Can PIR go on top of a timber suspended floor, instead of between?

[scott08]

What you haven't explained, is how you are going to access the room from the existing room. Will you have a step up into the extension?

This extension is coming off the kitchen. The existing kitchen has a step down into it (100mm), as the back of the original house is lower, so by raising this entire room I effectively remove the step and have it level throughout.

Presume a lintel of sorts above this? Take everything out back to the underside of that.

Yeah. Taking out all this brick also means messing with the roof (that's on now). Tbh if I went down that route it would be cheaper just to get a new bifold.

There's no problem with insulating over your floorboards then putting another floor on top. It would be just a floating floor. It needs a vapour barrier immediately under your new floorboards.

I'd recommend 22mm chipboard for a floating floor rather than 18mm, so it's stiffer.

But no idea what your Wunda thing is. Check with the manufacturer. Obviously it needs to be on top of the insulation.

More insulation is always better, I don't see what you need to calculate.

Great, thank you! I was curious about u value, as if I went down this route, I wouldn't also put insulation between the joists (save the time/ money/ effort). Would just need to know what PIR thickness get's me to the required 0.18.

 

[catlad]

You can put PIR on the floor to bring your height up.

 

[stevie888]

It may be that the brickie took his level from the existing floor and didn't make a mistake per se. How clear were the drawings?

 

[^woody^]

How on earth could the bricklayers get the DPC or FFL wrong?

It's very common to have people who lay bricks, but who are not actual bricklayers.

 

[noseall]

It's very common to have people who lay bricks, but who are not actual bricklayers.

I get that. But there must be people responsible at critical stages, surely? Unless the client was running things I guess and this is as you say, a zero responsibility site gang...?

 

[scott08]

I get that. But there must be people responsible at critical stages, surely? Unless the client was running things I guess and this is as you say, a zero responsibility site gang...?

It's possible the DPC is lower than I think, i'll be able to confirm in a couple days. If there is an issue with it i'll have to call the builder responsible for the shell, however likely not a lot that can actually be done about it now if the DPC level is wrong.

In any case, I still have the bifold height to deal with. I can step over it to get out, or have a near flush threshold internally by having a floating floor ontop of the joists, instead of insulation between. Is this acceptable BC wise or would there be issues? In my head it's basically the same as a warm roof - would be the same cost in material as between joists, but so much easier labour wise and gets rid of the height issue. Possible better performance too with no bridging.

 

[noseall]

It's possible the DPC is lower than I think, i'll be able to confirm in a couple days. If there is an issue with it i'll have to call the builder responsible for the shell, however likely not a lot that can actually be done about it now if the DPC level is wrong.

In any case, I still have the bifold height to deal with. I can step over it to get out, or have a near flush threshold internally by having a floating floor ontop of the joists, instead of insulation between. Is this acceptable BC wise or would there be issues? In my head it's basically the same as a warm roof - would be the same cost in material as between joists, but so much easier labour wise and gets rid of the height issue. Possible better performance too with no bridging.

Position of door threshold relative to the internal floor is irrelevant to BC. They would be more intrested that you have say 150mm upstand from exterior FFL up to DPC.

 

[noseall]

In any case, I still have the bifold height to deal with.

Not sure what you mean? The threshold height?

The internal finished extension floor level (FFL), the extension DPC, the existing finished floor level and the existing DPC and your doors thresholds should ALL be the same level.

The height of the doors is irrelevant in that respect.

 

[scott08]

Not sure what you mean? The threshold height?

The internal finished extension floor level (FFL), the extension DPC, the existing finished floor level and the existing DPC and your doors thresholds should ALL be the same level.

The height of the doors is irrelevant in that respect.

Yeah the threshold height.

The existing kitchen joists and new extension joists are the same height and as far as I can tell, the DPC is 10mm above where the finished floor height should be. The bifold is then sitting on top of this level, which effectively means the threshold + 10mm or so above FFL - it's a much chunkier frame than i was expecting, so makes this 110mm or so threshold to walk over. The step i previously mentioned was coming from the hallway into the kitchen, which is the existing structure.

Whilst not ideal, is this 10mm DPC discrepancy a massive problem? I thought the bigger issue was the bifold should have been sat lower so the threshold was more flush.

Not 100% sure what you mean about upstand? I was suggesting overboarding the whole floor.

 

[noseall]

Yeah the threshold height.

The existing kitchen joists and new extension joists are the same height and as far as I can tell, the DPC is 10mm above where the finished floor height should be. The bifold is then sitting on top of this level, which effectively means the threshold + 10mm or so above FFL - it's a much chunkier frame than i was expecting, so makes this 110mm or so threshold to walk over. The step i previously mentioned was coming from the hallway into the kitchen, which is the existing structure.

Whilst not ideal, is this 10mm DPC discrepancy a massive problem? I thought the bigger issue was the bifold should have been sat lower so the threshold was more flush.

In that case, the doors should have sat on a course lower. Or if this was too much, removed two courses of bricks and replaced them with a brick-on-edge. This would have sat the door sill about 35mm lower than the DPC. We have done this previously with doors that open out and bifolds.

Not 100% sure what you mean about upstand?

It's an external comment. External paving to be set 150mm below DPC...etc.

 

[scott08]

In that case, the doors should have sat on a course lower. Or if this was too much, removed two courses of bricks and replaced them with a brick-on-edge. This would have sat the door sill about 35mm lower than the DPC. We have done this previously with doors that open out and bifolds.

It's an external comment. External paving to be set 150mm below DPC...etc.

Yeah, this is what should have happened. A brick course below for the bifolds would have been fine, but I can't do much about it now without buying a whole new bifold - if I remove the bifold and sit it lower, i will have a massive gap at the top, which I don't think I can make look right due to the brick soldiers.

For the external paving, there will be a step on the external side and patio will be 150mm lower than DPC. I'm only really concerned about the internal threshold height, as the garden slopes so external steps are necessary regardless. My thoughts were to just have a step over for the threshold internally, or overboard the whole floor with PIR instead of putting it between the joists (it's not insulated yet), but unsure if there are any better options?

 

[aveatry]

From what you are saying it seems a perfect plan to me the use PIR on the internal floor to bring it level with the bifold threshold. And as you say another bonus in no step down from the kitchen. I can see the language description when you are saying door height - you mean threshold to step over and not the to ceiling height.
As for U value I am not sure what is required but if you need 50mm to get your height right then the appropriate amount between the joists would be calculated from that and you can fit 100 150 200? inbetween. But I am not a builder and dont know the requirements for over joists.
Also dont forget to take into account the intended floor covering+its underlay I had engineered oak and checked its height + underlay before the concrete screed was put down

 

[scott08]

From what you are saying it seems a perfect plan to me the use PIR on the internal floor to bring it level with the bifold threshold. And as you say another bonus in no step down from the kitchen. I can see the language description when you are saying door height - you mean threshold to step over and not the to ceiling height.
As for U value I am not sure what is required but if you need 50mm to get your height right then the appropriate amount between the joists would be calculated from that and you can fit 100 150 200? inbetween. But I am not a builder and dont know the requirements for over joists.
Also dont forget to take into account the intended floor covering+its underlay I had engineered oak and checked its height + underlay before the concrete screed was put down

Thanks! Seems like a few votes to say this would be fine.

I would plan to do the following in this order:

1. Joists (no insulation)
2. 22mm t&g chipboard, glued and screwed
3. 100mm PIR (floating)
4. Vapour barrier (or tape the PIR)
5. 18mm t&g chipboard
6. Wunda unferfloor heating mats
7. Finish flooring

Can anyone think of any issues I may have with this buildup? The original joists i'm connecting to are only 75mm so would be a pain to insulate - It seems like the above would be far easier to install, better insulated (no bridging), better joist ventilation, and improve the threshold at the bifold, at the cost of a little ceiling height, but not sure if there is some other issue i'm missing?