PIR Insulation - Differences & Overlapping

Dr_Colossus

I've a tiled bay window and while doing internal renovations have the plasterboard off so looking to insulate. Existing composition from the outside was tiles, cross battons, felt, plywood, 80mm fiberglass loosely fitted, plasterboard. As such fairly similar to the below timber frame construction and thus considering the use of PIR insulation boards.



1st Question: Is there much difference between the different types of PIR boards? For instance the above image from the Quinn product installation guide advises Quinn Therm QW boards between studs and then either Quinn Therm QW or QL-Kraft boards for over studs.

I usually get most of my material from the likes of Goodwins and other such builder providers so not sure where or if possible to purchase a single sheet of above. Goodwins do list Quinn Therm Isoshield on their website which is used for full fill cavities:
https://www.goodwins.ie/products/Cavity-Insulation-125mm-Full-Fill-1.6M2-Pack-Quinn-IsoShield.html?filter_set[]=1413,1425
but nothing in relation to Therm QW or QL boards. The Quinn product installation guide lists Quinn Term QW boards for use on partial fill cavities so I suspect this would be somewhat similar?:
https://www.goodwins.ie/products/Cavity-Wall-Insulation-60mm-3.78m2.html?filter_set[]=1413,1425

Would there be much different between those two types of PIR boards and then again with the likes of the sheets used in underfloor insulation which the Quinn installation guide refers to as Quinn Therm QF boards?
https://www.goodwins.ie/products/xtratherm-polyiso-rigid-foam-insulation-70mm-2-4m-x-1-2m.html?filter_set[]=1413,1426

Ideally I'd prefer to purchase the 2.4x1.2m boards and cut to size rather than the cavity wall packs which are comprised of smaller boards.


2nd Question: The studs between plywood and plasterboard are 120mm but the underfloor PIR boards available in Goodwins only seem to come in thicknesses of 25, 50, 70, 80, 100 & 150mm. Would there be any complications with doubling up the 50 & 70mm boards to get the necessary 120mm thickness and would you need to remove the foil backing from any overlapping sections?
Similarly could instead double up the 60mm cavity wall insulation boards but again any problems with that?


3rd Question: Rather than PIR boards would I be better off sticking and filling between the studs with Rockwool or any other advise from those more informed on insulation matters and materials?
https://www.goodwins.ie/products/rockwool-timber-frame-slab-140mm-1-2x570mm-2-74m2-1252111.html?filter_set[]=1413,2653&page=2

MicktheMan

Where is the air tight barrier?

In the absence of an air tight barrier (on the warm side of the insulation), it makes much less of a difference what insulation product or format you use because a significant proportion of the heat will be lost through air leakage anyway.

Calahonda52

Please post a few pictures of what you have now as its tough to see how you will reduce the thermal bridging.
the foil wont come off even if you wanted to take it off.
Is the felt the black felt from days of yore

Dr_Colossus

MicktheMan wrote: »

Where is the air tight barrier?

In the absence of an air tight barrier (on the warm side of the insulation), it makes much less of a difference what insulation product or format you use because a significant proportion of the heat will be lost through air leakage anyway.

Air tight barrier or some semblance of one will be foil backed plasterboard and plaster covering insulation/studs. House was built in early 2000s so it's far from a hermetically sealed sarcophagus but I am tackling the blatant leaks as time progresses.

Calahonda52 wrote: »

Please post a few pictures of what you have now as its tough to see how you will reduce the thermal bridging.
the foil wont come off even if you wanted to take it off.
Is the felt the black felt from days of yore

I realise thermal bridging will be near impossible to prevent in this set up but dry timber is far from a good conductor so is it that much of a concern given with what I'm faced with below:


This is what professional builders created:
Outer section beyond the noggins is PVC sheeting which had fiberglass overlay.
The cavity between outer brick and inner block is void but with a window on the ground floor underneath they'd dumped some 300mm of rubble over the lintel for reasons unknown.
With no block covering/closing cavity on ground floor (unlike attic) and wall vents left open (no piping between outer and inner leafs) air was freely circulating between joists pictured and under floor.
Inner leaf walls are dry lined with metal studs and 80mm fiberglass.

Confirmed, the felt covering the bay window section is the old bitumen type of yore, same as what's in my attic.

WildCardDoW

Dr_Colossus wrote: »

Air tight barrier or some semblance of one will be foil backed plasterboard and plaster covering insulation/studs. House was built in early 2000s so it's far from a hermetically sealed sarcophagus but I am tackling the blatant leaks as time progresses.



I realise thermal bridging will be near impossible to prevent in this set up but dry timber is far from a good conductor so is it that much of a concern given with what I'm faced with below:


This is what professional builders created:
Outer section beyond the noggins is PVC sheeting which had fiberglass overlay.
The cavity between outer brick and inner block is void but with a window on the ground floor underneath they'd dumped some 300mm of rubble over the lintel for reasons unknown.
With no block covering/closing cavity on ground floor (unlike attic) and wall vents left open (no piping between outer and inner leafs) air was freely circulating between joists pictured and under floor.
Inner leaf walls are dry lined with metal studs and 80mm fiberglass.

Confirmed, the felt covering the bay window section is the old bitumen type of yore, same as what's in my attic.

Unrelated but you look into extending the pipe work to come up out of the wall instead of floor. While you can type of thing.

Dr_Colossus

WildCardDoW wrote: »

Unrelated but you look into extending the pipe work to come up out of the wall instead of floor. While you can type of thing.

Yes, will be doing that as well as putting another radiator on the far side of the bedroom room. Radiator is somewhat inefficient in that bay window as it's like a little annex.
All the downstairs radiators have the pipe work from the walls but upstairs are from the floor so will change that while on the case.

Calahonda52

whats the outside?
what are the long screws through the timber sheathing holding?

concern is that too much insulation will bring the dew point in to the timber

Dr_Colossus

Calahonda52 wrote: »

whats the outside?
what are the long screws through the timber sheathing holding?

concern is that too much insulation will bring the dew point in to the timber

Outside are the vertical tiles and the long nails penetrating the plywood are from vertical battens which I suspect should have aligned with the studs but they failed miserably. Not sure yet what to do with these nails, would leave as it with rockwool but would prefer not to have multiple nails penetrating the foil of PIR insulation. Bending would result in their extraction impossible for any external repairs so may cut them or leave in situ.

As it happens came across this video (there's a few in the series) since of a chap that had similar ideas with regards insulating his bay window so will probably do similar:

air

If you have an oscillating multi tool you could use it to cut the nails off flush with the inside surface of the ply easily enough.

10-10-20

The way I see it is that you look at it as a flat roof, but on the vertical, so the challenges are the same.
You can't treat this like a wooden shed as the internal living space air is way too moisture-laden to allow for any cold surfaces within the structure.

You have three insulation options: one realistic, one not recommended and one technically difficult.
- Warm, where the insulation is pushed out past the plywood to augment or replace the PVC/fibreglass external panels.
- Cold, where the insulation is pushed into the 'studs', requires ventilation to eliminate the moisture and prevent sweating of the plywood. So unless you can design it in by perforating or gapping the plywood to the outside, you don't have the option of ventilation.
- You can't go hybrid (by fully filling the studs), or you'll end up with rot.

This guy has a good video on it.

If it was me, I'd be weighing up a 'cold wall' approach (insulation between studs and also and insulated plasterboard on top) but I'd be looking at completely sealing the woodwork and insulation using plastic sheeting and then PIR & plasterboard on top with a vapour-check in it (go easy on the nails/screws through the vapor-barrier). The way I figure it, the most moisture-laden air is at the top of the ceiling rather than at waist/foot height, so the risk of condensation should be less lower down (delta-T is less) - therefore the risk should be lower than doing this on a flat roof, if you get my thought process.

Lumen

The thing that bothers me about this wall buildup is the plywood and (possibly non breathable?) "felt". It breaks the rule of increasing breathability from inside to outside.

If any moisture gets into those studs it cannot escape to the outside, and filling the studs with PIR just worsens the cold bridging.

So I'd be inclined to use breathable insulation and an intelligent airtightness membrane behind the plasterboard.

I'm not totally certain of this advice so maybe consult an engineer.

Dr_Colossus

10-10-20 wrote: »

The way I see it is that you look at it as a flat roof, but on the vertical, so the challenges are the same.
You can't treat this like a wooden shed as the internal living space air is way too moisture-laden to allow for any cold surfaces within the structure.

You have three insulation options: one realistic, one not recommended and one technically difficult.
- Warm, where the insulation is pushed out past the plywood to augment or replace the PVC/fibreglass external panels.
- Cold, where the insulation is pushed into the 'studs', requires ventilation to eliminate the moisture and prevent sweating of the plywood. So unless you can design it in by perforating or gapping the plywood to the outside, you don't have the option of ventilation.
- You can't go hybrid (by fully filling the studs), or you'll end up with rot.

If it was me, I'd be weighing up a 'cold wall' approach (insulation between studs and also and insulated plasterboard on top) but I'd be looking at completely sealing the woodwork and insulation using plastic sheeting and then PIR & plasterboard on top with a vapour-check in it (go easy on the nails/screws through the vapor-barrier). The way I figure it, the most moisture-laden air is at the top of the ceiling rather than at waist/foot height, so the risk of condensation should be less lower down (delta-T is less) - therefore the risk should be lower than doing this on a flat roof, if you get my thought process.

Thanks for video, have watched a few more of his since and he explains flat roofs and their different compositions well.

I can't really go warm roof approach without rebuilding the whole front of the house and I can't see how I can properly ventilate behind the plywood similar to a cold roof construction again without rebuilding with the window sill and window board being a limiting factor.

Thanks for your suggestion, as I understand your cold wall approach description is similar to that of the initial cavity walls: timber frame diagram or what am I missing. As I understand it this approach would be quite common and especially in cladded buildings provided the membrane coating the existing plywood is breathable allowing any moisture escape.

Lumen wrote: »

The thing that bothers me about this wall buildup is the plywood and (possibly non breathable?) "felt". It breaks the rule of increasing breathability from inside to outside.

If any moisture gets into those studs it cannot escape to the outside, and filling the studs with PIR just worsens the cold bridging.

So I'd be inclined to use breathable insulation and an intelligent airtightness membrane behind the plasterboard.

I'm not totally certain of this advice so maybe consult an engineer.

I understand the non breathable felt is an issue and I fear this is what I'm dealing with as that's what's in the attic and professional builders will take any available shortcut with what's at hand. I may try to remove some of the wall tiles at the weekend to better access the felt/membrane and outer construction.

However why is the plywood a concern? All timber frame construction and garden rooms are seemingly built with OSB and the studs then filled with insulation. Agreed overboarding the studs with further PIR or insulated plasterboard will lessen the cold bridging but it's not always done especially in garden rooms/sheds.

I'm off the opinion of doing something right and doing it once but frustrating why this has become such a problem for a common design bay window.

Lumen

Plywood is structurally good where there may be contact with liquid water, but has lower vapour permeability then OSB, particularly than vapour open OSB products like Elka Strong board (which I've used myself).

Consequently, if you're aiming for a breathable (vapour open) build up, prefer OSB but ensure that it's suitably protected from liquid water by breathable membrane.

That's not to say breathable is better, you can make a perfectly fine house using non breathable products, but the details are different, and you have to be particularly vigilant to avoid details which risk interstitial condensation.