Loft insulation question

[ban-all-sheds]

My loft has plenty of insulation under the floor which does a splendid job of keeping heat in the house. The problem is that the loft gets very cold in winter; sub-zero temperatures are not unknown.

I'd like to put some insulation under the roof itself to try to prevent the wost extremes. My thought was to put some kind of fibre-based or polystyrene batts between the rafters, covered with sheets of thin ply.

Directly under the tiles is a membrane of bitumen/paper layers which is mostly sound, but in places is holed or torn.

Are there any condensation/ventilation issues which should concern me?

 

[masona]

Are there any condensation/ventilation issues which should concern me?

Yes,under building regulation the back of the roof rafter must have a 50mm air-flow from top to bottom.
Normally the roof rafter is 100mm and you put the 50mm wall insulation slab flush at the front of the rafter which will give you 50mm air -flow behind at the back but make sure you leave a gap at the top and bottom say 50mm to allow for continuity of air circulation.If you're going to put thin plywood on front make a few ventilation holes. On my roof I've put 8'x4' sheet hardboards with holes (I think they're called peg boards?) or you can use vapour moiture paper roll sheet which is normally use for shed/summerhouse and I think I've seen them in Wickes.

 

[ban-all-sheds]

Thanks, masona - like the idea of pegboard. Pretty sure my rafters will be good old fashioned 101.6 x 50.8....

 

[breezer]

it IS called pegboard, but so far a search has revelaed all types of hooks / fittings to go on it and even a game, but no pegboard itself

 

[DAZB]

Try Howarth Timber for Pegboard. I know my local branch in Leeds does it.

 

[GwaiLo]

ban-all-sheds
The obvious question to ask is why you are bothered about the loft space getting cold. It is designed to do so after all.

Apart from what masona said being correct in regards to the building regulations, you must realise that the entire roof space, including the rafters, purlins, struts and ceilings joints, are designed to be ventilated in a traditionally built pitched roof. If you block the ventilation to any part then you run a risk of increasing the moisture content in the timbers and developing wet or dry rot.

What you are considering doing really only makes any sense, practical or economic, if you intend to heat the loft and ventilate the space by some other means (e.g. convert the loft and put windows in with trickle ventilators).

However, you should ensure that any pipes and tanks in the loft space are well lagged to prevent freezing. If they are already well lagged and you still have a problem with freezing (which I would find hard to believe anywhere in the UK) then the solution would be trace heating.

 

[ban-all-sheds]

I use the loft as storage space, and some of the things I keep up there shouldn't be exposed to temperature extremes. (No, not herbal tobacco plants! ).

There are no pipes or tanks to concern me, and even when they were there they were positioned above a non-insulated area, and they never froze. I've logged the temperature in the loft and it has been below zero at times, and I would like to keep it from dropping below 10ºC. I'm hoping that insulation will be enough to accomplish this - I could not effectively heat the space if it doesn't.

It was the ventilation/condensation aspect which prompted me to pose this question in the first place. The ceiling joists are already buried under layers of insulation and 18mm chipboard. Does the 50-year age of the roof make any difference to the timber moisture problem?

 

[GwaiLo]

The age of the roof doesn't make any difference.

Within your house you produce gallons of moisture every day. This is held by the warm air and much of it rises through the house to end up in the roof space. Unless the roof space is ventilated then that moisture has nowhere to go. The moisture in itself isn't a problem, but as the air moves into cooler areas it it more likely to release the vapour and cause condensation and rot.

I am afraid your choices are limited to:

Don't store things in the roof that are moisture or temperature sensitive

or

Remove the insulation from the loft floor and insulate the loft as has been described elsewhere. Effectively this means you will be heating the roof space. If you don't maintain the temperature of the roof space at similar levels to the rest of the house and you remove the ventilation then condensation will occur and rot is very likely to result.

 

[ban-all-sheds]

OK - understood.

Is it likely then that Masona's idea of

a) A gap between the insulation and the tiles
b) An uninsulated area at the apex and by the eaves
c) Sheet material with holes in (e.g. pegboard)

would still result in insufficient ventilation?

I'm beginning to wonder two things:

1) Would any installation of insulation that worked (i.e. prevented heat loss) have as a corollary the prevention of ventilation?

2) Why do they sell insulation for putting between rafters?

 

[GwaiLo]

masona's idea is fine if you intend to remove the insulation from between the top floor of the house and the roof space. That will then be heating your loft because the heat will easily rise into it. If the area is heated it does not need to be ventilated as condensation cannot occur.

Alternatively leave the current insulation in place and put a radiator in the loft when you insulate between the rafters.

The key thing is that anywhere where condensation may occur must be ventilated to remove excess moisture or it could cause timbers to rot.

Rather than me try to explain this you should comply with Part F2 of the building regulations which states that "adequate provision shall me made to prevent excessive condensation:
A) In a roof or
B) In a roof void above an insulated ceiling"

The approved document to the Regulations shows you how you can achieve this and you can find it here http://www.odpm.gov.uk/stellent/groups/odpm_buildreg/documents/page/odpm_breg_600480.pdf

 

[ChrisR]

Coming at this from the other side, as it were, the heat losss though a ceiling with several inches of insulation is approximately bu**er all. People often double the insulation thinking they're saving money, whereas they're actually saving half of b a .
So there should be no heat getting into te loft , for any practical purpose. If you want stuff to not freeze up there, perhaps you could make a volume like a tent, and use a frost thermostat and some form of gentle electric heat, installed thoughtfully because of the obvious fire risk. Perhaps the matting intended for putting under floor tiles. Light bulbs do get used but aren't very good heaters.
I've known people use small radiators or zig zags of pipe off their heating system to make sheds/garages stay above freezing. It just needs a frost stat and a 2 port valve. Old radiators are always to be seen on the council tips!

Foil-backed plasterboard could be used to divide off the unfrozen area, perhaps.

 

[ban-all-sheds]

Perhaps I'll fix a few of these to the purlins:

Only joking!


Thanks to all for replies.