Can you use plasterboard for window jambs, reveals, sills?

[elbow]

hey guys - i've been here asking a few related questions and some of the helpful answers are now leading me to this question.

I had new PVC windows fitted but i refused to have any timber surrounds.

After browsing the internet, most suggestions are to use plasterboard - But now it has been pointed out that any moisture resting on the plasterboard is not healthy so i would like to ask for advice on what i should use/do?

There are also suggestions to use insulated cavity closers, but i cant see how i would fit them as the windows are attached to the outside wall (double-brick). I don't want to use wood anywhere - Can i use PVC or something moisture resistant instead of the plasterboard?

As always, your help is massively appreciated and my hurting head will be forever thankful 🙏🙏😁

[elbow]

Hey Dave, love your project and thanks for sharing, even the mistakes. 😁

From what i can see, your main concern is about water sitting on top of the sill - are you assuming that would be from say, a spillage or even condensation running off the window onto the sill?

[Noyade]

Hi @Dave-1 

"hahahha I just realised you were asking for dimensions for the window sill and edge"

Yes - the dimensions on the window as indicated - was what I was after. 😁

Do you have them @elbow ?

My vote would be...no - to plasterboard/gyprock.

If we're voting. 😁

[elbow]

yeah the window to the inside edge is 100mm and there is another 40mm to the edge of the outside edge of the wall - total 140mm from out wall edge to window

so, no plaster anywhere?  ☹️

[TedBear]

Hi @elbow , I can see that you certainly have a problem to solve here. Those windows are normally mounted in a window opening has already been lined (usually with timber, but could be another type of cladding).

If the possibility of dampness on the bottom sill is what is making you hesitant to use plaster, then maybe you can create that edge at a slight angle, so that it slopes downward, inward a little.   

Water should be kept out of that area by the sealing of the windows, so there shouldn't be much, apart from a possible few drops of rain if the window was left open.

Perhaps you could render that inside opening with cement to form flat surfaces to which plasterboard strips could be attached.  (Glue some mesh over the cavities between the walls so the render has something to stick to. Liquid Nails.)

You could seal the brickwork before rendering to prevent moisture from the walls if that's a concern, but given that your existing walls aren't affected (I assume) then you probably don't need to.  If you are concerned about moisture traveling in the opposite direction, then seal the cement render before plastering over it.

Then, when all joints are filled, paint the lower sill at least, with a waterproof paint.

(Ideas created without seeing the actual job, of course.)

[elbow]

thanks for coming back.

I originally thought to just use plasterboard for all sides but i saw mentioned about about moisture created via heating on inside and cold outside temps or cold air coming through the gaps in the wall cavity which is why i posed the question. I don't want to be doing a job to then have to rip it back down in a yr or 2 because the plasterboard gets ruined by moisture over time.  From what i'm understanding, you are suggesting plasterboard would be ok in normal circumstances?

[Nailbag · Accepted solution]

Hi @elbow what about filling the cavity with expanding foam and trim. Line the inside reveal with this aluminium flashing for increased moisture retardant. Then use Aquacheck Gyprock which is designed for wet areas like bathrooms and square-set finish the reveal. This is essential the process I used on a small window that copped every bit of the weather on the open side of a hill. The outside wall was flashed and rendered.

Regards, Nailbag

[elbow]

Hey Nailbag

thanks for your response. i like this.

Re the foam - when you say 'trim', do you mean cut off the excess?

Do you have a link to the aluminium flashing? i'm not sure what i would need.

And when you say, "square-set finish the reveal", what do you mean exactly? 

Apologies, i'm way down on building language so i just want to be clear what you mean  🙏

[Nailbag]

Apologies @elbow 

Yes spot on with the expanding foam and if you haven't used it before check out a few YouTube videos so you can see what little you need and how much it will expand. I would tape up the inside of the window frame and use some cloth tape over the cavity openings as this will assist containing the foam as it will continue to expand until dried. 

You have two options with the flashing, plastic or what I use which is the aluminium as I can use it on other projects and bend it to suit the applications.

And lastly to "square set" basically means to have the edges of the plasterboard exposed which gives it a clean modern look over tradition architraves around a window in your case or cornice. So the picture I supplied was the window finished off by a plasterer ready for painting.

Hope that all make sense 😁

Regards, Nailbag

[elbow]

Hey @Nailbag 

thank you so much for your explanation and links - super helpful!

one question tho - re the square-set - from what i understand that is to push the plaster upto the window frame which i had intended but i wasnt quite sure if your were talking about that edge or the edge facing into the room - i was going to use some beading for that?

[EricL]

Hi @elbow 

I'm thinking putting beading would make the edge stronger rather than just having it as plain plasterboard, but if it gets chipped it would still require the same type of repair procedure. Let me tag @Nailbag to get their opinion.

Eric