Sash Window DG Retrofit

[Fitzroy]

Morning All,

One of the jobs on the job list is having a crack at putting double glazed sealed units into one of our existing sash windows. With a spare hour (turned into four, oops) on the weekend I got stuck in and pulled one apart! From my investigations so far I'm pretty sure it will workout, the sashes are good and deep with spare depth to route out a deeper (rebate?) for the new sealed unit to sit in. Going from a 6mm single pane to a 4/6/4 unit the extra mass wont be too much to balance with a few extra weights.

However, I've a couple of specific questions that I was hoping some of you could advise on.

1. Seal pile thickness: I'm assuming that ideally you select the pile depth depending on the gap you have. Having stripped down my windows the sashes are very tight between the beads. For the internal sash I can adjust the staff bead to give me the correct fitting. However for the outer sash that is between the parting bead and the outer edge of the window I'm concerned that the sash with the pile seal will be too tight. do I need to worry about this, and/or try to adjust the sash/window frame to give a looser fit?

2.Meeting Rail: The meeting rails of the upper and lower sashes are profiled to interlock, see image at end, they seem to fit together well but I doubt they will be as wind tight as the brush pile. I'm loath to plane them off and fit a pile seal as they are original to the window and very nicely done! Any thoughts on how I could improve the draft 'proofness' of them?

3. Pulleys 1: When I was taking the window apart I noticed the pulley was running but not very smoothly. I didn't have time to investigate the pulley before I temporarily refitted the window. I'm about to order new beads/cords etc. and wondering if the old pulleys are serviceable or should I be adding some to the order?

4. Pulleys 2: Only the top sash is counter weighted. With the window apart I could see that there had never been pulleys installed for the bottom sash, nor has the bottom sash any rebate for cord attachment (easy to see as the top sash has amazing cord rebates with a large screw at the centre, the cord end is then spliced into a eye that fits over the screwhead). Does anyone have any info on why only the top sash would be counterbalanced? BTW the window is in the kitchen on the ground floor in a 1875 Scottish house.

5. DGU Fitting: I'm planning to leave a 2mm gap around the sealed unit and install spacers at the base. I've read about different options to fit the unit.
- Glazing tape, timber beads and a drained bottom rebate.
- Fully bedded, un-drained low mod silicone with external timber bead.
It seems to me there is a movement from the first option to the latter, driven by improved silicone technology facilitating what is a much easier install. I will check with the sealed unit supplier that their edge seal is compatible. So I'm planning to go the second option, does that seem sensible or have i got it all a-about-face?

Thanks Fitz.

PS. Not sure if this should have been in general woodworking or another forum, feel free to move if required

 

[mikefab]

Hi Fitzroy,

I don't have vast experience of this but have built the window that I posted a picture of over the weekend and have also made some replacement sashes (yet to be fitted). There has been a lot of brain ache as I figured out the necessary clearances and glazing methods etc so hopefully I can spare you some of that pain!

You need to consider the height as well as the depth of the rebates. Most DGUs have spacer bars which are 12mm, and my understanding is that they need to be tucked away out of the light. Even if that isn't true in terms of protecting the unit it looks a bit naf if you see the spacer protruding beyond the mouldings.

The extra weight will be significant as DGUs are heavy. For the replacement window which I made (also changing from softwood to sapele sashes), the new sashes weighed 38lb each. The old ones were 20lb each. I put new weights in but have kept to old ones for smaller windows.

Onto your specific queries:

I've found that 3mm is about right for the seals. Maybe 3.5..... That is using Sofseal P from Reddiseals but it seemed to be similar using the brush weatherpile stuff. The Sofseal seemed to slide a bit better but is quite a bit dearer. You could consider putting seals on the outside edges of the sash stiles, each side of the cord (or just one side I suppose). That makes them run nicely! You could still put the seals on the PB and SB. And also maybe a compression seal (Aquatic 63 or Sofseal P) on the top and bottom rails.

Meeting Rail. Not too sure here. I used a plant on the top MR which protruded out past the parting bead and left a 3mm gap to the bottom sash and put some Aquamac 21 wiping seal on it which works well. Perhaps you could square up the join and use a compression seal?

Pulleys. It's not an enormous amount of extra work to replace the pulleys while you have to whole thing apart so why not change them if they are shot?. No idea why your bottom sash did not have cords/pulley but surely worth doing?

DGU fitting. I tried the Butyl tape and low mod silicon as per some instructions from Hodgsons online (search for Method B6). I modified this by only using silicon for the bead on the outside as I was tight for space. The tape was a total pain to trim back to the edge of the moulding. I posted here for hints and came up with a blank except a few people saying they just silicone the units in. In future I will go for a 15mm high rebate with the DGU on 3mm packers and the appropriate silicon to bed it in.

Hope this helps!

 

[Phil Pascoe]

One point springs to mind - make sure before you waste any time that you have the physical space for the increased weight. I remember many years ago an acquaintance doing this alteration to his sashes then finding the weight he required wouldn't actually fit in the frames. Of course nowadays you could use springs.

 

[mikefab]

Good point Phil!

Mighton do some 38mm section lead weights which are designed for that situation.

 

[Fitzroy]

Hmm... I was mid way through writing a reply that I'd thought about this and it was no issue, but found my mental model was off the mark, I'd though a small bit of lead would easily counterbalance the 3kg increase in sash weight. Actually, although sashes are going from single 6mm glazing to double 4mm glazing, so gaining only about 3kg per sash or 1.5kg on each sash cord, based on the Mighton website this equates to approx 150mm long additional weights. Hmm that is more than I thought, I guess the window may not open quite as far as it did before, we'll see.

 

[RobinBHM]

Are they big sashes? 6mm glass is quite thick for single glass in a window.

I would suggest low mod silicone. Butyl tape is difficult.

If you are using timber beads, they will need to be fully bedded.

They could be face puttied with dryseal mp which js recommended for slimlite units. It takes quite a bit of practice though.

 

[mikefab]

Robin, could I ask you to clarify whether fully bedded means silicon all the way up to the top of the bead? This is what I have done (cutting away the squeeze out after it had cured). But having looked at some other professionally made ones where this clearly wasn't the case I wondered whether I was making work for myself by putting too much silicon in.

Thanks!

 

[Fitzroy]

Sashes are only 880x650 glass size. The were originally 6 pane sashes but must have been changed to single panes long long ago as I can see the original astragal mortises fixed up, but the new glazing is wobbly so it's not float glass. All the glass is 6mm in the whole house, no idea why, but many of the building practices here in Aberdeen are a little out of kilter with the rest of the U.K.

 

[RobinBHM]

Robin, could I ask you to clarify whether fully bedded means silicon all the way up to the top of the bead? This is what I have done (cutting away the squeeze out after it had cured). But having looked at some other professionally made ones where this clearly wasn't the case I wondered whether I was making work for myself by putting too much silicon in.

Thanks!

Fully bedded beads need a full coat of sealant on both edges. I think it is usually done with a MS polymer sealant or acrylic putty like dry seal mp. The beads are often cut short so the ends are also filled.

Fully bedded glazing relies on a complete seal, so what you have done is right, even jf it was messy!

Drained and vented systems dont require the bead to be fully coated with silicone, just a continuous bead of silicone between glass and timber rebate before the beadd are pinned on.

 

[mikefab]

Thanks Robin. Clue was in the name really!