Renovating 30s bungalow - underfloor insulation?

[-crashd]

As per a previous thread 

We're in the throes of adding a loft conversion and a rear extension to this property. We're also renovating/modernising bits as we go. We're thinking about what to do regarding the existing ground floor flooring situation. Our plan is to lift the cheap click flooring + underlay, pull the floorboards up (some of which will definitely need replacing as there's a certain damp smell in the kitchen/bathroom) and insulate underneath then lay overboard UFH and new flooring on top.

 

Our builder suggested that if we take up floorboards completely and instead lay down floor grade TNG board, underlay, overboard UFH (the Wunda board system) and then new flooring, and it's all done correctly, we'd save money on labour + materials and wouldn't notice the difference. I feel like this is probably an ok solution but seems like the 'easy out' from our builder - what do you all think?

 

We'll also be putting in EWI, new boiler, double glazing etc... for reference.

 

D

[JohnMo]

For clarity is the builder suggesting not to insulate?  Other that what are the differences?

[markc]

What’s under the floor? Seems a shame to go EWI, UFH and no floor insulation. If it’s just a void how about taking out the wooden floor and filling in with plenty of insulation and screed with UFH.

[-crashd]

1 hour ago, JohnMo said:

For clarity is the builder suggesting not to insulate?  Other that what are the differences?

 

That's right - their suggestion is to not insulate underfloor and replace the existing floorboards with a TNG chipboard of some kind.

 

 

[-crashd]

1 hour ago, markc said:

What’s under the floor? Seems a shame to go EWI, UFH and no floor insulation. If it’s just a void how about taking out the wooden floor and filling in with plenty of insulation and screed with UFH.

 

It's a timber board / joists / void construction. I did think about screed but cost + lack of ventilation worries me a bit ...

[Ferdinand]

A couple of things don't quite add up imo.

 

1 - UFH with zero insulation underneath.

 

How much heat do you lose? Short answer: half of it, unless you have something in place to stop, such as no ventilation underfloor void and your EWI goes 600mm underground around the edge, in which case you will get a warmer patch of soil underneath after 1-2 years. 

 

2 - How do you stop your floor joists rotting if you stop the underfloor being ventilated?

 

If you do ventilate it  there is an insulation free heat path direct to the outside, except for 1. But 1 will mean that 2 then applies, and your joists can rot when moisture gets in. You can do it by completely sealing it, and then you have a dry void you can leave empty or fill with insulation (traditionally people use poly beads, but it is less popular now).

 

In short no insulation underfloor is a touch ludicrous, as you have to put a more expensive sticky plaster somewhere else.

 

And 2 questions: Where are your services running? What spec are you renovating in to?

 

Your renovation is not *that* deep - I live in a bungalow which the previous occupant reduced to three walls and a hole, and he put solid slab floors and ufh throughout. Cost him more than he got back as it was sold to us in 2013.

 

(Further post with an idea to follow)

 

F

[Ferdinand]

I used a strategy on a reno 5 years ago, on a wooden floored bungalow of almost identical size and semi-detached layout, where I created an insulated envelope as you are - plus underfloor ins - and ran all the services inside.

 

At that time electricity was about 10% of current cost. The economic balance has now shifted to even more insulation for less reliance on volatile electricity prices. I did not insulate my walls internally or externally as my walls were already cavity-insulated which was suitable to get me a C76-C77 ish EPC. But I did allow for 200mm EWI in the future with extended windows sills etc, but when I got to it the numbers said itw as not necessary.

 

I lifted every 3rd plank of the existing floor, and put 100mm rockwool between joists (staple gun), took out the old services, and replaced non-sound floorboards. Then a membrane. Then 25mm celotex on top in a framed out floor. Then an 18mm osb floor, then a high quality (QuickStep 25yr guarantee) click-floor or underlay + carpet. I left an underfloor void along some internal walls to run my services (*), plumbing to rads on internal walls, and wiring to sockets on internal walls.

 

It was going to be 50mm celotex on the floor, but I couldn't find doors I could trim by 80-90mm. So had to compromise a little or start taking the top out of doorframe; I found Cottage Oak doors that could be trimmed by ~65mm. Really you want 50mm in case 15 or 22m pipes need to cross (which they might), we had to did a little hole in the old floor in one place.

 

The beauty of it is that everything can be accessed without making any holes, or dismantling, anything. As long as you cut your overfloor in the right shapes. Essentially I left a 300mm wide void in the added floor which ran along internal walls as far as the socket locations.

 

That sandwch could be adapted to fit your Wunder system in. I put markers where things went using football pitch line marker paint before putting the floating finish down.

 

(The one thing I put right under the old floor were ducts with pullstrings for telephone and TV wires in the future so the Virgin Orang-Utan would get to pull on a rope rather than apply his 2ft long 18mm drill through my beautiful thermal envelope).

 

My reno, including replacing 1/3 of the roof, cost just under £40k. But I bought the bungalow for under £100k. This is not Twickenham !

 

Two threads: we discussed the floor strategy, services and insulation in Boffin's Corner here when I was thinking about future replacement of gas by ASHP (long term investment). The piccie just showing is the floor sandwich:

There is a detailed reno thread here:

 

ATB

 

Ferdinand

[-crashd]

Thanks @Ferdinand - I'm an absolute newbie to this stuff so it's good to hear that my thinking is along the right lines. I'll pick through the Boffins thread and see if I make hide nor hair, and I'll be having a good chat with the builder next week

[-crashd]

1 hour ago, Ferdinand said:

And 2 questions: Where are your services running? What spec are you renovating in to?

 

Your renovation is not *that* deep - I live in a bungalow which the previous occupant reduced to three walls and a hole, and he put solid slab floors and ufh throughout. Cost him more than he got back as it was sold to us in 2013.

 

 

Currently run down the side of the house and in through the back, then underfloor. We're taking it all out and replacing with new though so it's in our gift.

 

[Ferdinand]

4 minutes ago, -crashd said:

Thanks @Ferdinand - I'm an absolute newbie to this stuff so it's good to hear that my thinking is along the right lines. I'll pick through the Boffins thread and see if I make hide nor hair, and I'll be having a good chat with the builder next week

 

Perhaps the renovation thread first, as that sets the context and will alert you to wider things to think about.

 

Make sure you take the time to reflect. It is always worth spending time before you spend money - replacement time is free.

 

For me this was for a long term rental investment over 20 years plus, so simplicity and maintainability are important.

 

There was also a brief thread on Green Building Forum - the "other place":

http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=14711

[JohnMo]

If you look at some of my recent replies to other threads there are some basic heat calcs that illustrate the heat loss with and without insulation 

 

[SimonD]

2 hours ago, -crashd said:

That's right - their suggestion is to not insulate underfloor and replace the existing floorboards with a TNG chipboard of some kind.

 

4 hours ago, -crashd said:

we'd save money on labour + materials and wouldn't notice the difference

 

On that note, I'd suggest you find yourself another builder. If that's their attitude to underfloor insulation, I dread to think how they'd approach the installations and detailing of EWI and everything else. They seem to be ignorant of fabric first thermal envelope. Building Regs might also want to differ due to the extent of renovation and there then being a requirement to upgrade the thermal performance of the floor.

 

Your other option not mentioned so far is to full fill the underfloor void with eps beads, depending on how deep it is of course. Having just completed a 1920s bungalow floor retrofit, that would definitely be my preferred approach now.

[-crashd]

37 minutes ago, SimonD said:

 

 

On that note, I'd suggest you find yourself another builder. If that's their attitude to underfloor insulation, I dread to think how they'd approach the installations and detailing of EWI and everything else. They seem to be ignorant of fabric first thermal envelope. Building Regs might also want to differ due to the extent of renovation and there then being a requirement to upgrade the thermal performance of the floor.

 

Your other option not mentioned so far is to full fill the underfloor void with eps beads, depending on how deep it is of course. Having just completed a 1920s bungalow floor retrofit, that would definitely be my preferred approach now.

 

Thanks Simon - they seem pretty reasonable other than this suggestion but I take your point

 

With the EPS beads, does that not compromise the airflow in the void?

 

Edited September 10, 2022 by -crashd

[SimonD]

1 minute ago, -crashd said:

With the EPS beads, does that not compromise the airflow int the void?

 

Yes, but with the full fill, you then don't need the ventilation. The advantage with it is that the floor structure becomes a warm floor as the main part of the insulation is outside the structure - a bit like a warm roof. You then also reduce cold bridging. You'll probably need a VCL above the joists, below the subfloor deck . This is an approach that may need explaining to your BC officer who may ask for ventilation anyway, but it's a discussion to be had.

[Ferdinand]

2 hours ago, SimonD said:

 

Your other option not mentioned so far is to full fill the underfloor void with eps beads, depending on how deep it is of course. Having just completed a 1920s bungalow floor retrofit, that would definitely be my preferred approach now.

@OP

 

Yes and that requires the underfloor void to be dry and sealed. In effect you created a shaped plastic bag from your DPC and fill the inside of that.

 

You don't do it if there is a chance of water coming back - high water table, flood risk, standing water in the void etc.

 

When I did my bungalow the BCO was nervous as it was not familiar, so for simplicity I went with insulation below floor level to the bottom of the joists to let any dampness escape, and made sure the cross-ventilation was good below that level. At one place I had to replace the air brick with a periscope version to get it below the new insulation, and 150mm above the extra concrete which had been added to the drive at some stage leaving the air brick at ground level.

 

On EWI, it is *really* important imo to use a specialist long-term in the business (not a general builder or one who can't show you 3 previous EWI projects where you talk to the customer) as there is a lot of detail that can be got wrong, and it is a bugger to redo afterwards. A simple example is if they don't put the 2G in first, as the EWI has to overlap the frames with a suitable detail. And *everything* (drainpipes, satellite dishes, lights ...) mounted on the wall has to be mounted outboard of the EWI or considered carefully - otherwise cold bridges. 

 

A good cost up here (North Notts) in 2018 was around £110-£130 per sqm of wall area.

 

Read this thread for some thinking: Perhaps @Adsibob can tell us how it went and identify some elephant traps.

 

Also on EWI you tend to get recommendations with a low amount of insulation - say 100mm. The material is cheap, so I would go with 200-250mm of EPS as a compromise - some argue woodfibre as it is moisture permeable. Plus you need to be sure where the dew point is at your temperature range, to ensure you get no interstitial moisture condensation (more complex if your cavity wall is insulated), and need to be sure that any moisture that makes it in can make it out again. It is easy for the structure inside the wall to show through to the outside. 

 

Also of course, it's a semi so you have the step and the coal bridge to next door through the entire cross section of your external wall.

F 

 

 

 

[Ferdinand]

And don't forget to consider ventilation alongside your insulation if it is airtight.

 

(You may have done that on the other thread which I have not reread.)

[-crashd]

2 hours ago, Ferdinand said:

On EWI, it is *really* important imo to use a specialist long-term in the business (not a general builder or one who can't show you 3 previous EWI projects where you talk to the customer) as there is a lot of detail that can be got wrong, and it is a bugger to redo afterwards. A simple example is if they don't put the 2G in first, as the EWI has to overlap the frames with a suitable detail. And *everything* (drainpipes, satellite dishes, lights ...) mounted on the wall has to be mounted outboard of the EWI or considered carefully - otherwise cold bridges. 

 

Really useful, we've already engaged and got quotes from a few specialists, our general contractor isn't doing the EWI. We've gone with one who has lots of residential but also does commercial, and specialises in EWI. Seems legitimate (although who can even tell).

 

[Ferdinand]

32 minutes ago, -crashd said:

 

Really useful, we've already engaged and got quotes from a few specialists, our general contractor isn't doing the EWI. We've gone with one who has lots of residential but also does commercial, and specialises in EWI. Seems legitimate (although who can even tell).

 

 

Remember to make the EWI thick enough and to discuss permeability and ventilation.