Insulating a 1960's suspended floor - how can I best instruct my builder on best practice?

[Keeko]

Hello all,

 

We are in the process of installing a heat pump and underfloor heating on our ground floor (~55m^2). Heat pump is in, next job is to get the ground floor insulated before the UFH pipework and new skirting and flooring goes down.  We have a helpful builder, and he has spec'd 90mm of Celotex to go in between the floor joists with a 10 mm gap above it for ventilation (there seems to be a total of ~200mm below the floor joists). While we are at it we are also seeking to internally insulate some external walls with Celotex between batons.

I understand there are different approaches to getting the work done and with this being something of a once in a lifetime chance to get this right, I wonder if anyone can provide me with advice that I can respectfully share with our builder?

I've heard about the need to avoid thermal gaps, and terms like "thermal bridging" and continuous insulation.  When I took IR photos last winter, I saw particular cold spots where the floor meets the walls but is there a balance to strike between insulation while maintaining ventilation?

 

So can anyone share tips or advice. Must do things? Things to consider, or avoid? What would a specialist do, that our builder could replicate?

I would be very grateful to hear from you, as I say, once the pipework down, that's it - we won't get another chance to insulate this part our home.

 

Many thanks in advance.

 

David

[LiamJones]

I’d get rid of the suspended floor and start again. With 200mm below the joists I’d assume you’d have atleast 300mm with the timbers out the way. 
 

You’d have to inspect what the sub base is like and work out how much, if any, hardcore you need to put in before building up with insulation and concrete/screed. You can put a dpm down too and cover up the air bricks. 

[JohnMo]

90mm is no where near enough insulation for UFH. Should be at least 150mm more if you can.

[jayc89]

Replacing suspended timber with a new concrete slab is a mammoth task, I've done it. Unless it's rotten I wouldn't do that. This is the guide I typically use when working with suspended timber - https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors

FWIW, I used 100mm PIR on mine, it works fine, but wish I added more whilst I was going to all that hassle anyway, like @JohnMosays. If you're sticking with a wool insulation, you'll need to nearly double it again to reach a similar U value. 

[IGP]

I’ve just done the ecological building system approach above, the floors are amazingly warm compared to before. I just had to be mindful of the attention to detail, some of the labourers did better jobs than others when it came to taping to the wall. If I found any loose tape or gaps, I’d foam (FM330) or Orcon F to get as good a result as I could.

[Keeko]

Thanks for all the replies. 

@LiamJones - that would have been my preference, but I've been told it would be an even more disruptive and expensive job.

 

@JohnMo I'll definitely see if we can install more than 90mm. I think I might have to remove a fair amount of rubble under the floor on the ground surface as there was all sorts in the area that I have seen.  Is there a minimum gap we should leave between the ground and the bottom of the insulation?

@jayc89 That's very a useful guide and something else to consider - thanks @IGP for sharing your experience with this too.

More for me to think about, happy to hear about the experiences of others too. 😊

[HughF]

On 14/09/2023 at 09:30, IGP said:

I’ve just done the ecological building system approach above, the floors are amazingly warm compared to before. I just had to be mindful of the attention to detail, some of the labourers did better jobs than others when it came to taping to the wall. If I found any loose tape or gaps, I’d foam (FM330) or Orcon F to get as good a result as I could.

Do you think it would be possible to do a reasonable job of a floor by copying the EBS system but using standard roofing membrane for the bottom layer, and air tightness membrane for the top layer? I’ve got a small section of floor to do over an alleyway in our bedroom and I’ve got enough breathable and non-breathable roofing membrane left over.

Edited September 21, 2023 by HughF

[IGP]

Absolutely, I actually ran out of Solitex plus for the breather layer for the last room, so bought some standard roofing breather layer from screw fix and has done the job. 

[IGP]

In fact, the pro-clima stuff while bomb-proof, I’d say you don’t need to bother membrane wise for the expense. Just use appropriate vapour checks and breathers in the right place. 
 

However the tapes and primers etc are top drawer and I’d highly recommend still using them. 

[HughF]

Smashing, I’ll go with what I’ve got then.

[MikeGrahamT21]

Minimum clearance underneath the timber is 150mm, that said parts of mine are as small as 110mm in places, but having cleared all of the rubbish out from under the floor, and ensured the gaps in the sleeper walls are free of obstruction it gets good airflow.

 

one tip I learnt along the way, if you aren’t using a membrane to sit the insulation in, friction fitting PIR can be problematic with suspended timber due to movement, and I’ve had it drop through, so make sure you have something to hold it in place, I used metal clips every 300mm both sides staggered 150mm which has done the trick and foam the gaps up as any air movement negates your insulation.

 

I totally get not being able to use enough insulation for underfloor heating, my maximum is 100mm. Couple of things you can consider… if you are doing the whole ground floor, you could consider adding another timber on top of the existing to increase height, either 50 or 100mm glued and screwed to existing, or (and) use an insulating product on top of your subfloor, so you can get boards which have grooves in for wet underfloor heating, or something like a marmox board to reduce the thermal bridge between subfloor and underfloor heating, depending on the type of UFH you are going for.

 

internal wall insulation, as long as your walls are straight, consider full boards of insulation direct to the wall, with battens on top of them to hold it in place, or use insulated plasterboard, which if memory serves me you can use adhesive to bond to walls, but check the installation guide to confirm that

[HughF]

We are insulating and bagging the floors just to get the thermals up. Still running radiators in the house. Extension is on ufh though, 150 spacing on 200 pir.

[TonyT]

Why install a heat pump and then insulate, you have reduced the heat loss of the property so the heat pump maybe oversized?

[HughF]

9 hours ago, TonyT said:

Why install a heat pump and then insulate, you have reduced the heat loss of the property so the heat pump maybe oversized?

It isn’t…. I’ve done the calcs based on the improved spec (post upgrades). There isn’t much in it, a drop from 7.2kW at -4 to 6.1kW iirc. Worse case I’ll just drop the flow temp at the cold end of the WC curve. Rads are sized for 45 deg at the moment.

 

EDIT: sorry, I see your question was aimed at the OP

Edited September 22, 2023 by HughF

[Keeko]

12 hours ago, TonyT said:

Why install a heat pump and then insulate, you have reduced the heat loss of the property so the heat pump maybe oversized?

Hi @TonyT

 

We had a Heat Geek Heat Loss survey done, and this came out as us needing a 3.5kWh Heat Pump only, so we're ok on that front. Thanks!

[Keeko]

Hi everyone,

We spoke to our builder, having shared the info above and the option of the rockwall on "hammocks" between the joists and a membrane over the top. We also discussed that the joists are only 10cm deep, so could potentially cause a cold bridge. His suggestion was sticking with PIR between the joists (to take advantage of the greater thermal properties vs the same depth of rockwall, and applying a layer of 25mm PIR over the top of the whole lot. This could also remove the potential for drafts associated with gaps between PIR and joists (the PIR is bound to not fit snugly throughout).

This will obviously reduce our ceiling height by about 3cms. Apart from that, does this sound like a good option to you?

 

Thanks, I appreciate hearing your views on this.

[Annker]

@Keeko Interested to know how you got on with the floor insulation job?