ICF and window sizes

[Markuz]

We are just getting our plans drawn up for an icf extension and question came up around window sizes. 
 

Planning to have a 1.5mx1.5m window for a window seat and a 3 meter sliding door. The rest I would say normal window sizes, meeting any regs that required. 
 

Are there any restrictions in terms of size when you use icf? Or can you go crazy ?

Anything to look out for when designing and installing windows in to icf?

 

If you have large windows could you post dimensions please. 

 

 

 

Edited May 18 by Markuz

[ToughButterCup]

4 minutes ago, Markuz said:

...

Are there any restrictions in terms of size when you use icf? Or can you go crazy ?

...

 

Restrictions ? Yes

Crazy? Within reason - almost

 

 

The most difficult windows for us were those that are about 300mm wide : the issue was manufacture of the windows, not how to insert them in ICF.

(In our case Durisol).

[Markuz]

4 minutes ago, ToughButterCup said:

 

Restrictions ? Yes

Crazy? Within reason - almost

 

 

The most difficult windows for us were those that are about 300mm wide : the issue was manufacture of the windows, not how to insert them in ICF.

(In our case Durisol).

Thanks very much. 
 

is that a sliding door you have there? Looks great. 

[Russell griffiths]

 

how big do you want to go. 

[Markuz]

19 minutes ago, Russell griffiths said:

 

how big do you want to go. 

thanks very much.

 

In my view 3 meters sliding door is not too much Just have been asked the question why icf and a comment was made on window sizes.

 

 

[Nick Laslett]

@Russell griffiths, @ToughButterCup, great looking houses, gentleman. Well done. I might have seen pictures previously, but in any case, good to see them again. 
 

 

Edited May 18 by Nick Laslett

[ToughButterCup]

On 18/05/2025 at 10:41, Markuz said:

... Is that a sliding door you have there?

 

Yes. It's been open all day for the last month.

 

The other unexpected but massive advantage is that it stops teenage door-slams dead.

[Conor]

On 18/05/2025 at 10:28, Markuz said:

 

Are there any restrictions in terms of size when you use icf? Or can you go crazy ?

Anything to look out for when designing and installing windows in to icf?

 

 

This is a question for your structural engineer. We have several large openings, 2x2m corner window, and 4.5m wide bi-folds. The 2x2m was constructed with the ICF and extra rebar in that area, plus a corner post. The 4.5m opening needed a custom made steel box beam with welded plates and a thicker column to support.

[FM2015]

No restrictions really but at a certain stage and under specific conditions, an engineer might move towards steel lintels rather than cast in place.

[Markuz]

On 21/05/2025 at 07:40, Conor said:

 

This is a question for your structural engineer. We have several large openings, 2x2m corner window, and 4.5m wide bi-folds. The 2x2m was constructed with the ICF and extra rebar in that area, plus a corner post. The 4.5m opening needed a custom made steel box beam with welded plates and a thicker column to support.

Thanks, we will be talking to SE. 

house is not wide enough for 4.5m opening and everything else. 😂

 

Sadly, we can’t have corner window as on the side there is only a fence to look at. Maybe next time in another house. 

[Muellar]

On 18/05/2025 at 10:28, Markuz said:

Are there any restrictions in terms of size when you use icf? Or can you go crazy ?

We have an 8400 opening. The SE has specified a 457x152x52 beam and we are confirming with window suppliers on their abiltiy to have a sliding door with two sliding panels.

[Gus Potter]

3 hours ago, Muellar said:

We have an 8400 opening. The SE has specified a 457x152x52 beam and we are confirming with window suppliers on their abiltiy to have a sliding door with two sliding panels.

This is interesting from a design perspective. I clocked this as it's a good span in ICF.. and then we have a steel beam. 

 

My thought process is roughly as follows:

 

ICF is basically an insulated reinforced concrete wall with insulation. Reinforced concrete, if you have a decent height of concrete over an opening can be really efficient structurally. The wall can't be too thin but say it's "normal" 150mm. We don't know what the loads are from above but the 457 deep  beam is likely designed for deflection (critical design condition)  rather than strength. 

 

So the SE has introduced steel. But I then think as designer.. what about fire protection, insulation detailing, cold bridging, insulation fixing to the steels, differential movement between the materials and impact on the external and internal finishes and buildability. All that is going to add to the cost. 

 

This is where you really need to think things through.

 

Scenario.. often the  SE, (after Grenfell) will pass the fire protection to the Architectural Designer and then they will put some random note on the drawing for the ICF folk to make it work and they will pass it on to you as the Client.. you see the problem? 

 

My gut feeling is to understand why you need steels. If you have a two story building then sometimes we can hang the beam over the door from the ICF wall above, even if there is a modest window above. If only single story then is ICF the right solution?

 

And 8.4 m door is going to be big bucks so head deflections are critical. 

 

To finish. Your design team need to get together and say.. this is a big area of moving glass so let's start to talk to each other and produce a well thought out design. 

 

For the technical minded:

 

On concrete buildings we can use a principle called a Vierendeel girder where we look at the whole wall say on a two story building. This is where ICF has advantages over say a timber frame. If it all looks good SE wise you then need to make sure someone can built it and ideally save money.

 

Again the SE stuff for the steel is probably straight forward but the detailing is where is could all fall to bits. ICF is simple in this respect.

 

 

Edited September 2 by Gus Potter

[JohnMo]

8 minutes ago, Gus Potter said:

why you need steels

That was my thoughts also, one of the key selling points is you don't need steel, other than rebar.

 

3 hours ago, Muellar said:

8400 opening

Other thought was why do you need an 8.4m wide opening.

 

3 hours ago, Muellar said:

two sliding panels

Other was this, for an 8.4m opening?

[Gus Potter]

6 minutes ago, JohnMo said:

That was my thoughts also, one of the key selling points is you don't need steel, other than rebar.

Agree John. 

 

There a loads of folk that use ICF as self builders and sometimes I look at ICF designs and think.. that does not stack up cost wise. It's forgivable as often folk convince themselves that it's right for them, but with my hard nosed hat on I know that's not the way I

design as I can see it more expensive. 

 

But other times ICF is absolutely the right way to go. As a designer I often look at projects and think, how can I get this built at the budget? It takes experience, it's not that hard though. 

 

One of my key principles is to understand the Architectural design intent, identify what is important in terms the Client requirement, what matters to them, and then check to see if that fits with the planning requirement. That's the way I design as an SE and general designer. The big problem is that folk just don't want to pay for this skill.

 

One skill a good SE should have is to be able "knock heads together" , bring discipline to the design process. Architects are not good at this in general. 

16 minutes ago, JohnMo said:

Other thought was why do you need an 8.4m wide opening.

Because that is a Client requirement, what is wrong with realising your dream to have something that is different and a wow factor. Now fair enough, it might turn out that the detailing and buildability is going to be prohibitive cost wise but you have to explore all the design options. 

 

In summary folks if you are thinking ICF also look as how much extra performance you get out of it for a bit of extra rebar. It's almost free money!

 

 

[Markuz]

On 02/09/2025 at 17:18, Muellar said:

We have an 8400 opening. The SE has specified a 457x152x52 beam and we are confirming with window suppliers on their abiltiy to have a sliding door with two sliding panels.

Thats the width of our bungalow

 

Anyway, bit interesting to introduce the steel beam, but i am sure SE knows their stuff.

Do send us a picture once you got it all sorted please.

[Muellar]

On 16/09/2025 at 11:34, Markuz said:

Thats the width of our bungalow

 

Anyway, bit interesting to introduce the steel beam, but i am sure SE knows their stuff.

Do send us a picture once you got it all sorted please.

We went for such a wide opening as the orientation looks out over Lough Neagh and on a clear day (ok, it is Northern Ireland...) you can see the mountains in the distance. Winter months get the sunset on that side too.

 

The SE has dealt with a number of ICF schemes and has done this before so it gave us flexibility on that opening so we decided it was worth it. Plan is to start build mid 2026 so as soon as I get some photos I'll post them.

[Muellar]

On 02/09/2025 at 22:50, JohnMo said:

That was my thoughts also, one of the key selling points is you don't need steel, other than rebar.

 

Other thought was why do you need an 8.4m wide opening.

 

Other was this, for an 8.4m opening?

 

As @Gus Potter sets out it became our requirement. Once you stand on the plot and can see the view you'd realise and i appreicate that's not easy here. We've had good friends point out the issues to us but once they've been up at it a couple of times they appreicate it and why we've gone for what we have.

 

We even found a guy out Lisburn way, Lifesize plans where he projected the plans at full scale onto the floor and at first we said the opening was too large so he added some 'polystyrene' walls and when we sat there or stood in the kitchen we saw how big an impact it had on the overall space and feel.

 

I've added a couple of photos to show the outlook we'd have from what would be the Kitchen/Dining/Living where we'd spend most of the time and is where this mammoth opening is in case that helps with understanding the decision we made.

 

I'll ask the SE why he didn't spec rebar, but I accepted his position as i'm not an SE nor experienced enough. We have used rebar in the rear bedroom to created a cantilevered beam so we can have a clear glass corner window as the rear of the house would have a view out over rolling hills.

[Gus Potter]

9 hours ago, Muellar said:

it became our requirement.

Yes, stunning view so worth paying a bit more to get the large uninterrupted opening. 

 

9 hours ago, Muellar said:

I'll ask the SE why he didn't spec rebar

Do that out of interest. It could be you have a height restriction over the head of the opening which would make the rebar in the ICF too congested for example. One question that can be a bit of an elephant in the room is fire protection and here ICF has advantages at times over steel. 

 

With that view come weather exposure no doubt so it's really important to get the weathering details nutted out. 

 

All the best and keep at it!

[Muellar]

On 02/09/2025 at 20:29, Gus Potter said:

My gut feeling is to understand why you need steels.

I got a response from the SE and we could go with a reinforced concrete beam but the depth would increase to 650. With the ridge height being tight I'd prefer to have the 457 UB and pack at bottom with a timber stud then ICF so the window installers can fix into. Same for the ICF, I'm thinking we could pack the UB with timber and then fix the ICF into that as the UB will basically fill the cavity so I can't see much concrete getting into the gap - but I am a rookie here on the build out so maybe it's what happens?

[Gus Potter]

29 minutes ago, Muellar said:

I got a response from the SE

It's good to get an update and some feedback, thanks for that. 

 

Structural depth difference 650 - 457 = 193mm. Pity it's not working here, can see you have a height restriction. 

 

A few questions, observations:

 

1/ Your beam will need an end rest at each end. I'm just checking, is your 8.4m the clear opening width? You'll want to insulate the reveals thus the structural opening will "grow" a bit. Can you get a beam delivered to your site at the length you need without splicing the beam, in other words is there an access / delivery to site issue. Check with your SE that the UB can be continuous. Any splices (depending on how the are designed) can potentially intrude downwards and that can thwart your glazing detail. Flush splices are expensive to fabricate. 

 

2/ I don't know how much load there is on the beam but if single storey is there a way to reduce the roof load by say transferring part of it to an internal wall? 

 

3/ For steel the devil is in the detail. How close to the sea are you? If less than 2.0 km LABC and NHBC can raise questions, your beam might need to be galvanised and the fixings more durable. Keep an eye out for that. The beauty of ICF is that you can change the grade of concrete and mostly the cover to the rebar if near the sea. If you are near the sea can you check your ICF designer SE has taken this into account? Sometimes folk miss this in the heat of battle, so just check your wall thickness and rebar cover.  

 

 

47 minutes ago, Muellar said:

but I am a rookie here on the build out so maybe it's what happens?

 

Post a couple of sketches if you wish and lots of folk on BH will help you with the detailing. What you are doing is not run of the mill, every day is a scoool day even if you have been doing it for decades, cut yourself some slack!