How to do loft insulation correctly

How to do loft insulation correctly

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Discussion

dazm

Original Poster:

158 posts

173 months

Monday 23rd October 2017
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Recently bought a 1930's semi which needs a complete refurb and has just got planning for a large extension to the side and rear.

The house is absolutely freezing, so looking for advice on what can be done to insulate the loft.

When the extension is done the hip end will be opened up when the new roof attaches to it, and at the same time we will replace all the existing roof tiles.

The current roof make up is: clay tiles, battens, 100mm rafters. No sarking or felt layer at all. The floor is 18mm chipboard over 100mm joists with patchy fibreglass insulation between them.

In the loft we have a brand new unvented cylinder, controls for the solar PV system, a new boiler going in sometime during the extension works, and a load of crap being stored up there. It will never be a habitable room.

Been buying up building materials on ebay and at my disposal I have loads of the following if they are of any use: TLX gold, 100mm earthwool, 50/100/140mm celotex.

What is the best way of insulating what I have, ideally at rafter level, without causing problems with interstitial condensation.

Want to get this right and not have to re-do things when we do the roof, so if its better to wait until then I am happy to do so.

Over to the PH experts for advice...

SantaBarbara

3,244 posts

107 months

Monday 23rd October 2017
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What about heat loss through the walls as well?

dazm

Original Poster:

158 posts

173 months

Monday 23rd October 2017
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Planning on EWI on the remaining external walls once the extension has gone up

Gtom

1,593 posts

131 months

Monday 23rd October 2017
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50mm PIR between the rafters with 100mm PIR over the top.

magooagain

9,911 posts

169 months

Monday 23rd October 2017
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Blown in 400mm fibre glass insulation if the loft is never to be used. If you board it before hand then it's safe to walk about up there after it's been blown in.

Eddieslofart

1,328 posts

82 months

Monday 23rd October 2017
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What depth are your rafters?

I have TLX Gold and 140 between rafters, with the correct airgap. Apparently, that gives me a U of .16.

It’s also extremely temperature stable.

[Edit]

Apologies, just noticed 100mm.As previously suggested, 50, and a 50 insulated platerboard.

TLX is a bonus, but a cow to work with.

Edited by Eddieslofart on Monday 23 October 21:27

dazm

Original Poster:

158 posts

173 months

Tuesday 24th October 2017
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Does the insulation not have to be 70% below and 30% between the rafters? I'm sure I read something like that, to prevent the risk of interstitial condensation.

If I did the 50 between 50 below, is it worth adding the tlx later when the old tiles come off? Also how does the ventilation gap work if I add the tlx later? Do I still need to maintain a 50mm gap or should the insulation then fully fill the rafters and be touching the tlx?

the_g_ster

374 posts

194 months

Tuesday 24th October 2017
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Hey, don’t worry about condensation, you have no felt, you no doubt have lots of through flow up air into the loft and under the tiles and through. I have never seen any issue in ours.

Think carefully on:

1. Will you want to store stuff up there? If so, then celotex etc is expensive and you won’t get through the hatch without cutting up, then through and back to lay, that will take you ages
2. If you not gonna store stuff, loads of roll insulation, easy, quick, just watch cables, light cables are fine, high current cables for showers etc etc should be kept on top of insulation
3. If you are gonna store stuff, loft legs or extra 4 by 2 perpendicular to existing rafters to lift levels of boards

We have a lot of insulation in the roof, none in the walls. The house is roasting, it annoys me as heat just seems to rise upstairs and it usually gets too hot up there even in winter, then downstairs isn’t quite as warm. I preferred living in a Victorian place with a bit of insulation and what felt like more fresh air in the house.

Also, don’t take short cuts now re storage, it’s a real pita working in a loft, so better to do once right rather than changing your mind in a few years time when circumstances change, have more stuff etc etc etc.

dazm

Original Poster:

158 posts

173 months

Wednesday 25th October 2017
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What about when we replace the tiles? Surely then we will be adding some kind of membrane under them, if not the tlx, which will reduce the ventilation through the gaps in the tiles (not that there are any, they seem really tightly fitted together)?

As I said, the loft is only ever going to be used for storage and as a kind of plant room, housing the unvented cylinder, boiler, pv controller, and eventually the solar thermal gubbins and maybe an mrhv system.

I was thinking about lifting the flooring and redoing all the insulation between the joists, then laying another layer of 4x2 with insulation between them too, then boarding over the top. I do want to insulate at the rafters though, as the cylinder and boiler will be up there along with lots of pipework.

austinsmirk

5,597 posts

122 months

Wednesday 25th October 2017
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I've a 1924 house, 2 attics- sounds similar to yours. strangely there were only wisps of ancient insulation up there- although fully boarded.

I stripped the boards, cleaned up the old debris between ceiling rafters- added loft leg feet and put in 400mm of insulation- rockwool type.

re-boarded.

I'm a bit obbessed about gas/electric meter readings: anyway- the resulting lack of heat loss meant a £17 a month saving. Without a doubt the materials paid for themselves and you could feel straight away how much warmer the house felt.

The roof is still a cold roof construction, so bare roofing felt and tiles are still on show: we've lots of stuff stored up there and it doesn't seem to harm it.

Eddieslofart

1,328 posts

82 months

Wednesday 25th October 2017
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If the loft space is to remain as a loft, put the insulation on the ceiling. Keep it pulled back 6” from the eages though. Save the TLX for the extension.

dazm

Original Poster:

158 posts

173 months

Thursday 26th October 2017
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Good to hear it made a noticeable difference. The house was painfully cold last winter. As soon as the heating goes off the temperature drops.

The reason I'm more interested in insulating at rafter level is because of the cylinder and boiler, I'd like to keep all that at a sensible , steady temperature. Also with the extension it has been drawn as a warm roof, and it would be easier to tie that in with an existing warm roof rather than some kind of hybrid setup.

Edited by dazm on Thursday 26th October 09:59

Gtom

1,593 posts

131 months

Thursday 26th October 2017
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So just got with 50mm between to the bottom of the rafters, giving 50mm air gap and then 100mm over that. Use silver foil tape on all the joints and expanding foam (get a gun and use Screwfix's no nonsense stuff, it goes off in ten minutes and doesn't expand wildly) to fill all the holes and gaps you may get.

Not putting in insulation at ceiling level will keep your electrician happy!

dazm

Original Poster:

158 posts

173 months

Friday 27th October 2017
quotequote all
If I do the 50 between 100 under will that be ok when I replace the roof tiles and add a membrane under them?

Also read somewhere about bringing it up to building regs standards if replacing more than 30% of your roof. Is that the case, and if so will the 50/100 celotex solution meet regs?

Elysium

13,764 posts

186 months

Friday 27th October 2017
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austinsmirk said:
I've a 1924 house, 2 attics- sounds similar to yours. strangely there were only wisps of ancient insulation up there- although fully boarded.

I stripped the boards, cleaned up the old debris between ceiling rafters- added loft leg feet and put in 400mm of insulation- rockwool type.

re-boarded.

I'm a bit obbessed about gas/electric meter readings: anyway- the resulting lack of heat loss meant a £17 a month saving. Without a doubt the materials paid for themselves and you could feel straight away how much warmer the house felt.

The roof is still a cold roof construction, so bare roofing felt and tiles are still on show: we've lots of stuff stored up there and it doesn't seem to harm it.
This is the right approach.

Gtom

1,593 posts

131 months

Friday 27th October 2017
quotequote all
dazm said:
If I do the 50 between 100 under will that be ok when I replace the roof tiles and add a membrane under them?

Also read somewhere about bringing it up to building regs standards if replacing more than 30% of your roof. Is that the case, and if so will the 50/100 celotex solution meet regs?
No issues at all. It gives you the correct ventilation gap and somewhere between 10-15mm over the required regs (unless they have changed recently).

I would agree about putting it in the ceiling had you not been using the loft space as a boiler room.

barryrs

4,376 posts

222 months

Friday 27th October 2017
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For reference B&Q have 3 rolls for £14.95 on mineral wool at the moment; just went in a picked up 39 rolls!

CoolHands

18,498 posts

194 months

Friday 27th October 2017
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Another option, Could you insulate at ceiling level but build an insulated cupboard around the boiler etc

SantaBarbara

3,244 posts

107 months

Friday 27th October 2017
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The cold is probably coming from under the ground floor floor boards as well, if you have a void space under there.