airtight detail at internal window reveal

enmac

I'm using a 38mm insulated slab on the inside of the external walls.

At the windows I had originally thought I would use the insulated slab in the reveals with an airtight tape at the window frames but now I'm in 2 minds.

The other option that I'm now considering is taping the window frames with an airtight tape and embedding the tape in a sand/cement plaster. This would mean that all the reveals would be sand cement but the walls would be insulated slab - my main reason for considering this option is that I feel the insulated slab in the reveals with an airtight tape would not give a sufficent airtight seal

Any views out there on the best solution ?

reefray

enmac

I think a combination of both of your options would set your mind at ease.

If you first stick the internal tape to the window frame and blockwork as per usual and then plaster a tight coat (6mm-ish) of sand and cement over the tape to seal it, you should still be able to dot & dab the insulated board to the reveal after.

Ray

ronaldo84

you could if u wanted 2 go to the trouble of sticking the airtight tape to window and block work u can buy metal plaster beading to screw 2 the window that has a channel to take a 12.5mm board screw a right angle metal frame section 2 the inside of thermal board then cut ur plaster board and slot into the channel at the window and screw to the other side of the metal right angle section. i take it u left room on the inner leaf to accomadate the insulation on the thermal board?? this when done proper is very neat detail and all ur reveals will be square level and plumb with the same margin around ur window frame. remember to leave enough thermal board past the window to square plumb and level up. make sure ur windows have sufficent expanding foam around da edges. this is the way i would do it on a new build or my own and have done it before on council buildings. mixing a insulated pasterdoard and sand and cement is messing and a poor job

reefray

ronaldo84

mixing a insulated pasterdoard and sand and cement is messing and a poor job

Maybe I wasnt clear enough with my previous post. I meant that enmac should wait until the sand/cement render had hardened sufficiently before dot & dabbing. Maybe you thought i meant to dot & dab onto wet render...

Applying sand/cement render over an airtightness tape is the accepted technique for ensuring airtightness and as enmac is intending to fit insulated slabs onto the interal faces of his walls he most likely will be applying a parge coat to these wall first to make them airtight.

this is the way i would do it on a new build or my own and have done it before on council buildings.

Council buildings or any other state or semi-state bodies have spec'd for using a parge coat under dot & dabbing for quite a few years now...

u can buy metal plaster beading to screw 2 the window that has a channel to take a 12.5mm board

Is screwing a drywall edge bead into a window frame gonna do any favours for it's u-value?

Ray

ronaldo84

Is screwing a drywall edge bead into a window frame gonna do any favours for it's u-value?

Ray[/QUOTE]
i dont know is it??

enmac

thanks for the replies

I hadn't considered a parge coat - i thought the skim coat would look after the airtightness?

reefray

I'm afraid not enmac

Only insitu concrete (ICF) or sand/cement render are airtight enough without going down the membrane route.

enmac

there seem to be a few specialised products out there - Airtite plaster by Gypsum etc.

Do i need to go down this route or would sand cement be OK - what depth sand cement is recommended ?

reefray

2mm depth of parge coat is thick enough for sealing a wall but it may be easier to aim for a little more as this will give you peace of mind that the depth is sufficient over the blockwork's inconsistencies.

It doesn't have to be dead straight and flat as you are only sealing the blockwork so it's possible to do yourself easily enough and save some $ in the process.

I don't know an awful lot about Airtite plaster esp it's price but generally speaking sand and cement bought by the ton work out cheaper than bagged gypsum products.

Ray