Hardboard loft SIP...

[Archer]

Merry Christmas everyone. 

 

I know this is a bonkers idea before I've even typed it out from my sherry addled brain. Anyway, let rip, I can take it. 

 

Thinking goes something like this...

 

1. Want to board a section of my loft for easier access and light storage;

2. Head height is very limited (about 1.4m at the apex;

3. Rafters are about 8cm deep at 50cm spacing (I think, need to remeasure)

4. Worried about adding too much weight; also want to make sure that any insulation and boarding is vapour permeable

5. Want the easiest DIY option possible - loft is not a nice place to work

 

Now obviously loft legs with mineral

wool underneath and 18mm OSB is the obvious choice. However, I've been idly thinking that finding a way to screw an insulated panel directly to the rafters would be more straightforward than faffing about with the loft legs, balanced between rafters... There are PIR options for loft floors with foam insulation laminated to a single sheet of 18mm OSB. The weight of these is 12-15kg/m2 so I might only be able to safely store another 5kg doing this... Also, I'm not sure how rational it is but it feels risky to encase a section of the rafters in impermeable foam in case moisture vapour from below rises and gets trapped. (Any thoughts on this risk?)

 

So... I've been wondering whether, structurally speaking, you could DIY a breathable "Loft SIP" by laminating 2x face sheets of either 3mm or 6mm hardboard either side of a 80mm cork insulation core using D4 adhesive or something similar.

 

3.2mm hardboard is obviously incredibly flimsy by itself, but am I right that in a sandwich panel, the thickness of the face material is less important, because it's acting like a flange (ie. in a metal SIP for eg.). The panels would only need to support the weight of a person across a pretty short span... Still, don't want people dropping though the ceiling! Is there any straightforward way to calculate this or is it just flat out a terrible idea? I was thinking to mock up a single cork SIP panel with some offcuts and test it but happy to get some science before going further! 

 

Cheers

Edited December 25, 2024 by Archer

[joe90]

I love thinking outside the box as you have.

1 hour ago, Archer said:

either 3mm or 6mm hardboard either side of a 80mm cork insulation core

I have never worked with cork and no idea of its structural integrity and I think this is the crux of the mater. Hopefully someone here knows its strengths and weaknesses and will be able to advise on this matter. Regarding the hardboard as long as you don’t wear high heels up there or any point loads you MAY be alright but I would prefer say 10mm OSB for the top layer.

[G and J]

What stage are you at?  Are you having to put materials up through a loft hatch?

[Archer]

7 hours ago, joe90 said:

I love thinking outside the box as you have.

I have never worked with cork and no idea of its structural integrity and I think this is the crux of the mater. 

 

Again, really limited understanding of the structural theory... But I thought that the core material mostly acts like a web to keep the two face materials apart. Hence being able to use quite weak stuff like EPS. Cork insulation is quite strong in compression, but not in tension. 

[Archer]

5 hours ago, G and J said:

What stage are you at?  Are you having to put materials up through a loft hatch?

It's an older 70's house so not a new build. Loft hatch is very limited in size. I can get all the timber and cork cut to size though

[SteamyTea]

Knock up a section and try it.

 

You can spend more time thinking and debating it than it will actually take to make a prototype.

 

I worked in a place that loved meeting, the more people there the better the meeting was.

We were discussing a production problem, after 20 minutes of debate, I excused myself (they assumed I was going for a smoke), came back 7 minutes later with a prototype made they way they said it could not be made, and I had a fag.

[G and J]

3 hours ago, Archer said:

It's an older 70's house so not a new build. Loft hatch is very limited in size. I can get all the timber and cork cut to size though

So it’ll be a well ventilated loft.  
 

12 hours ago, Archer said:

3. Rafters are about 8cm deep at 50cm spacing (I think, need to remeasure)

So even if you fill between the rafters with soft and fluffy stuff the risk of condensation on the warm side of your walkway I imagine will be negligible.  I don’t think you need to worry about vapour permeability of your walkway, especially if it’s only going to be a couple of feet wide.  
 

So how about thin ply, then a nice slab of PIR, then the thinnest ply on top?  Not as environmentally friendly but best insulation level for given headroom.

[SteamyTea]

2 minutes ago, G and J said:

Not as environmentally friendly

Tell that to the trees that are having their skin ripped off.

 

Everything has an environmental impact, just depends where you put the most value.

[Archer]

8 hours ago, G and J said:

So it’ll be a well ventilated loft.  
 

So even if you fill between the rafters with soft and fluffy stuff the risk of condensation on the warm side of your walkway I imagine will be negligible.  I don’t think you need to worry about vapour permeability of your walkway, especially if it’s only going to be a couple of feet wide.  
 

So how about thin ply, then a nice slab of PIR, then the thinnest ply on top?  Not as environmentally friendly but best insulation level for given headroom.

Yup, very drafty loft for sure. Between the rafters are already filled with mineral wool. We will be boarding 50% of the width of the loft and half of the length, so much more than a few feet. The risk I was imagining is warm, moist air from below rising and getting trapped on the underside of the insulation and possibly rotting the rafters. I don't know if this is imagined but there a few horror stories floating around online and surveyors recommending that foam type insulation is removed from lofts... I think problems are probably unlikely and rare and that I'm overthinking, but still.

 

You're right though, PIR would be twice as effective thermally for the same depth of insulation. They used to sell a perforated "vapour open" foam board, but I can't find it online any more

Edited December 26, 2024 by Archer

[Archer]

Ok, if it's of use to anyone else - there does still seem to be a breathable, perforated EPS insulation board available, which could be another option for me. Baumit Open Therm

 

https://baumit.co.uk/products/external-wall-insulation/opensystem-breathable-ewi/baumit-opentherm

[SteamyTea]

Why not fit a VCL while you are at it.

It is only a sheet of polythene after all.

[ProDave]

This sandwich SIP panel you are describing is exactly what caravan manufacturers have been doing for decades to produce lightweight, thin and strong floor wall and roof panels with foam insulated cores.  It would be worth a search to see if you can buy for instance caravan floor panels ready made?

 

Here is one such product  https://www.panelsystems.co.uk/news/2019/04/lightweight-panels-for-a-variety-of-applications

 

I think the challenge will be finding something for sale in a small quantity at an affordable price?

[Archer]

On 26/12/2024 at 23:08, ProDave said:

This sandwich SIP panel you are describing is exactly what caravan manufacturers have been doing for decades to produce lightweight, thin and strong floor wall and roof panels with foam insulated cores.  It would be worth a search to see if you can buy for instance caravan floor panels ready made?

 

Here is one such product  https://www.panelsystems.co.uk/news/2019/04/lightweight-panels-for-a-variety-of-applications

 

I think the challenge will be finding something for sale in a small quantity at an affordable price?

Thanks for the suggestions, this is really useful. I've abandoned the idea of laminating something myself... Working out the number of panels that I'd need and it's a lot of work with uncertain results. The company linked above looks pretty much exactly what I'm after with the XPS and 4mm ply facings. The "Tekwarm" loft boards are similar but with PIR and a single 8mm OSB facing.