UKH

This isn't remotely related to climbing but seeing as we're all stuck indoors I hope no one minds a cheeky DIY question.

I'm finally getting around to sorting out insulation for the attic floor. I want to use PIR boards but there are so many different brands - Kingspan, Celotex, Ecotherm etc. Some with foil, some without. There are also websites selling unbranded seconds. Anyone got any tips on what's good and what to avoid?

There's also a huge range of thicknesses available (25mm to 150mm it looks like). Obviously thicker = more insulating, but what's a good thickness to use for an attic floor? I'm going to put plywood boards over the top for storage space.

I'd read quite a lot about moisture movement and timber and rot before doing anything too drastic.

I'd always put down something breathable. 300-400mm in total depth, put down in layers over lapping, or criss crossing, so no direct line for air through.

Watch where you walk! 

Do you really need foam, it's compact if you need the space but overall a very expensive fix for the insulation it provides and it has to be well fitted or draughts under it will rob you of almost all its value. Also it doesn't breathe so that needs considering. Wool type products are messier and bulkier but generally better suited to attics and pretty cheap. I just raised the boards in the attic a few inches to get loads of rockwool in.

I buy seconds if I'm buying a batch. I get mine delivered from somewhere on the Welsh borders (they used to be local to me but moved) https://www.secondsandco.co.uk/ the boards so far have been fine, the paper covered ones the paper was a bit misaligned, wrinkled and loose in places but 100% uesable. The Insulated plasterboards were perfect, I can only assume overproduction or a dropped and parted out pallet.

jk

As others have said, solid foam is a bloody expensive way to do it unless you have a tiny space to cover.  Loose rolled fibre wool will be much cheaper, although it's horrible stuff to actually install - and good luck getting hold of a decent mask right now!  

Use loft legs (Wickes sell them now) to build a raised floor above the insulation.  

And yes, give some thought to breathability. Especially important in the space above occupied bedrooms and showers, where there is more moisture riding with warm air and permeating into the insulation.

Foam can also be rather flammable, rockwool less so.

I have done 4 lofts with the fibre wool stuff, which is a horrible job, but worth it in the end.

3 of them I did by raising the floor with cross members of 90mm x 45mm timber on edge, so a total of 180mm high, plus the original depth, bringing it up to 270mm, which at the time was the standard. Then boarded on top of that. Basically, 3 layers of 100mm insulation criss-crossed, if that makes sense.

Did the other with the mentioned Wickes loft legs - much more screwing involved and actually slightly more expensive for the area I did.

Either way, worth having 2 cordless drills - one for drilling and one for screwing, although you're only into pine and don't really need pilot holes if you use decent screws.

Do it NOW, before it gets any warmer! And yes, you'll need a mask, if you can get one.

Cheers all. The reason I wanted to go with PIR boards over fibre wool was that it is quite a small space (the space above 2 bedrooms and narrow corridor - just under 20 metres squared). Also my back of an envelope estimations was that the cost of raising the joists + fibre wool would only be a bit less than the cost of the PIR boards.

I didn't know about the loft legs though, I'll look into them. Thanks. 

The moisture issue though - what's the solution to that? Does the fibre wool stuff just breathe better than boards?

There is non itchy fibre insulation available, made from recycled PET bottles. It works.

This sort of thing: https://naturalinsulations.co.uk/product/supasoft-insulation/

There is always the old-fashioned solution of filling your loft with crap.

Gloves, long sleeves and legs, mask.  Not on a sunny day. Start at the furthest point to minimise contact with the horrible stuff.  Take a very cool shower and thorough shower afterwards 

Good replies above- also think about retaining ventilation at the eaves so that loft can breathe..

https://www.swishbp.co.uk/design/building-regulations/

I've done a couple of lofts and I have to say of all the DIY I've ever done, it's the number one thing I'd go straight to paying someone to do. Dirty, sweaty, fiddly, banged heads, skinned shins and knees and that's before getting on to the insulation itself.

I may just be a bit whingey though.

I don't know why you're getting dislikes for this, it's a fact. 

PIR turns into smoldering, noxious, dripping death. Rockwool is made from molten rock spun like candyfloss and is very fire-retardant. 

PIR is roughly twice as good at insulating (e.g. compact rockwool panels of 100mm will insulate to a similar standard as 50mm PIR. Loose rockwool will need to be thicker still). 

If you're going the PIR panel route, you'll want a load of squirty foam also to fill in all the gaps, and it'll get really messy. 

As others have said, mineral wool is the better way to go for a loft, where space is not at a premium. 

I used rockwool and fitting that was very quick. Rather stupidly I didn't use mask or gloves but it wasn't too bad (I do know its risky for skin and lungs). Made a real difference to house warmth.

What took the time was putting down 18mm chipboard (didn't worry about exact fit or always linking the tongue and groove as wanted to be able to lift to access cables if necessary), and other extra work to prevent roof spread. Second the power drill recommendation.

I didn't use the regulation thickness of insulation and stuck to the 4" joist thickness. Possibly the chipboard adds a little to the insulation value and might trap warm air more.

The old stuff certainly was. We took some out of the loft (damp/collapsed) and more recently out of the walls at work and that was how I remembered it from my youth, it gets everywhere in the air, on your skin, in your eyes and itches like hell.

The modern glass/rock wool I put in the loft and my floors is like cotton wool by comparison, hardly and dust and no skin irritation at all. It's almost nice.

jk

I think the "old stuff" is likely to have been fibreglass and the "modern" rockwell. In fact When I did our loft recently, without PPE, there was very old and flattened fibreglass and I decided to keep that in place and to use rockwool over it rather than have to remove and dispose of it

Top advice. When you get hot your pores open and the fibres stick in causing irritation.

I'm properly confused about the breathability thing now. Some websites say "insulation must be breathable" and others say "you need a vapour barrier between house and attic". Sounds like people here are for the first option? 

Builder here for what it's worth. We lift existing 3 inch / 75mm joists up with a second layer of 3 inch / 75mm timbers screwed on top and infilled between with what we call Earth wool, which has been rebranded as Glass mineral wool' rolls by Knauf. It's non irritant but always use gloves and masks anyway.

Between the roof and the plasterboard we use the foil backed kingspan slabs at 50mm to allow an airflow between the roof underlay and the insulation slabs. We always use a foil backed plasterboard as well to prevent small amounts of condensation or roof leaks damaging the boards

Ventilation will either be roof vents which are liked a vented tile, 2 per pitch but mainly we use a vented soffit at the ends of the roof eaves to ventilate the area.

Hope that helps.

You can have breathable and vapor-proof

Get any pitched roof fitted now and it'll come with a breathable, water-proof membrane under the tiles. 

Yes, works a bit like Goretex. We use a monofelt.

Oh god, what! But for insulation on the attic floor, does that have to be breathable too?

Bless someone still thinks this is a climbing forum, it's actually a pool of knowledge about just about any subject you can think of with a small nod towards climbing. 

If you insulate your loft with a layer of anything above the ceiling then the space above it becomes a 'cold' space in so far as it will become a lot colder after you've put in the insulation. This means that any moisture in the air is more likely to condense on the wooden rafters, potentially leading to rot.

To prevent this you need to ensure the loft has adequate ventilation to prevent excessive moisture levels. This means the eaves of your roof (the bottom where it rests on the walls) should be clear and unobstucted so a decent draught can blow through.

Ideally the roof itself should have a breathable felt below the slates/tiles. Obvioulsy no felt at all is even more breathable but you might get a wet loft if any slates/tiles slip!

A vapour barrier acts to prevent moisture getting through something, in this case the ceiling, thus reducing the potential for condensation. If you insulate with Kingspan type insulation (Celotex, Xtratherm etc, they're all the same stuff) then the foil acts as a pretty effective vapour barrier anyway which will reduce the amount of moisture getting up there.

If you go for a rockwool type insulation more moisture will get up there. It will be easier to fit but easier to inadvertantly block the eaves and the airflow needed so take care not to block them.

Some info here

https://www.sixstargroup.com/blog/roof-ventilation-and-interstitial-condens...

This sounds like you're describing a 'proper' loft conversion with plasterboard screwed to the undersides of the rafters. If I'm reading the OP correctly that's not what he's doing, he just wants to insulate his loft to keep the house below warm so plasterboard doesn't really come into it.

Whatever parts of my advise he takes is entirely up to him.

It is, but I thought telling him about a method used in a full on loft conversion without making it clear that's what you were describing could be confusing.

We got a new roof recently....previous was unlined and from 1920s.

Well known local roofing firm. I was emphatic I wanted eaves and ridge ventilation. While working roofer proudly explained how good modern membranes were, they overlapped the felt over the ridge and the plastic ridge system went over that. I questioned how membrane allowed air flow but was told it was fine because "its breathable". Just before they finished I thought:"That's c..p, , goretex doesn't allow significant air flow". Checked ridge manufacturer's website and found membrane must NOT be continuous beneath ridge. Contacted the boss of the firm (and sent him ridge instruction pdf) who said he'd never heard that, roofer said he'd never done it and none of his mates had either. We reached a compromise where I got them to cut a slit in the membrane from inside the loft below the ridge. I'm sure they, and a few other roofers are still happily installing ridges with useless ventilation, though saving themselves considerable time.

Got to be honest, we've always continued the membrane over the ridge but what you're posting makes total sense. Learnt something.

We used sheep's wool in rolls (like glass fibre), which we bought on a whim when a local DIY store was closing which made it very much affordable. Previous occupants had done all of the small lofts, but one and it made an immense difference in our bedroom. The whole process was much more pleasant. Not sure it would be that affordable at full price. 

Presumably chemically treated (thinks field day for clothes moths). 

No moths here so I presume so ! This is a similar thing

https://www.thermafleece.com/uploads/pro_20160304160731.pdf

Having heard some horror stories about this & moth infestation would be very nervous !

Although meant to be better treated now..

Thanks for the link, was useful to see the diagrams. I understand the cold roof ventilation stuff and the air flow via the eaves. What I'm confused about is the above-the-ceiling/attic floor insulation.

If you use loads of insulation (and a vapour barrier) on the attic floor, would that not cause condensation issues at the ceiling, or the joists above the ceiling, because the vapour can't get through? I think this is what jk and Jamie were referring to above?

Don't try to do it on a hot day. That's all I have to say. It's been said already but it can't be said enough.