Sealing window fill timber frame.

[junglejim]

I’m building a timber frame home and trying to achieve airtightness. I’ve taped external windows top and sides and will be taping internally as well as adding low expansion foam. I’m wondering about the bottom of the outside of the window? I’m sure I read not to tape the bottom to allow any condensation/moisture a route out. Is this correct? 
Thanks

[jayc89]

1 hour ago, junglejim said:

I’m building a timber frame home and trying to achieve airtightness. I’ve taped external windows top and sides and will be taping internally as well as adding low expansion foam. I’m wondering about the bottom of the outside of the window? I’m sure I read not to tape the bottom to allow any condensation/moisture a route out. Is this correct? 
Thanks

 

Depends on the frame and sill. Concealed drainage typically needs a matching sill to route any water away from the frame, and you'd seal and foam beneath the sill, so not a problem, whereas face drainage discharges down the face of the frame, so has a clear egress path anyway. 

We went for the latter because we had existing stone sills, and weren't using PVC sills. If we had concealed drainage, there'd have been nowhere for any water buildup to escape after we'd sealed and foamed up around the frame. 

[junglejim]

3 hours ago, jayc89 said:

 

Depends on the frame and sill. Concealed drainage typically needs a matching sill to route any water away from the frame, and you'd seal and foam beneath the sill, so not a problem, whereas face drainage discharges down the face of the frame, so has a clear egress path anyway. 

We went for the latter because we had existing stone sills, and weren't using PVC sills. If we had concealed drainage, there'd have been nowhere for any water buildup to escape after we'd sealed and foamed up around the frame. 

Thanks @jayc89 we’ve got aluminium windows (sheerline prestige) which I think have concealed drainage. I’m not sure about the route for this or whether there’s a reason not to seal underneath regardless… I was wondering whether the bottom should be open in case of any condensation. Probably way over thinking this but want to get it right. 
thanks for the advice.

[junglejim]

On 25/02/2025 at 16:29, jayc89 said:

 

Depends on the frame and sill. Concealed drainage typically needs a matching sill to route any water away from the frame, and you'd seal and foam beneath the sill, so not a problem, whereas face drainage discharges down the face of the frame, so has a clear egress path anyway. 

We went for the latter because we had existing stone sills, and weren't using PVC sills. If we had concealed drainage, there'd have been nowhere for any water buildup to escape after we'd sealed and foamed up around the frame. 

On closer inspection I think the concealed drainage comes out at the top of the cill. So presumably ok to tape underneath the cill to timber frame junction. Feels counterintuitive to leave an air gap on an airtight house but also counterintuitive to block any moisture exit. I’d prefer to tape all around though I think and prevent moisture from entering in the first place. Any further comments welcome.

 Thanks

[jayc89]

2 hours ago, junglejim said:

On closer inspection I think the concealed drainage comes out at the top of the cill. So presumably ok to tape underneath the cill to timber frame junction. Feels counterintuitive to leave an air gap on an airtight house but also counterintuitive to block any moisture exit. I’d prefer to tape all around though I think and prevent moisture from entering in the first place. Any further comments welcome.

 Thanks

 

Yep, concealed drainage + compatible sill and you should be fine to seal right around it. When fitting the sill avoid any adhesive around the drainage channels between it and the frame.