Rigid insulation boards installation advice

m4r10

I'm looking for some advice regarding laying down some rigid insulation boards in the attic between joists. The attic will be clear of any old insulation (fibreglass) and the boards would replace it. Should I glue the insulation boards to the floor or just lie them down and only use some expanding foam between the joists and the boards to seal up any gaps?

My main concern is that cold air would get in between the boards and the floor at the eaves. I'm toying with the idea of at least sealing the boards close to the eaves t prevent that.

Thanks in advance for any ideas/tips.

raspberrypi67

If the attic is clear then what do you mean by glueing the boards to the floor.? Is it not bare tile from the roof ceiling below.?

IMO I think the loose insulation for an attic is best.

m4r10

I meant glueing the insulation boards to the attic floor, which is the roof ceiling of the floor below.

I know expanding foam should be used on the sides between joists and boards to seal up any gaps, but not sure about the side of the board towards eaves. I'd say that by glueing that side too would make a better seal between the floor and the insulation board.

IMG_20230810_174610.jpg

10-10-20

What depth do you have at the joist? And why did you decide to get rid of the glasswool?

I have 80mm or 100mm rigid boards on the ceiling between the joists in my attic, but I wasn't happy with the seal as you mention, and I ended up topping them off with 100mm of glasswool. They just aren't suited to that application as they don't fit tightly in any aspect and even using expanding foam doesn't resolve the issues.

If I was you I would leave the boards about 50cms shy of the eves and run glass-wool out and wrap it over the top of the inner and outer block making sure to leave ventilation space. Then top off the rigid boards with 100mm of glasswool too. That's how I got it to work here.

m4r10

The joists have a depth of 120 mm and the insulation boards are 100mm. For venting purposes, there's going to be a 20mm gap down the middle of the attic between the insulation boards and loft flooring panels (for storage space).

The glasswool was removed as I kept having condensation issues in all upstairs bedrooms close to the outer wall. The outer edge of the rooms (about 20 cm deep) were full of condensation during the winter period. Just to mention that the air vents were/are all open in the bedrooms, so that was not a (major) contributing factor to the condensation.

Doop

No offence but you really are making this difficult for yourself ... you definitely do not need to glue the boards down, they should be friction fitted but unless your measurements and cutting are very precise then you will get gaps... Rollout insulation is ideal for the application you want. Modern rollout insulation will be way better than the previous fiber glass you removed, but you could have just rolled over it to save the hassle of removing.

If you haven't already bought the rigid boards I would seriously rethink using roll out insulation.

10-10-20

I'd have to agree with @Doop, glasswool is the better product here.

Also you don't need that 20mm vent space, I'd try packing it out with loose-fill or 50mm glasswool.

m4r10

The boards are bought and on-site already. It would've been much easier with roll out insulation, but after some research, I opted for the rigid ones. I know it would be a difficult task, but hope it'll be worthwhile in the end.

Knowing my cutting skills, there'll be gaps for sure, hence the need for expanding foam.

There were already two layers of glass wool in the attic, laying another layer would've been impracticable.

10-10-20

https://www.boards.ie/discussion/comment/120990746#Comment_120990746

Understood. They are a pig to fit into place.

Try foaming the gaps, but they don't need to be adhered to the ceiling.

raspberrypi67

https://www.boards.ie/discussion/comment/120990746#Comment_120990746

So I guess if the stuff is already there you may as well go with it so. Its a lot of work and joists are certainly never straight. I've been there.

I see were you are coming from ok on the expanding foam but they are usually friction fitted as someone pointed out. You need to leave a couple of feet at the eves foe ventilation as somebody else pointed out. Just use glasswool around here if I were you.

m4r10

https://www.boards.ie/discussion/comment/120990737#Comment_120990737

I'm not that worried about that 20mm gap down the middle of the attic, but rather the potential gap between the insulation boards and floor.

m4r10

https://www.boards.ie/discussion/comment/120990821#Comment_120990821

Will the eave vent protectors not be enough to keep the attic properly vented? I wasn't going to push the boards all the way to the eaves, but just enough to cover the floor and maybe to overlap a bit the piece of timber at the edge.

Shoog

Apparently there is now a new edge seal tape which you stick to the edge of the boards, its a bit like a sponge with tape to stick to either face, its more complient than the insulation board itself and thus takes up any gap. Its going to become compulsary in new building regs where ever rigid board is going to be used. I would say if you can source it it will be a much better job than spray foam for gap filling.

I tend to agree though that roll insulation is a much better job than rigid in this application. With roll though you need to get it up to the eaves without compromising the ventilation and there is only really one product that achieves this and no one in Ireland seems to stock it.

https://www.roofingsuperstore.co.uk/product/manthorpe-refurbishment-eaves-vents.html

On a retrofit I am currently working on I will be going for 12" of rockwool roll with these Manthorpe eaves vents.

jaykayphd

https://www.boards.ie/discussion/comment/120991280#Comment_120991280

LOFT VENT TRAY 315X700MM 10/Pack (5P/Box) : SIG Ireland

Gapotape

m4r10

Thanks for that. Those eave vents come from UK, so I'll try to source some closer to home. I'll check the old ones first as they seem OK.

m4r10

Thanks for that. Those eave vents come from UK, so I'll try to source some closer to home. I'll check the old ones first as they seem OK.

m4r10

That gapotape, although seems very useful, is very expensive.

Shoog

https://www.boards.ie/discussion/comment/120991465#Comment_120991465

Thanks for that, but those are much more expensive and they only come in 315 widths which would make them even more expensive for my 600mm gap.

m4r10

https://www.boards.ie/discussion/comment/120991280#Comment_120991280

Where did you buy those vents from, if I may ask?

Shoog

https://www.boards.ie/discussion/comment/120992005#Comment_120992005

I haven't yet - but they can be got off ebay or from the a roof supplier in England. However when I reached out to the English company they never got back to me which points to a post brexit unwillingness to trade with Eire.

Ebay maybe my only source and they at least sort out customs up from that way. Cost goes from about £57.00 for 50 to over €120.00 once everything is done. That still works out about half the price of the Irish supplier.

Update: It ended up costing me €125.00 buying a box of 50 from Ebay in the UK.