How can I make ceiling joists airtight?

soldsold

Hi,


My bungalow has 7x2 ceiling joists about ¾ filled with rockwool. Last year I started to prepare the attic to use as a spare room and in the interest of soundproofing I nailed 1 inch timber strips just under the top of each joist (i.e. 3/8 inch below the top of joists) and fitted lengths of 3/8 inch plasterboard resting on these strips and fitted tightly between each joist so that the plasterboard was flush with the joists making what looked like a level floor.


The plasterboard was sealed with silicon sealant [FONT=&quot]on every edge [/FONT]to avoid airgaps where sound could leak through. I then put down ¾ inch plywood sheets, with silicon sealant on the edges. The plywood only covers the finished floor area – i.e. the crawlspace just has rockwool. The finished floor will be carpeted and the attic walls/ ceiling will have 75mm PU sheets tightly fitted between rafters with gaps sealed, leaving a 50mm air gap between the sheets and the roof tiles, with 38.5mm insulation backed plasterboard beneath. The rafters are 125mm and the roof height is pretty low so that’s all I can fit.


Before I finish the floor I need to stop air/ wind travelling under the plywood floor, so I plan to install noggins - 7x2’s between each joist at the perimeter of the floor and seal with silicon/ mastic.


My questions: Am I missing anything – I can’t find much info on how to make ceiling joists airtight when retrofitting.



1) Should I consider insulation backed plasterboard below the ceiling as an extra airtight/ heat saving measure? I will DIY all the work including the skimcoat so its just material cost I’m faced with.



2) Would I be mad to lift the rooftiles and put more insulation on the roof?



3) At what point should I get an airtightness test done?



4) How will I make the ceiling airtight – 9mm OSB or 500 gauge PE (condensation issues I expect…) or more expensive airtightness membrane?


Thanks for any advice!
Steve

soldsold

I think my last post got no replies as it was too complicated!

Simpler version:

Bought a bungalow with 7x2 ceiling joists and a cut attic roof so there is enough space to convert the attic.

Insulated the 7x2 ceiling joists about 3/4 full with rockwool (not full as i was more concerned with sound insulation at the time)

Currently the 7x2 joists sit on the wallplate and are tied to the rafters there (ie normal cut roof).

I have floored the attic out as far as the knee walls with 18mm plywood, glued and screwed.

I have fitted 70mm kingspan between the 125mm sloping rafters.

I have bought enough 38mm insulation backed plasterboard to insulate under the sloping ceiling.

I need to make the eaves, sloping ceiling and under the floorboards airtight as there is a howling wind throuh the house in winter.

how do I plan to fix this?

1) Pack rockwool tightly under the plywood floor at the perimeter of the attic floor (ie under the kneewall).
2) Put a layer of 500 gauge PE sheeting under the insulation fitted between the sloping rafters, and put insulation-backed plasterboard under this.
3) Insulate the knee wall in the attic and insulate the floor of the remaining crawlspace (ie the strip at the edges of the attic) with 400mm rockwool.

Any issues with this or other advice?
Ive searched the main forums but cant get an answer that allows me to use the insulation backed plasterboard that I bought before I studied these forums...

Thanks!

Steve

Carlow52

Having read both posts what follows might be wrong:)

I presume the soffit is vented so there is an air supply across both sides of the house between the felt and the insulation in the 50mm gap.

What is not clear is how far down to the eaves this gap runs or did u stop at the knee wall.

You can deal with AT in the room section as u please, what follows relates to the crawl space only

The following detail is 99.9% based on a post by Sinnerboy some time ago:

where the cut roof intersects at the wall plate cut and fit , say a 4 by 2 longitudinally at the correct angle so as to take the point out of the triangle I]my variation[/I

square face longitudinally on rafter, angle cut against rafter

Then snugly fit full depth noggins in the floor joists along just inside the above 4 by 2: fill any gaps with silicone

then take an AT membrane and fit it along the slope, down over the 4 by 2 and out along rafters to the noggins where it is sealed.

This detail will make the crawl space and floor space AT and still let the air pass up under the felt, what u do insulation-wise is ur call.

From a dew-point condensation perspective I will allow the others to suggest if u can use the 38mm composite with this detail.

I would floor the crawl space as I hate the dust from the loose insulation

sinnerboy

Carry the attic flooring from wall plate to wallplate . This floor should be 1/2 hour fire resistant - including the parts you don't walk on , on day to day basis .

This is my approach . A lesser approach is possible looking to Homebond manual* / regs - but bugger that imo . Safety first .

So - at the pinch point of the roof /attic - you can use AT tapes / membrane to seal the underside of the rafter "cosyboard" to the topside of the ( imo properly extended ) flooring .

another essential point - you must maintain a 50mm vent space between your roofing felt and the top side of the between rafters insulation

and
have a 25mm ventstrip at eaves
and
have a 5mm ventsrip at ridges / hips

( google glidevale vents )

* Page 408 2008 manual

.

soldsold

Thanks for the replies.

sinnerboy wrote: »

So - at the pinch point of the roof /attic - you can use AT tapes / membrane to seal the underside of the rafter "cosyboard" to the topside of the ( imo properly extended ) flooring.

Sinnerboy:

Will this allow cold air to get from the eaves under the floorboards?

sinnerboy

Good Q - yes .

to solve you need to remove your facia / soffit eaves "box" and extend/ drape the attic floor insulation over the wall plate + down the wall face . And then insert ply or osb "wind cheater" inserts between the rafters .

Sorry you asked ?

Point is - buildings in Ireland are rarely built to these standards .

soldsold

Nope - happy I asked because I just know that any moment someone will come on with a simple fix that can be done from inside the crawlspace that will give good airtightness and windbreak...

10...9...8...7...6...

Carlow52

In many houses where this issue arises in a retro case I have found that the space between the rafters over the wall plate has been filled in with concrete right up to the felt which makes access from the soffit box more difficult and also negates the value of any air vents in the soffit board, usually fitted when the timber ones are 'upgraded' to PVC so just check this in ur case

soldsold

No concrete thankfully, the rafters are just resting on the wallplate.

sinnerboy

soldsold wrote: »

Nope - happy I asked because I just know that any moment someone will come on with a simple fix that can be done from inside the crawlspace that will give good airtightness and windbreak...

10...9...8...7...6...

Haven't done this - but you could make "sausage rolls" 300mm diameter made up of Tyvek wrapped around rockwool . And squeeze between the joist into the attic floor cavity over the wall plate .

Firestop , windcheat + thermal continuity

soldsold

Good idea.

I have already done this as a temporary measure but without the tyvek.

Adding tyvek might be the best option, although I really dont mind having to do a bit of rooting to get a 100% fix if another method is more airtight as I may end up using an MHRV system.

To stop the "sausage rolls" from getting out to the cavity I cut lengths of 70mm thick PU sheets and wedged them in between each of the ceiling joists directly over the wallplate.

Will be full-filling the cavity soon, maybe this will help to allow an easier solution?

soldsold

I am still not sure of how best to make the sloping ceiling airtight.

It has 70mm kingspan tightly fitted in between 125mm rafters.

I have 38mm+12mm kingspan/ plasterboard sheets ready to attach below the rafters.

Can I just use 500 gauge PE sheeting in between the two layers of insulation?
I would guess this would still allow the rafters to breathe?

Steve

sinnerboy

Polythene wont allow breathing

Staggering the insulation joints should suffice

Carlow52

soldsold wrote: »

I am still not sure of how best to make the sloping ceiling airtight.

It has 70mm kingspan tightly fitted in between 125mm rafters.

I have 38mm+12mm kingspan/ plasterboard sheets ready to attach below the rafters.

Can I just use 500 gauge PE sheeting in between the two layers of insulation?
I would guess this would still allow the rafters to breathe?

Steve

Can u describe the make up of both layers: eg is there any foil?
thanks

soldsold

Thats the bit I dont understand....

Kingspan sheets (or Quinn etc) with foil backs are airtight/ vapour tight but the joints are the weakness.

So I dont understand if you want the joints to be air/ vapour tight or not?

My understanding is that PE film will be 100% airtight and vapour tight. Kingspan sheets may be both if done well but probably will leak a bit.

Would it be better to either have fully sealed PE or part/ mostly sealed kingspan?

On staggering the joints - the insulated plasterboard sheets will have to meet at rafters so I cant see how they can be staggered?

Sorry for all the questions but I dont want a leaky roof or a mouldy roof so I need to get this right.

Thanks,

Steve

soldsold

Sorry Carlow52 just saw your last post:

The 70mm sheets currently between the rafters have foil on both sides. They are wedged in tightly and expanding foam is filling any gaps.

The 12mm plasterboard is stuck to 38mm PU sheets with no foil (will need to double check when I get home this evening but dont think there is foil backing).

Thanks!

Steve

soldsold

Just checked the insulation-backed plasterboards and these 12mm plasterboard/ 38mm PU insulation boards have no foil on the back, just paper.

So the question still remains, should I use a PE air barrier between this plasterboard/ insulation layer and the rafters that are filled with foil backed PU sheets between the rafters?

Or would a better solution be to attach the insulated plasterboard, then put up a 500 gauge PE vapour/ air barrier and then add another layer of plasterboard?

Or some other solution? ie do I need to use an intelligent membrane?

Thanks!