Attic/loft insulation (yes another)
 

Lo,

I'm planning on insulating my loft. It has some insulation
from when it was built circa 1966. The loft is used for storage
and has a central heating/hot water boiler, along with all the
pipes.
As I'm worried about burst pipes, I thought I'd insulate the roof,
rather than the floor of the loft. Having researched a bit, it
seems that I have to have 50mm clearance between the insulation
and the roof tiles, for ventilation.
There is plastic sheet under the tiles, but I doubt that it is
permeable.
I have vents in my eaves, which currently seem to be covered.
(And have been for 40+ years). I'll also be putting a wood floor
down in the loft before I start. This floor won't extend into the
eaves, just to the where they start ie above the outer wall.

I thought I'd follow this design:
Kingspan Insulation - Self Build

ie the K7 with possibly the K18.
So, for the questions:
1) How far from the apex of the roof should I stop boarding,
to allow for ventilation?
2) Should the eave end of the insulation touch the new floor
or should I leave a gap?
3) Will the insulation have any benefit given the gap in the
apex?

Thanks

John

Hello John and welcome to ER! Glad to see another project going. :thumbup:

Do you have a ridge vent there? Its hard to tell. I'd recommend adding in that 50mm vent space all the way to the peak. Then, I'd insulate the back side of that vent all the way and encase the entire room. You should be able to do this so you don't have any leaks. Leaks are the #1 way to loose heat. Once you seal everything up then you can start worrying about how to insulate it the best you can.

You're welcome. Thanks for the quick reply!
The sudden interest in this is because my central heating packed up
the other day and I'm still waiting for a new part for the fan. I've fixed
it temporarily, all very heath robinson. The upshot is I can hear the fan
starting up and it seems to do it too often.
That and the last quarters gas and leccy bill came to 380 GBP. ie best part
of 550 USD.

No. No ridge vents at all, just a vent in each eave (I have a gable end)
that runs the length of the eave.

I don't quite follow. I was going to put board between the rafters and I presumed (heh) that they wouldn't join at the peak. ie there'd be a gap at the
ridge. So, say it's twelve foot from the ridge to the gutter, for example,
I'd start the board a foot from the ridge and finish it a foot from the gutter.
I thought the theory was that hot warm air gets into the loft (obUS: attic)
and would need a gap at the ridge to allow it to flow down to the eaves.
Or something.
I'm guessing obv, not having a clue :)

What do you mean by "back side of the vent?"

John

Right, I've googled a bit and been on some suppliers websites. It seems that the rafter/joist interface needs to be sealed. I'm a little unsure as to what happens at the ridge. If I seal it there too, where does the water vapour go that gets into the loft from the house? Or am I supposed to seal it that well that none gets into the loft?

Well, if you maintain the air gap all the way to the peak of the roof, and install a ridge vent or regular vents it just keeps the air flow trapped in that gap. I'd use a vapor barrier on the bottom face of the roof's joists to prevent moisture from getting into the insulation.

Here is a real rough picture of what I mean:

http://ecorenovator.org/forum/attach...1&d=1234182269

Aye, I was hoping I wouldn't have to do that tho. What I think I'm going to do is drill large bore holes in the vertical plank that's at the ridge so there will be a route from eave to eave. The waterproof covering is a good idea. There is one beneath the tiles but one on the other side of the rafters wouldn't hurt. Although looking at your diagram the vapour barrier is on the other side of the insulation to what I mean. I was going to do:
tiles->barrier>joists->barrier->insulation
but the diag does:
tiles->barrier>joists->insulation->barrier

I presume that with no ridge vent the first is the better option.

Which side should it go on?

John

Vapor barrier should always go on the warm side of insulation to stop water from getting into the insulation. The warm air holds more water and as it cools off the water condenses because the cold air can't hold as much water.

I'm not sure what you mean about tiles.

Ah, I see. I presumed that I'd need some on the cold side as there'd be damp there too. Ok, I got it. By tiles I mean slates, roof tiles.

Out of interest how much is insulation board in the US?
2400mm x 1200mm x 150mm is around 55GBP (about 80USD) on the net over here...

John

I'm not sure they even sell foam board that thick here. My 1" thick (25mm) polyiso 2400x1200 was a little over $10 per sheet. Regular polystyrene is a bit cheaper.