Green Building Forum - Loft insulation - ceiling joists or rafters?

Loft insulation - ceiling joists or rafters?

Arf measures — Wed, 28 Jun 2023 16:12:50 +0100

I moved into a new house earlier this year (typical early 20th century with solid brick walls, old uPVC double glazing, uninsulated floors etc). My plan is to improve the thermal efficiency over time as funds allow and potentially put in an ASHP at some point. The loft is currently unboarded and due to the need for alot more space to store stuff this is where I'm going to have to start! I'm pretty new to this but have already picked up lots of useful info from reading old posts on this forum - no doubt I'll have more questions in due course...
I'm trying to decide between insulating between and above the ceiling joists (to give a total of ~300mm mineral wool insulation) and insulating between and below the rafters (giving a total of 150mm foam board insulation).
There is clearly a difference in material costs (Â£800 for the ceiling joist option including plastic legs to support the new boards vs ~Â£2,500 for rigid foam insulation boards to the rafters).
However, pretty much everything else seems favour insulating the roof (more usable space, boarding out first will make it easier installing the insulation, don't have to worry about insulating the loft hatch, and ventilation perhaps less critical?).
Are there other considerations I should be aware of? I saw a previous post about roof insulation boards shrinking over time - is that a significant risk?
Many thanks in advance

Loft insulation - ceiling joists or rafters?

Nick Parsons — Wed, 28 Jun 2023 18:32:35 +0100

If you need the space, and you need it dry, insulate all the way from eaves to eaves, but with a little dropped ceiling at the top to allow uninterupted cross-flow of air.

''ventilation perhaps less critical?''

No, just 2 separate lots, and they are both critical! 'cold side' behind the boards, and warm side, in the insulated space, to deal with all that warm moist air that will find its way up there.

Tape all joints and perimeters, preferably with 'proper' air-tightness tape, not (weak) foil tape which is often not strong under tension.

I have done nearly 2 dozen Victorian attic insulation jobs with PIR, and am not aware of shrinkage issues having occurred, but then I wouldn't be, would I...?

Scary, those pics. Does strike me as an anomaly, though. Shrinkage claims have tended to be more about phenolic than PIR AFAIK.

Loft insulation - ceiling joists or rafters?

tony — Wed, 28 Jun 2023 22:10:57 +0100

I think loft floor 400mm quilt 500 ventilation then any proposed deck.

Doing the rafter option increases the heated volume and hence heat losses and heating bills

Worse is that the loft will almost certainly be sit leaky, costing even more on unnecessary heat losses

Loft insulation - ceiling joists or rafters?

djh — Thu, 29 Jun 2023 21:00:38 +0100

Posted By: tonyI think loft floor 400mm quilt 500 ventilation then any proposed deck.

Presumably the 500 is a typo?

Loft insulation - ceiling joists or rafters?

tony — Thu, 29 Jun 2023 23:23:15 +0100

Yes sorry 50mm ventilation

Loft insulation - ceiling joists or rafters?

cjard — Fri, 30 Jun 2023 06:40:40 +0100

"alot more space to store stuff"

Do you really need the stuff?

Wool is considerably easier to install on a loft floor than board is on a loft ceiling. If I wasn't converting the loft I would definitely just use risers and thick wool.

Loft insulation - ceiling joists or rafters?

tony — Fri, 30 Jun 2023 17:11:51 +0100

Not a problem to store stuff in the cold

Loft insulation - ceiling joists or rafters?

Arf measures — Sun, 16 Jul 2023 23:18:24 +0100

Thanks for all the advice. After a bit more thought I've decided to go with the option of insulation between and above the joists. I just had a couple of follow-up questions..

The rooms below the loft are 2 bedrooms and a stair landing, all of which have lath and plaster ceilings (the loft hatch is above the stair landing). The only other penetrations in the ceiling are the cables for the lights which can be sealed well. I've seen people talking about adding a vapour barrier above the ceiling and joists. How important is this? I would have thought that the loft hatch is always going to be a much more significant source of air and water vapour leakage with its less than ideal seal (not to mention every time the hatch is opened..)

Also, I was planning on using plastic 'loft legs' to support the deck in the loft. The standard size is 175mm which seems to be based on providing the building regs minimum of 270mm insulation with 5mm spare. But this doesn't allow for the recommended 50mm ventilation gap between the insulation and boarding (or for a greater thickness of insulation). Is there another similar product that has a bit more height? Or another way of doing it? I'd rather not have to put in a load of additional joists up there with all the additional weight.
TIA

Loft insulation - ceiling joists or rafters?

Dominic Cooney — Mon, 17 Jul 2023 09:16:39 +0100

I donâ€™t think that 50mm is required under loft boarding?
The roof itself, yes.

Loft insulation - ceiling joists or rafters?

Mike1 — Mon, 17 Jul 2023 12:01:20 +0100

Posted By: Arf measuresI was planning on using plastic 'loft legs' to support the deck in the loft. The standard size is 175mm which seems to be based on providing the building regs minimum of 270mm insulation with 5mm spare.

Check out LoftZone StoreFloor, which has 279mm legs. They also have a BBA certificate - I think they're the only such system that does - that does specify leaving a 50mm gap. Although, like Dominic, I don't recall the Building Regs having a ventilation requirement below boarding.

https://www.loftzone.co.uk/downloads/

Loft insulation - ceiling joists or rafters?

djh — Mon, 17 Jul 2023 12:21:42 +0100

Posted By: Arf measuresThe rooms below the loft are 2 bedrooms and a stair landing, all of which have lath and plaster ceilings (the loft hatch is above the stair landing). The only other penetrations in the ceiling are the cables for the lights which can be sealed well. I've seen people talking about adding a vapour barrier above the ceiling and joists. How important is this? I would have thought that the loft hatch is always going to be a much more significant source of air and water vapour leakage with its less than ideal seal

Plaster is pretty transparent to water vapour, though not to air. That's one reason people add a vapour barrier or a VCL. But some paints are pretty good vapour barriers themselves (e.g. latex paints tend to be barriers, clay paint is transparent). Loft hatches are similarly variable, ranging from very leaky to airtight. Opening the hatch won't matter much as long as it isn't left open for periods. So everything depends on the particular circumstances.

Loft insulation - ceiling joists or rafters?

Osprey — Fri, 21 Jul 2023 18:47:57 +0100

Posted By: Arf measures

Also, I was planning on using plastic 'loft legs' to support the deck in the loft. The standard size is 175mm which seems to be based on providing the building regs minimum of 270mm insulation with 5mm spare. But this doesn't allow for the recommended 50mm ventilation gap between the insulation and boarding (or for a greater thickness of insulation). Is there another similar product that has a bit more height? Or another way of doing it? I'd rather not have to put in a load of additional joists up there with all the additional weight.
TIA

Loft Stilts are 205mm high, so allow some gap underneath with 275mm insulation and 100mm rafters. Also they are easier to install than Legs, having a clip to hold in place on the rafter.

Loft insulation - ceiling joists or rafters?

tony — Fri, 21 Jul 2023 20:40:29 +0100

â€œFig. 6.7: The loft insulated and the stools and rails in place ready for the raised floor with 50mm of ventilation space under it. This picture is from much later in the build process. â€

â€” Adventures of an eco builder: Designing and building a low energy home by Tony Cowling
https://amzn.eu/4bFzwV5

Loft insulation - ceiling joists or rafters?

tony — Fri, 21 Jul 2023 20:42:16 +0100

â€œThe floor, when it was built a lot later in the build process, was raised 50mm clear of the top of the insulation and so sat 600mm above ceiling level. This was done by nailing short wooden props made from offcuts of carcassing timber at 800mm centres and then fixing 47x50mm rails to them to lay the floor deck on. â€

â€” Adventures of an eco builder: Designing and building a low energy home by Tony Cowling
https://amzn.eu/81qkq0C