Foil Insulation for attic

[Cozzer]

Some advice please, boys and girls....

Victorian house.
Large attic, inside of the roof slates in view.
Floor mostly boarded, sandwiching x inches of the orange/pink skin-irritating roll stuff.

Foil insulation on a roll, to be stapled to the woodwork.
Fit horizontally?
Start low, so above layer overlaps the bottom piece?
How taut do I try and pull it?
Bother with taping the two lengths after stapling?
Know any five-year-olds that I can threaten to get in the corners?!

Not looking forward to doing this, so all advice from those who've already done it gratefully received, and thanks in advance!

 

[Jones]

What are you hopeing to achieve? Stapling multifoil to the rafters of an unheated attic space will not reduce heat losses from the inhabited upstairs rooms much. If you lay it flat on the joists it might work for insulation but will also work as a vapour barrier above the joists exactly where you don't want it. Best to add more cheap rockwool roll above joists making sure to leave airflow at eaves (you can get plastic bits for that) and raise boarding on stilts to leave clearance for insulation. If you are planning a room in the roof it's normal to use different types of insulation to keep costs down and achieve the desired insulation value, such as foil plus board or roll, foil and board.

 

[Cozzer]

What are you hopeing to achieve? Stapling multifoil to the rafters of an unheated attic space will not reduce heat losses from the inhabited upstairs rooms much. If you lay it flat on the joists it might work for insulation but will also work as a vapour barrier above the joists exactly where you don't want it. Best to add more cheap rockwool roll above joists making sure to leave airflow at eaves (you can get plastic bits for that) and raise boarding on stilts to leave clearance for insulation. If you are planning a room in the roof it's normal to use different types of insulation to keep costs down and achieve the desired insulation value, such as foil plus board or roll, foil and board.

"Help to keep your home warmer in the winter and cooler in summer with this SuperFOIL Insulation Wrap. Reduce your energy costs and protect your home from moisture, wind and dust. Perfect for lofts, sheds, walls and more ...." is what's claimed.
In truth, the only current use for the attic is to store bloody Christmas decorations! There are two enormous bedrooms and a 26' x something bathroom on the floor below, so that'll give you a clue as to the loft size. (Add a staircase and a dormer or two, you could easily adapt the attic to gain two bedrooms of a fair size - perhaps even making one en suite....but I suspect the cost would be prohibitive.
So the idea was to stop some of the floor below's heat escaping....as "simple" as that.

 

[Jones]

If you just want to add to the existing loft insulation then it's not the best product for the job. Tacked up to the rafters it will keep the attic space a bit warmer but still allow heat loss from below. If it's laid over the existing roll insulation it will add insulation but also act as a vapour barrier trapping moisture in the roll insulation which will reduce that's effectiveness and may cause other damp problems. The best solution is simply to add more insulation roll up to a total thickness of 10-12 inches. You may be able to get it done for free by govt or local council if you're over 60. You'll have to raise any storage platforms to allow for the extra thickness but that's not difficult.

 

[mikej460]

If you insulate under the rafters then you are creating a 'warm roof' typically used in vaulted or semi-vaulted ceilings, the room beneath i.e. your attic would then need to be heated as a room. If this isn't what you want then you need to insulate between the attic floor and the room below.

To achieve this I would take up the attic floor and fit 100mm rockwool or 100mm celotex between the joists then lay 100mm celotex across the joists i.e. at 90 degrees. You can then create a floating floor onto the celotex using chipboard flooring. This is what I did and it works really well and the attic floor is strong. Have look at YBS website for advice on using superquilt in this scenario as you need to avoid condensation rotting your joists.

Take a look at this, especially the Suspended Timber Floor section SuperQuilt-England-All-In-One.pdf (ybsinsulation.com)

More info on superquilt is on the YBS website Home - YBS Insulation

 

[Jake]

If you insulate under the rafters then you are creating a 'warm roof' typically used in vaulted or semi-vaulted ceilings, the room beneath i.e. your attic would then need to be heated as a room. If this isn't what you want then you need to insulate between the attic floor and the room below.

I do not understand why you say that the attic would need to be heated as a room. Perhaps you saying it will end up being heated the same as the other rooms because the heat will rise?

 

[Lard]

I converted the attic in our 1960s bungalow and, for the first time, used a mixture of 50mm celotex-type in between the 100mm rafters (ie allowing a void either side) and 19 layer multifoil fitted to the face of the rafters. I also insulated the floor with fibreglass to reduce the heat loss from the ground floor.
In our case the effect was quite dramatic (sound-wise too) and we have still not extended or fitted any heating up there, even though that’s where we now sleep.
Don’t get me wrong, it’s not roasting but certianly nowhere near freezing either which suits the pair of us.

It was my first experience of multifoil and I’m now going to use it in my detached garage conversion.

I wish we had ‘had’ a 5yr old too as it was a pain to fit both the pir and the foil right down low in the eaves.

Just for info - both my mate and myself coughed and coughed and coughed and coughed because of cutting the pir

In your situation I think I’d be a little hesitant simply because of the non-existent roofing underlay……but that just me.

 

[mikej460]

I do not understand why you say that the attic would need to be heated as a room. Perhaps you saying it will end up being heated the same as the other rooms because the heat will rise?

Correct but it really depends if this matters. Leaving it unheated would mean incurring unnecessary heat loss from rooms below but this would still be a lot less than through the current cold roof.

 

[eribaMotters]

We had a similar issue when we moved end of 2017. A near fully boarded loft used for storage, but under the boards was between 20 and 60 mm of pink/yellow fibreglass insulation that had settled with age.
I took everything out, took the opportunity to re-do the loft lighting, update the bathroom lights and run cables for outside lighting in the soffits. Between 340 and 400 mm of non scratchy rockwool type then went in. One layer along between the joists and the other across the top. Down the centre I Omitted the 2nd layer where I'd put an inspection runway in about 400 mm wide. The bungalow went from unbearably cold to very comfortable in winter and equally stays cool in summer.

Colin

 

[Cozzer]

Afternoon, chaps....hope you are all well.
Many thanks for your advice and suggestions.
I should've mentioned that I've already bought this foil stuff, but have held back since reading some of the above!
Just to complete the picture, the attic is unheated. One of the bedrooms beneath is famously cold, mainly as it's blessed with only one CH radiator. Ordinarily this isn't a problem, as nobody actually sleeps there since our nestling has fledged, but with all this work-from-home-when-possible business, my better half is using it as an office during the day.
(Curiously the other bedroom and Wembley-sized bathroom are fine warmth-wise - it's only the front bedroom that causes your nose to drip.... Not a good look on office paperwork)
Suspicion was that whatever heat in being generated in there is being lost upwards through the attic, hence the (wrong, according to most up above) thought about the foil across the attic rafters.
A quick and easy solution....or so I thought....

 

[scholar]

Interesting discussion.

Our house is Victorian, with extensive loft space. The loft(s) are not inviting places to be, so I have been slowly working on them on the “warm loft” principle. My concerns were to seal off the loft to deter the large numbers of flies etc that like to overwinter there, whilst keeping it vapour permeable (outwards).

My particular solution is as follows:

Ceilings:
- 100mm fibreglass in between the joists
- 50mm Polyboard (some kind of polystyrene designed for this purpose) laid over the joists
- 18mm chipboard flooring screwed through the Polyboard into the joists

- part of some of the ceilings is sloping into the eaves and there we have 50mm celotex insulation board between the rafters.

Roofs:
- these are felted with traditional roofing felt under the tiles
- 50mm breathable natural fibre insulation between the rafters (held away from the tiles with batten strips down the rafters)
- breathable “felt” laid over the underside of the rafters stapled in place to hold the fibre insulation boards.

It ends up with a neat and clean solution. I remain somewhat wary as to whether:
- the ventilation of the air gap between the underside of the felt and the top side of the fibre insulation is adequate - There are gaps between the overlapping felt that do provide airflow but I may get some ventilation grilles fitted under the ridge tiles when we get some roofing work done (not a small job)
- the ventilation generally of the loft space will be adequate when all the insulation is completed - I may need some air bricks in the gables.

I wanted to avoid foil insulation because in my experience it plays havoc with wireless signals.

It is not a straightforward subject, loft insulation…!

Cheers

 

[Andy Jones]

Will stapling the foil to the roof beams cause any damp issues.? Assuming a gap the width of the beams will allow sufficient ventilation. I don't know what I'm doing really, just wanted to make the loft a little less cold/hot for storage solutions. Insulation of the rooms in the house is fine just wanted to make the loft itself a bit more insulated from extremes

 

[Sideways]

The damp will come from inside the home, warm, humid bodies and make it's way outward.
If you have a vapour barrier, it should be on the inner side to stop damp making it's way into the insulation. Not on the outside where it will trap moisture inside the insulation.
I don't see anything wrong with draughty lofts if you're only using them as storage space.
Draughty tends not to be damp.
Pile up the thickness of the insulation on the reverse side of the walls / ceilings enclosing the living spaces.
I confess, I would like to know how much moisture actually makes it through a few coats of paint and a sheet of plasterboard to get into the loft insulation above my ceilings. I genuinely don't know if it's significant. They didn't have "vapour barriers" when our house was built.

This is my DIYer understanding of the issue, so my own improvements have been simply to remove old, settled and patchy glass fibre insulation and replace with a greater thickness of new Knauf loft roll. A rockwool type of insulation.

 

[Andy Jones]

Thanks for the quick reply Sideways. I'm getting LoftZone storefloor compact system to board the loft. This will leave a ventilation gap between the existing rock wool insulation and the loft boards. I was hoping the insulation foil would make going up in the loft a bit nicer but hadn't considered it causing any problems with damp etc, I'm fine with the draught and there is no vapour barrier. Just want to make the space suitable for storing books, photos and some old electronic stuff.

 

[Andy Jones]

 

[Sideways]

This will leave a ventilation gap between the existing rock wool insulation and the loft boards ....., I'm fine with the draught and there is no vapour barrier. Just want to make the space suitable for storing books, photos and some old electronic stuff.

Pretty much what I did.
After changing the fibre between the joists, I just boarded over the joists with chipboard, laid loose.
Fibre insulation tends to settle under gravity so you soon get an inch air gap under the boards if no already there to start with.
In summer, the heat in our loft is pretty high so I reckon this bakes out any slight damp that may have accumulated through the winter.

 

[Jake]

If you insulate under the rafters then you are creating a 'warm roof' typically used in vaulted or semi-vaulted ceilings, the room beneath i.e. your attic would then need to be heated as a room. If this isn't what you want then you need to insulate between the attic floor and the room below.

That's a cold roof. A warm roof is insulation over the rafters (it's the roof structure which is cold or warm).

Not immediately understanding the heated as a room point - what's the difference between insulation under the rafters and insulation over the ceiling joists apart from some height. I can see insulating both over joists and under rafters might cause a problem with the dew point being inside the attic though.

 

[Beau]

We insulated our whole place with mutifoil (Actis super 9)

Great stuff when done right even though there was some controversy in the trade about it's effectiveness some years back. Taping all joins sealing edges is a must IMO otherwise cold air can just bypass the insulation. I did use expanding foam to seal edges in a few awkward access corners. Also make sure you install it with the air gaps both sides as per instructions. I was pushed for space where I used it on one wall and didn't have the required 20mm gap both sides and the wall is noticeably cooler

Edit. Just seen the original OP was a long time back but will leave this here anyway

 

[RobinBHM]

That's a cold roof. A warm roof is insulation over the rafters (it's the roof structure which is cold or warm).

Yes I suppose that is true

a conventional pitched tiled roof is a cold roof, the whole roof space is the cold roof and therefore has to be ventilated to move damp air.

putting insulation in between the roof rafters or attached to the underside of the rafters will restrict the ventilation around the rafters and underside of tiles. In terms of insulating the loft it might help somewhat but the problem is the whole loft space is still fully ventilated to the outside and must remain like that

 

[Jake]

putting insulation in between the roof rafters or attached to the underside of the rafters will restrict the ventilation around the rafters and underside of tiles. In terms of insulating the loft it might help somewhat but the problem is the whole loft space is still fully ventilated to the outside and must remain like that

Between the rafters or between and below them has always instinctively seemed problematic to me, as who knows where the dew point ends up with the cold bridging rafters buried between most of the insulation with only a thinner layer under.

Completely under leaves the whole depth of the rafter just as ventilated as it is with insulation over the ceiling joists. The one difference is the face edge of the rafter to which the insulation butts up, but I can't see how that's an issue as all the sides of the rafters and the cold face of the insulation are clear and ventilated.

Bit over my skis and relying on 'common sense' (i.e. guesswork and my own priors) so may well be wrong. Interested to know though, because I did my loft under the rafters (not between)!