Attic Insulation (Again)

peterc14

Hi - I have read similar threads but dont seem to be able to get a clear answer.

New Build House - I intend to insulate the attic with 200mm wool insulation between the joists. Also insulate between the rafter and also over the rafters with insulated plasterboard. I also intend to put and extra 100mm insulation over joists, behind dummy wall. See Pic Attached. Will there be any condensation/other problems with this proposed idea.

The attic will generally be used as storage, however there is windows on one gable and it is my intention to convert approx 1/2 the attic space at a later date.

Also what are you best flooring the attic with?

morning delight

Have you taken a look at a system like this: http://www.aerobord.ie/roofproducts.html (Mods- please remove if it's favouring one product. However it's a well know company and, I'd have thought, a well known product).

The pdf and animation make it all look very simple.

I'm not in the trade so not offering an expert opinion. However looking at what you've proposed, it appears it's neither a cold roof or a warm roof.

Have you dormers?

peterc14

Thanks Morning Delight...

It is basically a warm roof in the sense that the rafters are insulated. However, I thought what is the harm in putting insulation in the joists at ceiling level also on the assumption that more insulation is better than none. Some people seem to think just one or the other. I thought that by putting insulation at ceiling level (between the joists) that the heat would be retained better on the first floor, while also helping the attic to remain at a stable temperature.

Any further thoughts or opinions?

peterc14

...meant to add that there are no dormers... just two small windows on one gable

sydthebeat

peterc14 wrote: »

Thanks Morning Delight...

It is basically a warm roof in the sense that the rafters are insulated.

no, a "warm roof" is insulated, usually between and over the rafters.
if its insulated between and under, then its a cold roof.

you have two options.

either consider the attic to be a cold vented space and insulate (sufficiently to meet regs) at joist level (usually 300mm quilted material).......
or consider the dormers space to be warm, and therefore insulate (sufficiently to meet the regs) either up the slope or along the dormer line.

using insulated plasterboard under the rafters is completely pointless if the attic is vented through.

if the attic is to be consider warm, then the ventilation needs to follow the slope of the roof, ie 50mm between the roof felt and the insulation between the rafters.

importantly, a warm roof does not need to be vented.

peterc14

sydthebeat wrote: »

using insulated plasterboard under the rafters is completely pointless if the attic is vented through.

if the attic is to be consider warm, then the ventilation needs to follow the slope of the roof, ie 50mm between the roof felt and the insulation between the rafters.

importantly, a warm roof does not need to be vented.

Thanks syd for your comments...

. What do you mean by vented?
. I had intended to leave gap between rafter insulation and felt ie part fill with insulated board over to help u value.
. Essentially I thought insulation to carry on from the external wall along sloping roof would be the best option and as an extra level of comfort/performance to put insualtion within the joists also. Is the insulation at ceiling/joist level pointless? I had thought this would help keep heat where you would want it most?

sydthebeat

peterc14 wrote: »

. What do you mean by vented?

roof constructions must be vented to exhaust water vapour and condensation.

peterc14 wrote: »

. I had intended to leave gap between rafter insulation and felt ie part fill with insulated board over to help u value.

you must ventilate either through the attic or along the slope of the roof.

if you intend venting along the slop of the roof then i would suggest focusing your insulation in the dormer shape. this insulation must meet regulations.

If you intend making the insulation at the joists your "building reg" compliant insulation, and you vent up the slope, and you use an insulated plasterboard in the attic, then you will get condensation problems on this insulated plasterboard.
The insulated plasterboard will keep the attic space warm, though colder than the first floor. Water vapour would usually be vented away in the attic before it condenses, but in your spec it cant. Therefore it wil lcondense when it hits the insulated board, and this space isnt vented, thus you will have problems.

if you focus the main "building reg" compliant insulation layer to be on the slope then you wont have this issue, but you will have a large attic space that is within the insulation envelope and is unheated (i assume). this wil ljust increase the loading on your heating system.

so my opinion is either to insulate at teh joist level and keep your attic vented (the standard construction)
or insulate on the slope and make the attic a proper heated space.

creedp

I was palnning to do much the same as the OP with a new build. I wanted to insulate at joist level and place a vapour barrier below the joists to stop heat and moisture escaping into the attic space. This joist insulation was to be 'building regs' compliant. As the attic space is quite large I proposed to insulate/slab/skim the attic space from the knee walls, i.e. not insulate the rafters in the crawl space maintaining the 50mm ventilation gap on the sloped roof and using a ventilated ridge tile and 2 ventilated slates. I don't intend to use this as a habitable space and will not be heating it to the same level as the main house but I will have a radiator there to keep the space from getting too cold. Essentially it will be used for storage/and HRV Unit. There are 3 velux windows within this space.


I was of the view that with the vapour barrier at joist level there would be very little risk of condensation in the attic space and anything that might exist could be vented using the velux windows or alternatively a HRV supply duct could supply fresh air to the space. Am I in danger of creating problematic condensation with this approach?

peterc14

creedp wrote: »

I was palnning to do much the same as the OP with a new build. I wanted to insulate at joist level and place a vapour barrier below the joists to stop heat and moisture escaping into the attic space.

I was thinking of using foil backed plasterboard on the first floor to help lessen condensation risk in attic.