate the standard joist spacings of 600mm or 400mm.

Ours are ~450mm.

So what I’ve started to do is place 400mm insulation. But this does of course leave a good 2cm either side.

In an ideal world, the guy should have measured the spacing, and cut (in the rolled package) to fit. But hey, apparently that’s just too hard.

Anyway. Is this going to be an issue?

Thanks!

km1500

it depends what you mean by an issue

it won't cause any problems if that is what you are asking but obviously it won't insulate quite as well as if it was full width

he only problem I can forsee is that if you ever come to sell the house a surveyor might point this out but no idea how serious that will be to any potential buyer

ThisIsWeird

Hi PC.

Why is that insulation a 'base layer', do you know? What's different about it compared to 'top' layers? Just curious!

If you have a look on FB Marketplace, you will almost certainly find folk selling off/giving away the odd part pack of insulation for next to now't - I'd personally get some more insulation to fill the 2" gaps.

A saw is unlikely to cut most roll insulations neatly, as the teeth will just pull at the fibres. So I think the easiest way to cut neat slices would be with two planks; unroll the insulation along one plank so their edges line up, and then place the second plank on top, but set 2" (or a bit more tobesure) in. Kneel on the top plank to squash the insulation flat, and run a craft knife along its edge = 2" inch slices. 

Why was your loft fitter so carp? Simply incompetent/unknowledgeable on loft insulation? Otherwise 'ok'? Worth letting them know the proper way to do this, so they don't mess up other folk's lofts? And, did he leave the eaves gaps clear - that's another crucial point for him to observe.

If he's anything other than, "Wow - thanks for that! Sorry, and I'll take it all on board!", then perhaps an honest review?

FreeBear

ThisIsWeird said: A saw is unlikely to cut most roll insulations neatly, as the teeth will just pull at the fibres. So I think the easiest way to cut neat slices would be with two planks; unroll the insulation along one plank so their edges line up, and then place the second plank on top, but set 2" (or a bit more tobesure) in. Kneel on the top plank to squash the insulation flat, and run a craft knife along its edge = 2" inch slices.

Whilst a hand saw won't give a clean, accurate cut, it is a quick way of slicing a roll up. Take the roll, still wrapped in its plastic packaging, measure roughly where you want the cut, and then get sawing. I needed a non-standard width yesterday, and this is just what i did. If you need (say) a 150mm strip, cut at 200mm to allow for blade wander and inaccuracies. The extra width is easy enough to lose when stuffing the insulation in to a gap.

OP - 150mm is generally regarded as the absolute minimum nowadays. 300mm being the preferred depth. If you want to use the loft for storage or need access to certain areas, get loft legs and boards up there. Yes, it is an additional expense, but does enable you to go full depth with the insulation and provide a safer surface to walk around on.

paperclap

It’s a base layer simply because it is 100mm deep, which fits flush to most joist depths (100mm or 4 inches).

The top layers are either 170mm or 200mm. Exactly the same stuff, just thicker. In order to bring the grand total of depth to the recommended thickness of 270mm (to meet adequate U values).

I’d nagged and nagged him to get the insulation done, so eventually came to do it (squeezing us in), and got it half done (and not correctly) in a couple hours. I think they did observe the eaves gap.

How much compression can / should this insulation take? My thinking is that it may be an idea to squeeze 600mm into the 450mm gap? The insulation is precut at the sizes mentioned in the original post… so could use a 400mm (that’s already laid) and an additional 200mm?

ThisIsWeird

paperclap said:

It’s a base layer simply because it is 100mm deep, which fits flush to most joist depths (100mm or 4 inches).

The top layers are either 170mm or 200mm. Exactly the same stuff, just thicker. In order to bring the grand total of depth to the recommended thickness of 270mm (to meet adequate U values).

I’d nagged and nagged him to get the insulation done, so eventually came to do it (squeezing us in), and got it half done (and not correctly) in a couple hours. I think they did observe the eaves gap.

How much compression can / should this insulation take? My thinking is that it may be an idea to squeeze 600mm into the 450mm gap? The insulation is precut at the sizes mentioned in the original post… so could use a 400mm (that’s already laid) and an additional 200mm?

Thanks.

How much compression? Clearly a lot - you only have to look at how it's wrapped up when you get it! It might, tho', bounce up in t'middle if you try and sideways-compress 600mm into 450mm. Just try - if it squishes in ok and stays there, then jobbie jobbed. Insulation value shouldn't be compromised, but it's just a bit 'wasteful', that's all.

born_again

paperclap said:

How much compression can / should this insulation take? My thinking is that it may be an idea to squeeze 600mm into the 450mm gap? The insulation is precut at the sizes mentioned in the original post… so could use a 400mm (that’s already laid) and an additional 200mm?

It is the air in the material that helps to insulate. So compressing is going to reduce it's efficacy.
If you want to walk on the joists, then better to get some loft legs & board, keeping the insulation at it's max depth.

In the area of our loft we do not use, we have insulation between the joists & running across them giving great insulation.

paperclap

I’ll take a look at using the 400mm + 200mm into the 450mm space tomorrow evening. But yes, it is very wasteful! That said, I’ve almost no choice in the matter, as the stuff that’s already up there (from the guys that did it) is obviously already rolled out. Can’t “cut to size now” 

I have bought some additional rolls though, where they hadn’t fully completed the job. So perhaps I’ll cut them to size. That said, there will still be wastage / awkward offcuts.

They’re 1200mm wide. So if I go and cut two 500mm (extra 50mm, for saw wander), I’ll still have an odd 200mm of scrap.

FreeBear

paperclap said:  How much compression can / should this insulation take? My thinking is that it may be an idea to squeeze 600mm into the 450mm gap?

if you take a strip of 200mm thick insulation and compress it down to 100mm, it will perform the same as (wait for it) 100mm*. Squeezing a 600mm wide strip in to a 450mm gap is certainly more preferable than leaving a 50mm space down the side. A little wasteful on materials, but more than made up for by the lack of air gaps.

*) Compressing loft insulation does result in a very small change in R-value, but not enough that the average home owner can measure.

paperclap

Thanks all. I think that may be the way to go. And I agree - the small waste in material (through the compression route) is far better than gaps everywhere!

I had a brief moment of thinking “With my new unopened rolls, what if I cut small 10cm slithers?”. No. Not even going there. Far too bitty and will only end in frustration. 

Maybe in the future I’ll top it up crossways with another 200mm. But, at £32 a roll, covering 5.5 square metres… I’ll probably need 10 rolls (£320). Maybe it’s still economically worth it? Don’t know.

Plus the expense of the loft legs and proper boards!

ThisIsWeird

FreeBear said:

if you take a strip of 200mm thick insulation and compress it down to 100mm, it will perform the same as (wait for it) 100mm*.  A little wasteful on materials, but more than made up for by the lack of air gaps.

I'd agree with that. I wouldn't expect measurable loss of insulation value by compressing 200 LI down to 100, in fact it could even improve it.

Yes, it's the air gaps wot do it, but if you look at high-value PIR insulation boards, the air bubbles are truly minute, but well sealed - no convection.

Fully expanded LI is probably the optimum balance between volume and insulation value, but the air gaps will be quite sizeable, and I would hazard that the top inch of this will be much less effective than the rest simply because it's so open to air flow. I cannot see compressing LI reducing its effectiveness at all.