I have 2 large alcoves either side of the fireplace and want to seal up, 2 reasons, one noisy neighbours and 2 I want one flush wall. Want to do the cheapest and easiest way, if Iput a frame around the edge and put up the cheapest plasterboard!!! or something similar!!! Please advise!
You can make a frame out of 2x4 timber and then sheet it with plasterboard as you have suggested. If you want to keep noise down you might want to put some insulation in the gap. You don't have to fill the gap but having some in there will help. It can be stapled to the framework before you sheet it.
Once its sheeted you would need to fill the joints and screw holes etc before decorating.
Depending on the width of the alcoves you might need to add extra vertical pieces to the frame to reduce flex in the plasterboard.
Thanks for the advice, how would I secure the extra bits of wood across, and should I nail or screw the plasterboard to the frame. Also should I make the frame and then secure to the wall, or can I do it bit by bit!!
The width of the gap is 43" and height is 99" high
with my experience of alcoves i would say build it in situ, because alcoves are rarely (if ever) square, plumb and true. it will be hard to make a frame then try to fit and have it fit exactly (though you can get around it using packers and spacers) fit two verticals, one on either side of the alcove. then put cross braces (noggins) across and just screw directly into your verticals with long screws, no need for fancy joints.
if you want to run an additional vertical you can screw it straight into to the floor. if concrete obviously you will need rawl plugs. if wood then great, its easy just watch the depth of the screw as there could be hidden pipes or cables.
fixing to the ceiling might be more of a problem unless you are lucky enough to hit a parallel joist (unlikely). if not and your joists in the ceiling run perpendicular to the back wall of the alcove then run another 4x2 along the ceiling and fix into the joists. this will give you somewhere to fix the additional vertical as well as giving extra support to the plasterboard.
to find the joists, spot drill holes up into the ceiling (again being careful with the depth to avoid any pipes and cables) until you find them. do this in the alcove so they will be hidden by the new wall to save making them good after.
you can nail or screw the plasterboard just make sure you have the right type of either. personally i always screw, its just as quick (if you have an electric screwdriver) and less likely to knock your timber framework out of position. good luck, let us know how you get on!