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How much do drywall screws impact insulation value of a wall?

nostos156

By nostos156
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nostos156 Member

nostos156

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I tried looking this one up, but results seem to be mixed. So looking for a sanity check.

 

For a quick background rundown: old wall was an exterior facing attic one, which was constructed in the old style: solid outer wall, with thin vertical battens that were attached to the wall by hammering in wooden posts right into the old mortar, then laths, then plaster. Pretty damn cold in the winter, and any wardrobes close to that wall had mould on the backs of them. (Although only at the bottom of the wardrobe and I did note draughts, so there was probably an issue with the old skirting as well). This wall has a chimney behind it (so probably the draught source) but when I tore the wall down it was completely dry. I bricked it up and drilled holes for ventilation to keep it dry, that the cavity will take care of.

 

Have since torn that old wall down and built out a new one with 2x2s, with a 50mm cavity from the stone. 50mm PIR was laid in between the studs (at 400 centres), and then to avoid cold bridging from the wood I put 25mm over the whole lot. Liberal use of foil taping. Finished by screwing down plasterboard directly on top (did not opt for battens to keep overall thickness at a minimum and did not need a service void).

 

I didn't know how many screws were appropriate, answers seemed to vary, so I erred on the side of caution and did them at 150mm around edges, 300mm in the field. This has evidently ended up in a fair bit more screws than I see some people use, especially around joins. It got me (probably over) thinking how much of an effect these screws are having on cold bridging, and if I goofed? 

 

Like I said, probably overthinking it, as I mainly insulated the wall heavily so that I could bang a wardrobe up against it once more and not worry about the appearance of mould any more. Just hoping those excess screws won't have defeated the point of all the insulation, particularly right behind the wardrobe where it sits at those joins. 

Edited by nostos156
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nostos156 Member

nostos156

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5 hours ago, Andehh said:

No difference at all, and be warned.... if you're that intent on details and micro worrying you will put yourself into an early grave!! 

 

Perfectionist by heart I'm afraid, and when you deal with stuff like insulation/vapour barriers and such where they over-emphasise how breaks in it can massively negatively impact things, it's the perfect storm, ain't it?

 

 

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Pocster Advanced Member

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6 hours ago, Andehh said:

No difference at all, and be warned.... if you're that intent on details and micro worrying you will put yourself into an early grave!! 

Yes on a self build it’s easy to fixate on apparently irrelevant things . But ! they keep you awake at night . Let the drywall screw mindset go …. You’ll find something else 🙄😁

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Iceverge Advanced Member

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The screws will have a very small cross sectional area. About 9mm2. 

 

You have about 70 screws per 1.2*2.4m board so about 0.02% or 1mm2 per 4,500mm2. 

 

To take your 50mm thick PIR layer the calcs look like..

 

(99.98% X 0.022W/mK)+(0.02% X 45W/mK) = an average thermal conductivity of 0.031W/mK so it does make a small difference. However in the real work you need to hold the wall together somehow. 

 

Your 50mm layer of PIR between battens at 10% wood will have an average thermal conductivity of about 0.034W/m2K. 

 

So your wall as built will have a U value of about 0.32W/m²K Vs about 0.27W/m²K without the effect of the screws. 

 

Assuming 20m2 of wall heated for 8hrs a day at 100days per year at a delta T of 12deg is 9.6kWh/year. 

 

At 15p/kWh that is £1.44 cost of extra gas per year. 

 

Feel free to loose sleep about that but frankly I think you did the bang on right thing by building securely. 

 

 

 

 

 

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nostos156 Member

nostos156

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7 hours ago, Iceverge said:

The screws will have a very small cross sectional area. About 9mm2. 

 

You have about 70 screws per 1.2*2.4m board so about 0.02% or 1mm2 per 4,500mm2. 

 

To take your 50mm thick PIR layer the calcs look like..

 

(99.98% X 0.022W/mK)+(0.02% X 45W/mK) = an average thermal conductivity of 0.031W/mK so it does make a small difference. However in the real work you need to hold the wall together somehow. 

 

Your 50mm layer of PIR between battens at 10% wood will have an average thermal conductivity of about 0.034W/m2K. 

 

So your wall as built will have a U value of about 0.32W/m²K Vs about 0.27W/m²K without the effect of the screws. 

 

Assuming 20m2 of wall heated for 8hrs a day at 100days per year at a delta T of 12deg is 9.6kWh/year. 

 

At 15p/kWh that is £1.44 cost of extra gas per year. 

 

Feel free to loose sleep about that but frankly I think you did the bang on right thing by building securely. 

 

 

 

 

 

 

Oh, this is great. I would also assume that the real-world impact is probably less than those theoretical values, because the basic calcs don't take into account that the fastener isn't exposed on either end, add on an R-value for air film, etc. 70 screws per sheet roughly sounds about right, yeah.

 

Did the cross-sectional area take into account a standard 3.5mm screw? I used the thicker 4.2x65mm ones for this.

 

Seeing the actual maths definitely makes me feel better—but honestly, so long as I can bang the wardrobe up against the wall without worry of any cold spots from fasteners leading to problems like I had in the past, then the new wall has achieved its purpose.

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nostos156 Member

nostos156

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7 hours ago, ADLIan said:

Correction to the U-value for the fixings adds about 0.006 W/m2K to the U-value in this instance. Bigger things to worry about!!

 

Your U-value correction is much different to Iceverges. How did you get this value?

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Andehh Advanced Member

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18 hours ago, nostos156 said:

 

Perfectionist by heart I'm afraid, and when you deal with stuff like insulation/vapour barriers and such where they over-emphasise how breaks in it can massively negatively impact things, it's the perfect storm, ain't it?

 

 

Yep... Me too. And boy have I lost sleep over the last couple years of our build. It's hard enough when your worrying about the finances, before you 'check up' on the build on the way home from work, mid January, and see the brickies have left small gaps between the insulation slabs along the entire length of wall.... To then end up spending 2 hours adjusting all the insulation for tight fit, in smart office wear freezing to death and late for dinner!!! 

 

My wife didn't see what all the fuss was about. :(

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saveasteading Advanced Member

saveasteading

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20 minutes ago, ADLIan said:

u-value calculation method including correction factor for mech fixings

They had nothing more important to do either. 

I wonder if they considered that it takes longer for the heat to travel along the thread than the core. Then when the room cools down it has to travel back in again.

Seriously,  the code writers had to consider it for thoroughness. We don't. And if you can add more insulation elsewhere to compensate ( 0.1mm of pir?) then do it anyway.

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ADLIan Regular Member

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The correction factor does mean more insulation is needed to offset the effect of the fixings. With wall ties and small screw fixings the effect may be minimal. An extreme case would be aluminium brackets used in rainscreen cladding that may add almost 0.1 W/m2K to the uncorrected U-value. An uncorrected U-value of 0.15 W/m2K becomes a corrected value of 0.25 W/m2K in this instance so additional insulation is needed to compensate and get back to the 0.15.

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saveasteading Advanced Member

saveasteading

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4 minutes ago, ADLIan said:

An uncorrected U-value of

In this extreme,  a wrong value. You would hope anyone responsible for such assessment would know to allow for the structure.

 

Your point is good though. The u value for cladding is often by testing an area of panel between the supports. 

 

If that is what the commissioning manufacturer asks for, it is all that is done by the lab.

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nostos156 Member

nostos156

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3 hours ago, ADLIan said:

EN ISO 6946 give u-value calculation method including correction factor for mech fixings.

 

Does it specify an amount based on standards (so more correction for overuse of fixings) or is it just a flat value?

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39 minutes ago, ADLIan said:

It’s specific depending upon the type and number of fixings penetrating the insulation and the construction of the floor, wall or roof

 

So 0.006 W/m2K for 70*3 (roughly) 4.2x65 fasteners in a wall? Huh, that's much lower than I'd ever expect.

Edited by nostos156
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