Fitting fibreglass attic insulation

Connacht

I get most of what to do about fibreglass attic insulation (depth, width, etc.). But two specific questions please folks. I already have insulation up to the top of the joists. I now want to lay another layer over that perpendicular to what's already there and the joists.
1. Looking at the diagram attached, do I need to keep the second layer in from the felt on the underside of the roof and, if so, by how much ?
2. Is there something about rising the insulation up the gable end wall (exposed concrete blocks) and, if so, how far up ?
Thanks for all your help.

Reyman

Connacht wrote: »

I get most of what to do about fibreglass attic insulation (depth, width, etc.). But two specific questions please folks. I already have insulation up to the top of the joists. I now want to lay another layer over that perpendicular to what's already there and the joists.
1. Looking at the diagram attached, do I need to keep the second layer in from the felt on the underside of the roof and, if so, by how much ?
2. Is there something about rising the insulation up the gable end wall (exposed concrete blocks) and, if so, how far up ?
Thanks for all your help.

The way it's shown in the diagram is fine. A few inches gap will allow air to flow in the attic and prevent dampness.
There's no need to run the insulation up the gable wall. You're not trying to insulate the inside of the attic, just insulate the attic from the rooms below.

Make sure you don't cover any electric cables with the insulation

Connacht

Thanks Reyman,
Was aware of the need not to cover electric cables, but the problem is they don't pull up enough to lay the extra insultation under them. Do I need to cut channels in the new layer then not to cover them ? Maybe 3 to 4 inches wide ?
Thanks for your help

MicktheMan

Connacht wrote: »

2. Is there something about rising the insulation up the gable end wall (exposed concrete blocks) and, if so, how far up ?
Thanks for all your help.

It depends on how the external gable wall is insulated below the attic. If this wall is drylined and insulated internally, then there is no need to insulate the gable wall in the attic as there should be minimal thermal bridging to the cold attic. If, otoh, the external wall is not drylined (f.i. insulation in the cavity) then I would insulate the gable wall in the attic to a height of at least a meter above the ceiling as this will reduce the thermal bridging effect from the attic to the room below.

Connacht

Cheers Mick,
The house is 13 years old and the external walls have the standard 2-inch-odd thick white dense stuff - the stuff your tv comes in for protection. Sorry don't know what it's called properly. Is it polystyrene ?

MicktheMan

Connacht wrote: »

Cheers Mick,
The house is 13 years old and the external walls have the standard 2-inch-odd thick white dense stuff - the stuff your tv comes in for protection. Sorry don't know what it's called properly. Is it polystyrene ?

Aerobord.

Reyman

That's a tricky problem with the cables. The only real solution is to extend the individual cables so you have more slack.
This means adding two junction box terminations for each cable. Or else one junction termination if you pull the cable back from the light bulb socket. Either way it's not too difficult provided you've only a few cables for lights up there.

I can't see how you can really leave a channel for the cables if you're laying the insulation in two directions. If you avoid the cable you won't be insulating the floor properly.

RKQ

Leave 50mm clear air gap minimum, between insulation and roof felt.

Don't insulate over cables - if the cables can't be extended - leave a gap to prevent cables overheating. (The gap isn't ideal but extra insulation overall is better than none!)

Good luck

Connacht

Thanks. Gap between insulation and felt comfortably over 50 mm.
The insulation I was laying over the weekend was a second layer, laid perpendicular to the 12 year old layer that is lying between the joists. All cables were already on top of the original layer, so there is insulation below them. So I dug the channels in the new layer, as the cables cannot be pulled up - no slack. Reyman, junction boxes sound like too much grief. Thanks anyway.

Reyman

Glad to hear you managed the cables. Just to note - watch out for the thicker cables (2.5mm2 or above) perhaps serving the immersion or some of the 3 pin sockets. These are likely to be carrying quite a high current and could easily overheat if insulated too much.

The 1.5mm2 thin cable used for lights is not so much of a risk particularly with the new low powered long life bulbs.

Connacht

Sound.