Insulation for cathedral ceiling retrofit - interior « Alternative Technology Assn Forums

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Insulation for cathedral ceiling retrofit - interior

(12 posts) (7 voices)

Started 4 years ago by Ether
Latest reply from wallsi

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Ether
Member

Hi, I am looking for advice on the best kind of insulation for a cathedral ceiling.

Existing roof is not great - it has tiles on top, blue/foil lining and then not much space between that and the pine lining shown on the inside for the bulk insulation that's crammed in there. It's a skillion roof angled approx 13 degrees to the South.

Seeing as I'm getting the room walls plastered (not by me) I thought I'd get a flush ceiling done too, creating a decent size space to better insulate.

Bear in mind this is an upstairs bedroom with recently fitted timber d/g windows and in insulated cavity brick wall. It faces North with Clerestories with some passive solar gain, so the bigger problem is more going to be overheating in summer. All windows are shaded on the outside in summer, but I'm hoping to reduce heat gain a bit through the roof.

Main questions are:

Should there be an airgap between pine lining and the plaster or should it be filled with insulation, e.g earthwool or similar?

Is there any benefit in having some type of reflective insulation to reject heat, that would sit above any bulk insulation, right below the pine lining?

Image showing the ceiling:

https://s13.postimg.org/akmosrjcn/BR-001.jpg

Posted Saturday 27 Jan 2018 @ 3:14:49 am from IP #
Annette
Member

The good thing is it sounds like you already have reflective foil and an air gap underneath the roof.

I can't see any advantage to having reflective insulation underneath the pine lining as the pine will block heat anyway. Also, heat is already being reflected at roof level.

By 'flush', do you mean attaching the ceiling plaster underneath the beams that are shown in the photo?

If so, just put the biggest batts you can fit in the space - to the correct standard (not compressed, no gaps, friction fit if possible so it’s tight but doesn’t 'bend' the batt).

However, insulating the ceiling will mean that, once heat gets in through the windows, it will be more difficult to expel the heat through the ceiling.

If you don’t have good cross ventilation (and even if you do),consider a ceiling fan. Edmonds (via Bradford’s) does these especially for cathedral ceilings. Haven't used them myself but have considered it.

If getting others to do the insulation, inspect their work well before the ceiling plaster is put on so there is time to fix things if the job isn't good.

Inspection is hugely important (I posted on a shocking job I saw a few months back on a high-end build)

Posted Sunday 28 Jan 2018 @ 1:50:03 am from IP #
Ruffy
Member

You'll need about 5cm per R-value for the batts. If you are in a cold climate go with R5 (leave 25cm), if the climate is moderate R4 should do the job (about 20cm thickness).

Bulk insulation works on the basis of the tiny trapped air pockets within the material, so as Annette said don't compress it.

Posted Sunday 28 Jan 2018 @ 10:46:26 pm from IP #
Ether
Member

Annette said:

By 'flush', do you mean attaching the ceiling plaster underneath the beams that are shown in the photo?

If so, just put the biggest batts you can fit in the space - to the correct standard (not compressed, no gaps, friction fit if possible so it’s tight but doesn’t 'bend' the batt).

Yes, that is what I meant - attaching plaster to the beams.

Annette said:

However, insulating the ceiling will mean that, once heat gets in through the windows, it will be more difficult to expel the heat through the ceiling.

If you don’t have good cross ventilation (and even if you do),consider a ceiling fan. Edmonds (via Bradford’s) does these especially for cathedral ceilings. Haven't used them myself but have considered it.

Yes, I had considered that problem. I intend to open the windows at night to let cool air in. I have two openable clerestories and one openable East facing window. They are all awnings though, so ventilation is ok but not going to be amazing.

Annette said:

If getting others to do the insulation, inspect their work well before the ceiling plaster is put on so there is time to fix things if the job isn't good.

Inspection is hugely important (I posted on a shocking job I saw a few months back on a high-end build)

Thanks for the heads up. The plasterer had offered to do the insulation, but I wanted to research what should go in and how it should be done. I'll now make sure to inspect it before the plaster goes up.

Posted Monday 29 Jan 2018 @ 2:30:25 am from IP #
Ether
Member

Ruffy said:
You'll need about 5cm per R-value for the batts. If you are in a cold climate go with R5 (leave 25cm), if the climate is moderate R4 should do the job (about 20cm thickness).

Bulk insulation works on the basis of the tiny trapped air pockets within the material, so as Annette said don't compress it.

Thanks for the info. I should have 25cm or more to play with.

I do have to wonder though given all the thermal mass and (perhaps) limited ventilation, whether by insulating I might create more problems in summer and little benefit in winter...unless it's overcast for days the room is rarely very cold - I've never used any heating in it.

Posted Monday 29 Jan 2018 @ 2:33:19 am from IP #
Ben1
Member

All the advice above is good, but no one has mentioned condensation. Your existing ceiling is a warm roof -- most of the roof structure is on the warm side of the insulation. By insulating in the rafter bays, you are changing to a cold roof. Your ceiling lining boards will be colder in winter than they are now and if moist air traverses the gyprock and fibreglass batts, you could get condensation and mould forming on the inside of your ceiling lining. You won't be able to see it. I would look at putting a vapour barrier between the gyprock and the rafters.

Posted Monday 29 Jan 2018 @ 5:04:10 am from IP #
Ether
Member

Thanks, I hadn't thought of that.

Posted Monday 29 Jan 2018 @ 7:42:37 am from IP #
Ether
Member

I have been reading up on vapour barriers. I have just remembered there is some sort of black plastic lining sitting directly on top of the pine lining (i.e. on the roof outside). So I'm thinking that is already a vapour barrier...

I found this comment: "The one thing everyone agrees on is that you never want to have more than one vapor barrier, as that will only trap moisture within the two of them."

Unless I have it wrong, it sounds like I shouldn't add another barrier inside.

Posted Saturday 3 Feb 2018 @ 3:52:51 am from IP #
David
Member

Ether said:
Unless I have it wrong, it sounds like I shouldn't add another barrier inside.

You could potentially use an extra permeable one

Posted Saturday 3 Feb 2018 @ 5:18:00 pm from IP #
jdaley
Member

You could install an exhaust fan through the wall up as high as you can go. I have the same situation and by using that fan at night I pull cold air in through the house by evacuating the hotter air in the high ceiling.

Posted Wednesday 7 Feb 2018 @ 6:02:14 am from IP #
Ben1
Member

Ether said:
I have been reading up on vapour barriers. I have just remembered there is some sort of black plastic lining sitting directly on top of the pine lining (i.e. on the roof outside). So I'm thinking that is already a vapour barrier...

The problem is that the vapour barrier is not in the right place to prevent moist air from reaching the old timber ceiling lining. Relative to the ceiling linings, you have moved the insulation layer from the exterior side to the interior side.

Posted Friday 9 Feb 2018 @ 6:14:24 am from IP #
wallsi
Member

I'm just retrofitting earthwool batts into the beam space of our lounge room cathedral ceiling. Wonderful what a difference it has already made.

The advice to not compress batts is over-simplistic. Yes, compressing batts reduces their R-value, but this may still be advantageous.

Using the excellent calculator at http://www.andersonenergy.com.au/roof-insulation-compression-calculator/, I worked out that the best bang for my buck in my ceiling was to slightly compress 2 layers of R5 batts into the available space for a compressed value of R8.94. The best uncompressed alternative was a layer of R5 batts and a layer of super high density (and cost) R2.7 batts - less effect for more money.

I staggered the layers of batts to minimise through-joins, and sealed them with a layer of Intello Plus (closed to air, differentially open to vapour).

Just doing the counter-battens for the gyprock now, but I'm also thinking about including a layer of Infinite-R PCM.

Posted Thursday 8 Mar 2018 @ 3:01:13 am from IP #

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