Insulating window reveal timber frame new build

[benben5555]

Is insulated plasterboard the way to go here? Any suggestions on thickness? Also shall I advise the timber frame company to widen the structural opening to allow for the insulation? I.e. for the timber frame opening to be wider than the brickwork to allow for the extra thickness of insulation?

[Iceverge]

What are your window frames made from?

 

If it's something thermally good like UPVC or a properly thermally broken Wooden frame the gains are very marginal if not negligible. 

 

Increasing the opening size, you'll end up with a slightly better psi value at the window wall join at the expense of a larger area of structural headers, jambs, and sills. 

 

I would concentrate on making sure all headers are thermally broken and all jambs and sills are continuously externally insulated or internally insulated.  

 

 

[benben5555]

The windows are timber (not sure if they are thermally broken). So just return standard plasterboard into the window then? There will be 140mm wool between the studs and 50mm PIR internally.

[Iceverge]

In or out opening windows? 

 

 

[benben5555]

Out

 

[Iceverge]

PIR to the reveals and then plasterboard on top. Much better chance of getting a continuous insulation layer and less waste too. Airtightness trumps any thermal bridging however. 

 

Have you committed to this wall build-up? @Thorfun did similar recently and I think would have changed their mind given the labour involved. 

 

 

I'll have a pop at a therm model tomorrow if I get a chance. 

[Roundtuit]

If it's a brick outer skin, you need to get the cavity closed properly to provide insulation and a fire break.  The timber cheek of the reveal shouldn't be an issue.

[benben5555]

I'm a bit confused. Does the below detail look correct?

[benben5555]

Forgot the cavity sock in the above post and I've set the window back to avoid the cold bridge

[Iceverge]

 

 

Have a look at this entry from @TerryE

 

The detailing of the Cavity Sock stuffed with rockwool is pretty good. 

 

 

[Iceverge]

Here are some mock-ups. 80mm thick simplified window. 30mm in the brick, 50mm in the cavity. 

 

No insulation inside or out. U value 0.6985. Lots of heat loss at the corner. 

 

 

 

No insulation inside or out. 

 

 

Edited September 21, 2022 by Iceverge

[Iceverge]

Additional 25mm PIR to reveals. U value of 2m section this time. 0.6238.  You can see more of the heat is being forced out through the wall and centre of the window. Better. 

 

 

 

[Iceverge]

With a 75*50mm cavity sock added. U value 0.6037 You can see the flux is becoming more dissipated. This is better.  

 

 

 

[Iceverge]

Double cavity barrier.  150mm *50mm . U value 0.5946

 

 

 

 

[benben5555]

That is great information. I've never seen these kind of models before. Really helpful in showing the best approach. What is the software called?

[Iceverge]

Exterior 75*50 cavity sock only. U value 0.6275

 

[Iceverge]

2 minutes ago, benben5555 said:

I've never seen these kind of models before. Really helpful in showing the best approach. What is the software called?

 

THERM. It's a PITA to setup and use but it is free for PC's. it gives a good idea graphically of what actually makes much difference if nothing else. 

 

From above from best to worse. 

 

1. 150mm sock and pir cheek.  0.5946 

2. 75mm sock and pir cheek. 0.6037  = best - 0.0091

3. pir cheek only 0.6285 = best -0.0292

4. 75mm sock only. 0.6275 = best -0.0329

5. Nothing. 0.6985 = best 0.0091 -0.1039  

 

 

All figures in W/mK. 

 

So if you have 100m of window perimeter to consider on a cold day with temp outside of 0 and inside of 20

 

Your difference in heat loss will be 100m*20deg*0.1039W/mK = 208watts of extra heating. 

 

At a guesstimate using 100 heating days per year and an average outside temp of 9 deg and gas heating

 

It'll equate to. 

 

100days*24hrs*9deg*208*15p per unit *80 % boiler efficiency........ = £93.60 extra per year going from no insulation to best insulation.  

 

[ADLIan]

Sorry but that model is wrong - therefore the numbers for the psi-value (not U-value) are probably wrong too. THERM is not particularly intuitive when pulling together the basic model (in fact it's downright difficult), the numbers generated need further manipulation to generate the psi-value and the calculation  must follow set conventions. Depending upon the geometry, cavity closer and position of the window I would expect a psi-value for the jamb to be approx 0.07 - 0.10 W/mK 

[Iceverge]

Hi @ADLIan. I'm only trial and erroring here so don't take any of the above for gospel. What have I got wrong?

 

The big numbers above are only U-values for the total 2m section of wall using a simple window. I used these as it's easy to read off the numbers from THERM. 

 

I didn't try to calculate any psi values but just the different overall U values between each buildup which should be representative of the different heat loss with and without insulation. 

 

 

 

 

 

 

 

 

 

 

[Iceverge]

@ADLIan @benben5555

 

Looking back over those THERM drawings yesterday I had a completely b*llocks value for the thermal conductivity of the window. Almost an order or magnitude worse than reality. 

 

Cover as much of the frame with insulation as possible remains the mantra. 

 

I'll try to post a better drawing shortly.