Insulating Easijoists

[D Walter]

Hi, does anyone have experience of insulating Easijoists?  We are using 254mm Easijoists at 400mm centres and were unable to get them pre-insulated.  It looks like a mission to force soft roll or simi-rigid batt insulation inside the Easi-joists and we were considering sarking and using spray foam insulation as an alternative.  We are aiming for a U value of 0.11 and will in any event be using approx 60mm PIR under the sarking.  Any thoughts would be appreciated.

[Nickfromwales]

Gloves, mask, goggles, and many, many handfuls of rock wool stuffed in there. Watch for the skin grafts off the metal webs. TBH I doubt many folk bother as the air gap is an insulator and the rest of the rock wool inserted in-between tends to push partially into the voids. Easiest would be to insulate it as normal and then look at filling the obvious gaps.

Consider making up the 'difference' by overboarding across the joists with 8x4's of rigid PIR foam insulation, and then plasterboard instead of just plaster boarding. That would get you a bunch of extra brownie points from the U crowd.   

Good luck ! 

[D Walter]

Thanks Nick.  That would be much cheaper than spraying, though spray foam will help with the air-tightness...

[Russell griffiths]

Have you bought them yet?

i changed my easy joists to i joist to make insulation easier. 

[ProDave]

I had to insulate the joists around the perimiter.

 

Another local self builder gave me several bags of all his insulation offcuts that I stuffed in the void.  Miserable job but I was in no hurry so I just did a bit every now and then.

[Nickfromwales]

1 hour ago, Russell griffiths said:

Have you bought them yet?

i changed my easy joists to i joist to make insulation easier. 

And the first fix 500 times harder

 

[Russell griffiths]

5 minutes ago, Nickfromwales said:

And the first fix 500 times harder

 

Not if it’s a roof, no services up there. All on the inside of the airtight layer in a service void. 

[Nickfromwales]

Just now, Russell griffiths said:

Not if it’s a roof, no services up there. All on the inside of the airtight layer in a service void. 

you should have added "roof" in your disclaimer  

[Russell griffiths]

I thought @D Walter was talking about a roof, do you need to go to such lengths for soundproofing of a floor

[Nickfromwales]

17 minutes ago, Russell griffiths said:

I thought @D Walter was talking about a roof, do you need to go to such lengths for soundproofing of a floor

He does say sarking, so probably digging another hole for myself here 

[Russell griffiths]

@Nickfromwales I would stop now, while you still have some credibility left. ???

[D Walter]

It is just the roof and there are will be no services running through the easijoists.  We did try for I Joists because we knew we would have an issue with the insulation but due to supply (based on Isle of Wight) and engineering issues were pretty well forced down the easijoist path. 

[Nickfromwales]

11 hours ago, Russell griffiths said:

@Nickfromwales I would stop now, while you still have some credibility left. ???

It may be too late........

[D Walter]

Having everything set up for Easijoists for our rafters we are now thinking again about the possibility of importing i beams from the mainland and using the Easijoists for the garage.  Our main concern is the potential cold bridging from the Easijoist metal webbing on our ventilated roof.  Does anyone have any experience/insights into the level of cold bridging the webbing could introduce?  It seems that the insulation manufacturers don't have the software to calculate this impact in their U Value/condensation programmes.

[ADLIan]

The amount of thermal bridging will be considerable;

• Series of 'metal fixings' penetrating the insulation layer which can be included in the U-value calclation
• Possibly no insulation in the depth of the joist unless you risk loosing fingers pushing mineral wool in there before insulating  between

I'm surprised the insulation manufacturers cannot model this. Perhaps also an issue for the easijoist manufacturer to look into this in more detail.

[D Walter]

We got more detailed information on the thermal bridging from the EasiJoist webbing from two sources and it looks much better than we feared.  We have decided on breather membrane on top of 254mm EasiJoists, spraying 154mm of foam directly under the membrane and an additional 100mm of soft roll underneath to fill the remainder of the EasiJoist space.  We will have an additonal 60mm of PIR under the EasiJoists.  There will be a ventilated 50mm batten cavity above the Easijoists with standard lay up for Tata above (18mm OSB, breather membrane and Tata).  This gives us a U value of 0.1 with no condensation risk.  It will also give us a really good head start on airtightness for the MVHR system. 

 

We did look at having 25-60mm of PIR (not foil backed) above the EasiJoists and spraying directly under that (with a view to countering the potential cold bridging from the steel webbing) but the calcs showed a risk of condensation.  On speaking to one of the technical teams we were told that because the spray foam bonds with the PIR there should be no increased risk of condensation as there is no surface for condensation to form on but the calculation programmes cannot cope with this level of sublety so to be safe and for guarantee purposes we are going with spraying under the breather membrane instead.

 

 

[ADLIan]

The 60mm of pir under the joist will mitigate the thermal bridging to a degree. The spray foam onto the breather membrane will make it non breathable but probably not a big issue. Can you post the details provided?

[D Walter]

Hi Adllan, I attach one of the calcs.  The other one was not quite as good but had less information.

The details of the EasiJoist values are on pp 21 and 39 of the manual:

https://www.wolfsystem.co.uk/media/114954/easi-joist-tech-guide-v8-6th-edition.pdf

On speaking to the BASF technical team the view was that it was possible to get better overall U values and mitigation with cold bridging with EasiJoists than timber rafters as the Walltite will completely fill the gaps between EasiJoist cords and the holes within the webbing.  The Wolf manual suggests no difference.

[D Walter]

We did check about compromising the breather membrane and we are promised copies of confirmation from various manufacturers that the breather membrane will still work.  The Walltite does allow some degree of vapour permeability though it is closed cell and whilst the breather membrane is primarily acting as a surface to spray against, the breathability, even if reduced by the Walltite, does have some advantage.

[ADLIan]

Thanks for that. Yes the 'breathability' of the breather membrane is swamped by the materials internal/external to it so it becomes irrelevant.

 

Just one warning the U-value does not include any correction for the metal webs (see dUf = 0.000 approx 5 lines from bottom of 1st page). All this metal bridging 2 layers of insulation will impact the overall U-value though offset to a degree by the continuous layer of 60mm PIR. The correction should be included in the calculation and may add 0.03 to 0.05 (?, more?) to the calculated value of 0.09 W/m2K.

 

[D Walter]

Thanks ADLlan.  The attached link is to the Ecotherm calcs which are not as good but still ok.  Their comment in the covering letter was: "This construction achieves a 0.10W/m²K U Value.  There is no risk of condensation shown in the construction so we find this construction acceptable from a condensation risk analysis point of view.".  I did discuss these calcs with the BASF technical team and they stood by their calcs as they said that the complete infilling of the EasiJoists with Walltite (including the holes in the webbing) reduces the EasiJoists' cold bridging.  They also said that the Ecotherm calcs were ok as the max condensation risk shown was in the 50mm ventilation void.

5K15CB4023.PDF5K15CB4023.PDF

 

 

[ADLIan]

No problem with the condensation risk analysis - generally you have decreasing vapour resistance materials from inside to out plus the ventilated airspace so all should be OK. As you have a cold roof there should be a vapour control layer on the warm side of the insulation, this will increase the margin for safety in the assessments.

 

However both sets of U-values do not include any correction for the thermal bridge created by the metal webs passing through the insulation layers. I've had a look at the easi-joist brochure (https://www.wolfsystem.co.uk/products/easi-joist.aspx - see page 39 of technical guide) which shows U-values including the effect of the metal webs and they add about 0.08 W/m²K to the basic U-value (higher than my guess of 0.03-0.05 above). Also the Ecotherm calculation has the chord width as 47mm when it should be 72mm for easi-joist and this increases the bridging from 12% to 18%!

 

Ecotherm calculation also does not correctly address spray foam between the easi-joist metal webs

Edited December 14, 2018 by ADLIan
Upday with additional comment

[D Walter]

Thanks ADLlan.  Very helpful.  I am not sure what the impact will be overall orhow to cure it, other than increasing the depth of insulation or replacing the soft roll with additional depth of Walltite or semi-rigid.  What we wanted to do was put some PIR on top of the joists (without any foil backing) but this is treated as creating a vapour impervous layer under which condensation can form, even though the Walltite is bonded to the PIR.

[Big Neil]

I may have misunderstood the intention here, but i found a product called insumate, link below. will this address your issue at all?

 

https://insumate.co.uk/product/400mm-joist-insulation-tray-1-bundle-of-10/

 

 

[ADLIan]

Nothing wrong with the basic construction The important issue for the U-value is to get it calculated correctly with;

• correct timber bridging for the top & bottom chords of the easi-joist (BASF only have it at 6% and Ecotherm at 12% when it is actually 18%)
• correct thermal bridging for spray insulation within the joist depth (between the metal webs)
• correct thermal bridging for quilt (perhaps not filling the metal web void)
• correction for the metal webs passing through some insulation layers (the 60mm PIR under the room will mitigate this to some extent).

U-value probably nearer 0.15 W/m²K rather than 0.10 W/m²K.