Hello all,

I'm on the hunt for some high-performance bifold windows for our build. I'm particularly interested in options that offer secure ventilation for cross-ventilation, like a tilt-turn section, top-hung section, or any other innovative solution that allows for ventilation without fully opening the windows.

This window is for a dormer in a warm roof. The build is mostly completed but still in progress, featuring 200mm of external insulation (including dormers) and 150mm of internal Rockwool, achieving a a U-value of around 0.096 W/(m²K). I'm aiming for high-performance windows, but I'm open to some compromise for added functionality.

I've been meaning to share the progress of our build here, as I've used a lot of ideas and solutions from the greenbuildingforum (and come up with some of my own, for better or worse), and I will do so once time allows! This forum has been a valuable resource.

Here's what I've looked into so far regarding window manufacturers:

Solarlux Highline: They seem quite pricey and haven’t been provided with any drawings despite a lot of time passing, but they assure me they can configure a tilt-turn on one side of the bifold setup. Contact has unfortunately gone quiet after I questioned their “time limited price” option (didn’t want the added pressure). U-value > 0.8 W/(m²K).

Alumil Smartia MF65: They say they can do a similar setup to Solarlux, but unfortunately, communication with their reseller has been an issue following the first quote, despite being initially very helpful. U-value = 1.1 W/(m²K).

Lacuna: I've just recently been in (indirect) contact with them after feeling like I’ve exhausted the above options. They seem to offer this solution—see this promising video: Lacuna Video >>> https://youtu.be/NX1yDLQYQgo?si=5tUMo1wrid9BDKtl&t=34

U-values seem to range from of 0.6 W/(m²K) to 0.85 W/(m²K), but not sure where suggestion will fall.

Sunflex: Their SF75 Triple Glazed 3+1 can be vented, which is great, but the venting is achieved by "shunting" the windows out slightly. Good, but I would have preferred a tilt-turn section for maximum ventilation when needed. U-value = 0.8 W/(m²K).

If anyone has any more suggestions to explore, that’d be great. Architecturally, the view from the dormer window is very much a "picture window," so a bifold feels like the best way to achieve the best blend of inside/outside experience…but they’re proving difficult!

Thank you!

I've seen bifold systems used as windows but only in GF applications. In a long dormer there is always going to be a issue cleaning at least one outside pane,-- I think.

Posted By: RigmaroleSo warmer days, when fully open, I wanted to achieve a panoramic view & (cross) ventilation, unobstructed by mullions. So purely an architectural/design decision, not for access. I’ve seen similar things in cafes/commercial buildings, not so much domestic.

Have you a photo of the kind of thing you're thinking of? I just don't understand at all what you're thinking of.

A quick update: I think I have found a way forward with this by way of Lacuna (a timber bifold door/window system). Their representative in the UK has been the most helpful, technically, out of all manufacturers contacted.

I am just conducting my own deflection test, at the moment, on our (possibly over-engineered) timber lintels (2x 165mm x 75mm, C24, nail laminated for a max span of 2900mm). They require max static deflection of 3mm and require the lintel to be able to carry 100kg per linear meter (though ours is less… as the windows are much smaller lighter)… so we want to be sure nothing will move significantly.

I am hoping not using Glulam (for dimensional stability), won’t be an issue, but it’s too late to change now.

Will update images when I we get and fit them and of my self tested static deflection rig!

Posted By: djh… very carefully to make sure I understood their operations before I did any business with them.

All good advice, thanks ever so much!