Airtightness approach for suspended floor PIR insulation?

[Sparrowhawk]

Because the joists and space below are shallow, I'm limited to 100mm of insulation so going to have to use 100mm PIR between our ground floor joists to get the best U value I can.

 

Previously I'd planned to use the Ecological Building Systems approach of loose insulation and 2 membranes:

 

I've come up with 3 ideas for PIR insulation, from least to most work:

1. Install boards using Gapotape (or home made equivalent as that stuff is seriously expensive. I'm experimenting with expansion joint foam and aluminium tape) and tape joints between boards with Tescon Vana. Don't tape between boards and joists.
2. As above, but then put an airtight non-breathable membrane VCL over it all. Any poor fitting boards can still let cold air come up to floor level
3. Put the airtight membrane VCL below the PIR boards and over each joist. No cold air can make it to floor level

 

 

Which approach has the least drawbacks?

Edited May 6 by Sparrowhawk
Correct name used for the membrane

[Redbeard]

Can I suggest a point 4? VCL over the lot.

[jayc89]

I'd still have a wind tight and airtight membranes at either side of the insulation. Doesn't need to be the expensive ones recommended by Ecological if you're going with PIR, but the benefits of both are still applicable. (So 2 and 3)

[Sparrowhawk]

4 hours ago, Redbeard said:

Can I suggest a point 4? VCL over the lot.

 

That would be idea 2 I think, with VCL being the proper term which I didn't know for "airtight non-breathable membrane" (something that keeps water vapour in the house and stops it from getting into the void)? I've edited my original post to clear this up.

 

22 minutes ago, jayc89 said:

I'd still have a wind tight and airtight membranes at either side of the insulation. (So 2 and 3)

 

Interesting, is that to be belt and braces? So 2 stops water vapour and air moving down and around the PIR, while 3 stops air and water vapour coming up from the void?

 

24 minutes ago, jayc89 said:

Doesn't need to be the expensive ones recommended by Ecological if you're going with PIR

 

Any brand alternatives you can suggest? I'm unclear if a cheaper VCL is just thick polythene in this case or it's something more fancy and reinforced.

[Iceverge]

Can you not just allow the lower membrane to drape below the level of the floor boards to fit more mineral wool? 

 

You could add rips of ply or OSB to extend it as low as you like. 

 

I'll do a sketch

[Iceverge]

 

You get pretty good thermal bridging mitigation too. 

 

 

[Iceverge]

5 hours ago, Sparrowhawk said:

Because the joists and space below are shallow,

 

Ah just saw this....

 

 

Will reread the problem shortly. 

[Sparrowhawk]

12 minutes ago, Iceverge said:

Can you not just allow the lower membrane to drape below the level of the floor boards to fit more mineral wool?

 

I'd love to, but unfortunately the joists are 100mm deep and then the void below is 100-130mm, less than building regs now require.

 

I'm loathe to dig it deeper because the foundations are super shallow, but more because we're living here and I've got too many other things to do I hate digging

Edited May 6 by Sparrowhawk

[jayc89]

33 minutes ago, Sparrowhawk said:

 

That would be idea 2 I think, with VCL being the proper term which I didn't know for "airtight non-breathable membrane" (something that keeps water vapour in the house and stops it from getting into the void)? I've edited my original post to clear this up.

 

 

Interesting, is that to be belt and braces? So 2 stops water vapour and air moving down and around the PIR, while 3 stops air and water vapour coming up from the void?

 

 

Any brand alternatives you can suggest? I'm unclear if a cheaper VCL is just thick polythene in this case or it's something more fancy and reinforced.

 

Presumably the void is/should be ventilated so the wind tight membrane will prevent cold air flowing through the insulation (as, again I assume, you'll only be taping the top of it). Airtight membrane on top to prevent warm, internal air going the other way.

 

No reason some cheaper polythene, or suitable gauge wouldn't do the trick. I'd probably go for something like a DPM for the bottom and a thiner sheet above, 

[Iceverge]

Don't get hung up on U values. There's really very little in it. The actual performance will be much more affected by the airtightness and wind tightness. 

 

Have a look at these examples. 

 

 

 

 

As the timber is such a large factor the type of insulation makes much less of a difference. If you were able to break the thermal bridge of the joists with a continuous layer you would fare much better but as you're limited with space no such luck. 

 

 

 

 

 

 

 

 

[Super_Paulie]

rightly or wrongly i did this.
Air tight Illbruck foam all round and aluminum tape over the edges. In places in the existing house where i was also limited to 100mm i did the same but with 2 lots of 50mm. If i could go back and pull it all out and replace with deeper joists/PIR then i would, but hindsight is a wonderful thing.

[Sparrowhawk]

On 07/05/2024 at 14:42, Iceverge said:

Don't get hung up on U values. There's really very little in it. The actual performance will be much more affected by the airtightness and wind tightness. 

 

I agree. Whether I notify BC or not I'm trying to get it to meet the required U-value of 0.25W/(m2K) and got myself in a right tangle with Ubakus

 

I will try and recalculate with other insulation materials, but I haven't understood properly the perimeter/area ratio stuff used in England for floors. Like perimeter of whole suspended floor, or just the current room?

[Nickfromwales]

1 hour ago, Sparrowhawk said:

I will try and recalculate with other insulation materials, but I haven't understood properly the perimeter/area ratio stuff used in England for floors. Like perimeter of whole suspended floor, or just the current room?

 

On 07/05/2024 at 14:42, Iceverge said:

Don't get hung up on U values. There's really very little in it. The actual performance will be much more affected by the airtightness and wind tightness.

The man has spoken, and his words are wise ones. "Heed" y'all!!

 

Airtightness is the money shot here for sure, but if possible also put some 10mm (or thicker) Marmox / similar insulation board between the flooring and the joists to kill the repeat thermal bridging, if you can lose that slight gain in floor height?

[Wil]

I thought the idea was to keep the bottom of the joists exposed to the external air so that the cross flow could/would remove any chance of moisture and rot? If you encase the joists in PIR but the ends are still in the cold wall isn't this exactly the worst case to be avoided?

 

I have slightly deeper floor voids. probably 500mm under the joists but can't see why I can't just 'bung' the vents and then fill the void with micafill or similar non-wicking insulation up to full depth of the joists? I've seen it posted on other sites but it seems to be held as a no-no and I can't understand why compared to putting insulation under and on top of the joists when you still can't do anything about the cold ends in the walls?

[saveasteading]

9 hours ago, Nickfromwales said:

10mm (or thicker) Marmox

Whatever the theory says, the conductivity improvement is obvious when standing on it shoeless.

Maybe it's only very local and short term  comfort, but I would do it. It's also easy to work with.

[Nickfromwales]

3 minutes ago, saveasteading said:

Whatever the theory says, the conductivity improvement is obvious when standing on it shoeless.

Maybe it's only very local and short term  comfort, but I would do it. It's also easy to work with.

Yup, and if the OP bites the bullet and covers the whole area with it, not just the joist tops, it'll form the upper vapour barrier if glued and taped etc etc. Even a 6mm board will give excellent results, but for the minimal cost of going for the additional 4mm (or more if they can afford / fit) there will then be an uplift on that.

[Sparrowhawk]

10 hours ago, Nickfromwales said:

The man has spoken, and his words are wise ones. "Heed" y'all!!

 

My wife agrees

 

I was explaining last night how I was trying to prove it'd meet building regs for BC and got that look.

 

"You're not planning to put in an application to insulate the floor, are you?", followed by "stop being a stickler for the rules and get it done".

 

Well that's me told.

[Sparrowhawk]

10 hours ago, Nickfromwales said:

Airtightness is the money shot here for sure, but if possible also put some 10mm (or thicker) Marmox / similar insulation board between the flooring and the joists to kill the repeat thermal bridging, if you can lose that slight gain in floor height?

 

I like this idea.

 

Is Marmox strong enough to replace chipboard in the floor build up, or needs that on top of it?

 

Longer term the floor level will be raised in this room by ~20mm; it's a half/half mix of old suspended wood floor and (we think) beam and block. Which is probably uninsulated. I thought lifting the floorboards would show me a cross section of the beam and block but they whacked 2 joists across the join so going to have to try harder to determine its make-up and if there's any insulation in it.

[saveasteading]

18 minutes ago, Sparrowhawk said:

this idea.

 

Is Marmox strong enough to replace chipboard

It's hard. Ish. I have some with carpet tiles directly on top and it's fine.

Spanning across joists though I doubt is ok.

[Nickfromwales]

7 hours ago, Sparrowhawk said:

 

I like this idea.

 

Is Marmox strong enough to replace chipboard in the floor build up, or needs that on top of it?

 

Longer term the floor level will be raised in this room by ~20mm; it's a half/half mix of old suspended wood floor and (we think) beam and block. Which is probably uninsulated. I thought lifting the floorboards would show me a cross section of the beam and block but they whacked 2 joists across the join so going to have to try harder to determine its make-up and if there's any insulation in it.

The floor material (say 22mm P5) gets laid over the Marmox, and you screw through the 2 and into the joists. Use x thickness of Marmox to get your levels correct, and you're off to the races.

[JoeBano]

21 hours ago, Nickfromwales said:

 

The man has spoken, and his words are wise ones. "Heed" y'all!!

 

Airtightness is the money shot here for sure, but if possible also put some 10mm (or thicker) Marmox / similar insulation board between the flooring and the joists to kill the repeat thermal bridging, if you can lose that slight gain in floor height?

Hit the nail on the head here. Did my lounge 2 years ago 100mm pir boards in between, Ufh pipes in a 40mm biscuit mix with cement board on top. 
when’s it’s freezing out side you can see the joists on the thermal camera 

feel a draft on the skirting board. Needed a vapour barrier sealing the floor to the walls really 

[Sparrowhawk]

Should the membrane between the joists and under the fibreglass be vapour permeable, in case the insulation ever gets damp (dripping pipe or whatever)? If so, what's the cheapest one I can get?

 

The layers I'm thinking are:

VCL (polythene sheet, stapled down) on top to stop any moisture getting into the space, and as a second airtightness layer.

Fibreglass between joists

Vapour permeable airtight layer (TBC, draped over joists and fixed in place) to support the fibreglass, prevent wind washing and add airtightness

Void

[Sparrowhawk]

OK with no feedback on a breather membrane at the bottom I'll take it that's a silly idea and use polythene sheet above and below.

 

Where this laps up the wall and is taped, you can't plaster onto polythene right? So it needs to be a small lap and plaster onto the Tescon Vana tape - or the wall replastered first and then this has to be done after?

[Iceverge]

On 11/05/2024 at 11:42, Sparrowhawk said:

OK with no feedback on a breather membrane at the bottom I'll take it that's a silly idea and use polythene sheet above and below.

 

Where this laps up the wall and is taped, you can't plaster onto polythene right? So it needs to be a small lap and plaster onto the Tescon Vana tape - or the wall replastered first and then this has to be done after?

 

I'd be very cautious of 2 layers of polythene as it has very low vapour permanbility. You will risk trapping moisture. You will be at risk of degrading the timbers. 

 

Fit the PIR as carefully as you can. Use a foam that won't shrink over time. Some FM330 would work but it's not the cheapest. This will suffice but let the joists open to the air below. 

 

Then put an airtightness membrane over the top. Seal it correctly and it'll really limit the amount of internal airborne vapour getting to the joists. 

 

You can use the tape to secure it to the wall or else lap it up the walls and nail some expanded metal mesh over the top and render for an airtight finish. 

 

 

[Sparrowhawk]

3 hours ago, Iceverge said:

I'd be very cautious of 2 layers of polythene as it has very low vapour permanbility. You will risk trapping moisture. You will be at risk of degrading the timbers. 

The buildup I had in mind was 2&4 from the first post :

VCL (polythene sheet, stapled down) on top to stop any moisture getting into the space, and as a second airtightness layer.

Glass mineral wool between joists

Vapour permeable airtight layer (or VCL TBC) draped over joists and fixed in place to support the fibreglass, prevent wind washing and add airtightness

Void

 

So the timbers should be fine, it's whether to make the glass mineral wool able to breathe.

 

While your earlier calculations used 0.32 glass mineral wool the stuff easily available here is 0.44 Knauf loft roll. It means the calculated U-value of the floor is 0.29, but is cheap.

 

3 hours ago, Iceverge said:

You can use the tape to secure it to the wall or else lap it up the walls and nail some expanded metal mesh over the top and render for an airtight finish. 

 

Good to know there's a way to plaster over it, thanks.