Help please with window installation position

johneym

hello folks,

Im doing a little timber framed extension and could do with a little advice if anyone would be so kind. Timber frame up, played and felted, no block work done.
6 inch timber framing, air cavity and 4 inch block outside. What is the window position? Have seen done where 2 inch battens are placed all around window frame so they are in the cavity so to speak, and then the window is placed on this, flush with outside of batten,so that it partially rests in timber frame and partly on batten. But then, if I am putting in window sill outside shouldn't the window rest on sill, which it cannot if resting on batten?
Not looking to do anything fancy, only looking for the simplest or straightforward way forward.

Help appreciated.

listermint

johneym wrote: »

hello folks,

Im doing a little timber framed extension and could do with a little advice if anyone would be so kind. Timber frame up, played and felted, no block work done.
6 inch timber framing, air cavity and 4 inch block outside. What is the window position? Have seen done where 2 inch battens are placed all around window frame so they are in the cavity so to speak, and then the window is placed on this, flush with outside of batten,so that it partially rests in timber frame and partly on batten. But then, if I am putting in window sill outside shouldn't the window rest on sill, which it cannot if resting on batten?
Not looking to do anything fancy, only looking for the simplest or straightforward way forward.

Help appreciated.

Whatever you do don't allow the cavity to be bridged by a sill.. I would recommend getting passive sill for below the window frame externally

bfclancy

in a timber frame the window is fixed to the timber frame via the timber fire stops fixed to either side of the window, it projects 50mm outside the timber frame and blockwork is built up tight to it, windows should be installed prior to beginning blockwork, cill is then positioned up under the cill with a bead of silicone on the upstand to form the seal with the underside of the window, this for standard cill, for passive cill or otherwise someone else might have the detail

johneym

thanks a lot guys,

BF I appreciate your help. Do I understand correctly that the window is pushed into the fire stops so that the outer window edge rests flush with the outer edge of the firestops. There is no firestop on the bottom window edge, only on the sides and top. And in place of the lower firestop is the cill which is pushed up under the window? How close to the timber frame sheathing do I push back the cill? ie how much of a gap between cill and sheeting or how much of the window frame is actually resting on the cill?

Thanks again.

bfclancy

the fire stops usually come as part of the window when delivered or are else screwed to the window before installing, the window is then fixed in place by fixing these fire stops to the timber frame, then foam around the window from the inside to fill gaps between window and timber frame.
the cill should be pushed right back into the sheeting (dpc between cill and sheeting), look online for detail but there is option to put thin strip of insulation between cill and sheeting.

listermint

johneym wrote: »

thanks a lot guys,

BF I appreciate your help. Do I understand correctly that the window is pushed into the fire stops so that the outer window edge rests flush with the outer edge of the firestops. There is no firestop on the bottom window edge, only on the sides and top. And in place of the lower firestop is the cill which is pushed up under the window? How close to the timber frame sheathing do I push back the cill? ie how much of a gap between cill and sheeting or how much of the window frame is actually resting on the cill?

Thanks again.

If your using a sill underneath then you are introducing cold bridge. Currently you are going through the trouble of specing high quality in terms of heat retention and air tightness why would you put a conventional sill on.

Firestop all the way around and passive sill on the front exterior