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[LC0112G]

With temperatures due to hit 35C again this week, I've been wondering about my loft insulation as you do. Currently there is (at best) one inch thick fibre-glass 'wool' roll laid between the floor rafters, and then chipboard loft flooring over the top. Wool roll is at least 50 years old, and I put most of the flooring down 20 years ago. I haven't measured them yet, but I suspect the rafters are probably 2 inches tall. House is a 1960's 3 bed semi, brick & breezeblock construction with cavity walls and plasterboard ceilings.

Plan A was to lift all the chipboard, rip out the existing wool, and replace it (yes I know it's glass and nasty on your hands so use a face mask and marigolds when handling it.) Then I saw this picture on the web :

https://www.bradfords.co.uk/xtratherm-xt-pr-rigid-pir-insulation-board-2400-x-1200-x-25mm?gclid=EAIaIQobChMI95eL-e-2-QIVN4xoCR1vpw0-EAYYByABEgKD7PD_BwE

I have been warned off insulating between the 'tops' of the A frames in the loft, or boxing in the eaves because of ventilation and condensation issues. But that picture appears to show exactly what I was told not to do.

Has thinking changed? Or am I 'safe' to put 2 inches of wool between the floor rafters, and another 2 inches of wool between the roof rafters (and cover it with a thin layer of summut (hardboard?) to keep it in place)? I'll probably use wool rather than hard sheet insulation unless there is a really good reason not to.

Also, can/should I stuff a bit of wool down the top of the cavity wall - say 4 inches worth? I know you can get to the cavity because I've stuffed ethernet and aerial wires down it in the past.

[LC0112G]

Sorry - meant to put the rafters are (probably) 4x2 - so 4 inches tall, not 2. So would use 100mm wool between all the rafters.

[Lizzie Zoom]

With temperatures due to hit 35C again this week, I've been wondering about my loft insulation as you do. Currently there is (at best) one inch thick fibre-glass 'wool' roll laid between the floor rafters, and then chipboard loft flooring over the top. Wool roll is at least 50 years old, and I put most of the flooring down 20 years ago. I haven't measured them yet, but I suspect the rafters are probably 2 inches tall. House is a 1960's 3 bed semi, brick & breezeblock construction with cavity walls and plasterboard ceilings.

Plan A was to lift all the chipboard, rip out the existing wool, and replace it (yes I know it's glass and nasty on your hands so use a face mask and marigolds when handling it.) Then I saw this picture on the web :

https://www.bradfords.co.uk/xtratherm-xt-pr-rigid-pir-insulation-board-2400-x-1200-x-25mm?gclid=EAIaIQobChMI95eL-e-2-QIVN4xoCR1vpw0-EAYYByABEgKD7PD_BwE

I have been warned off insulating between the 'tops' of the A frames in the loft, or boxing in the eaves because of ventilation and condensation issues. But that picture appears to show exactly what I was told not to do.

Has thinking changed? Or am I 'safe' to put 2 inches of wool between the floor rafters, and another 2 inches of wool between the roof rafters (and cover it with a thin layer of summut (hardboard?) to keep it in place)? I'll probably use wool rather than hard sheet insulation unless there is a really good reason not to.

Also, can/should I stuff a bit of wool down the top of the cavity wall - say 4 inches worth? I know you can get to the cavity because I've stuffed ethernet and aerial wires down it in the past.

And that is what I understand after watching many TV programmes warning against doing anything but placing the insulation, whatever you use, between just the floor joists, never in the areas you describe as it stops ventilation and promotes damp issues.  In fact apparently many people who had foam plastered in between the roof beams are now having to rip it all out because of the damp issues and mortgage providers refusing loans on properties with such insulation.

No doubt the builders on here, like Dave, will give more professional advice

[Marks DTM Calib]

If you look at the linked image then you can see they leave a gap between the insulation and underside of the tile structure (tile, felt, battens etc) to allow air to flow.

If it was me, the flooring would be lifted, some 8x2s added on top of the existing ceiling joists, insulation between, and flooring back down (but then we do use the loft a lot for storage)

[LC0112G]

If it was me, the flooring would be lifted, some 8x2s added on top of the existing ceiling joists, insulation between, and flooring back down (but then we do use the loft a lot for storage)

That means losing 8" of head room in the loft, and I'm 6'6 tall, and always banging my head on things. And like you, the loft is currently half full of highly collectable antiques (aka junk). This project doubles as an opportunity to chuck out stuff with no discernable value or use.

[LC0112G]

If you look at the linked image then you can see they leave a gap between the insulation and underside of the tile structure (tile, felt, battens etc) to allow air to flow.

Ahh, right, so if the rafters are 4 inch (100mm), then use 75mm thick material to leave roughly a 1 inch gap between the roofing felt and the top of the insulation?

[Marks DTM Calib]

If you look at the linked image then you can see they leave a gap between the insulation and underside of the tile structure (tile, felt, battens etc) to allow air to flow.

Ahh, right, so if the rafters are 4 inch (100mm), then use 75mm thick material to leave roughly a 1 inch gap between the roofing felt and the top of the insulation?

Effectively yes, rafters are probably 6 inch and you would use 100mm insulation and a 50mm air gap.

Reality is that the moisture builds on the underside of the felt/barrier material in the roof structure and hence the airflow between this and the insulation addresses the moisture issue. You do have to ensure you do not block the air flow so if you have eaves vents you have to make for certain that the ridge area is not blocked off.

[LC0112G]

Just ventured up into a stinkin' hot loft space. It's bin day tomorrow, so another opportunity to get rid of a wheeley bin full of junk.

Floor 'beams' are 4x2. Roof beams are 4x1.5.  So 100mm wool under the floorboards, and 75mm solid insulating board between the roof joists.

[dave the builder]

Local councils have grants available to home owners right now AKA "free money"
I was told last week( by housing inspector ) 100% funded ,so no home owner contribution 
grants for energy efficient homes for boilers, insulation , windows upto £10K

[TheBoy]

(but then we do use the loft a lot for storage)

I think we're currently using the "fill it with crap" method on insulation

[Migv6 le Frog Fan]

Same here. 

[Kevin Wood]

Same here. 

And me. Old scopes and radios are probably not that effective  as insulation  though.

[Keith ABS]

 Insulating between the roof joists is known as "hot roof". seen it on a few builds I have been on in the past and yes, an air gap was left.
Why not consider getting cavity wall insulation done? definatly makes a differance and with the hot roof, you would not need to take up the flooring to reinsulate that. But if you do, use rock wool instead of fibreglass as it is three times as good like for like thickness

[ronnyd]

I had 'free' loft and cavity wall insulation done about five years ago, from British Gas. Must say that it certainly made a difference. Would recommend anyone to get it done if house is standard construction, which many older ones aren't.

[LC0112G]

Insulating between the roof joists is known as "hot roof". seen it on a few builds I have been on in the past and yes, an air gap was left.
Why not consider getting cavity wall insulation done? definatly makes a differance and with the hot roof, you would not need to take up the flooring to reinsulate that. But if you do, use rock wool instead of fibreglass as it is three times as good like for like thickness

Cavities were done about 15 years ago. I'm not convinced they are a good idea in our area though. Lots of reports of problems caused by it bridging the cavity causing damp problems. Saw a map once of areas that were advised not to have it, and we were in it. Something to do with westerly aspects that see the brunt of the prevailing weather.