Insulation under supended floor joists

[Bruce]

We are about to insulate our suspended floors and we have a small problem, we have more joists than air to insulate as the joists are doubled and they only have about 300mm between them. This means that we will have loads of thermal bridging which I intend to mitigate by insulating under the floors with 70mm insulation. As there is only 240mm from the underside of the joists to the soil level I need to fix the underfloor insulation from above. The technique I intend to use is to use it so create "hangers" of packing band that I stick through the insulation and staple them to the joists and then to the hammock with breather membrane above the for the layer of insulation between the joists so in total I will get 170mm insulation under the suspended floor.  I have not seen this done before, am I missing something? The insulation will be of wool type so it will let moisture through if needed. No moisture will come from above as on top of the joists I will put a DPM.

[Super_Paulie]

Pretty sure the bottom of the joists need to be exposed to air flow, that's probably why you can't find any examples of it online. 

I was in the same boat in my original part of the house, could only get 100mm of pir into it. Not a lot I could do about it without major surgery.

[Roger440]

Are the joists located into the brickwork, or on joist hangers? Im assuming cavity wall?

 

If the former, then you do definetely run the risk of moisture forming at the interface with the cold brickwork which will struggle to evaprate, wool or otherwise

 

If on joist hangers, you may still have the problem.

 

Obviously you could pay someone to do a proper analysis, but my take is that it is a high risk solution. 

 

Its examples like this that just demonstrate the practical difficulties with insulating older housing stock. That you are asking puts you way ahead of the other 99% doing this stuff.

 

[MikeGrahamT21]

Your current method suggests you are essentially installing a VCL on both sides of the joists, a very bad idea.

 

Is there a reason those joists are doubled up? Rooms looks relatively small in size for all that timber. It may be more feasible to remove the doubling up bits, unless there is a large span which isn't visible on the photo. I can see a sleeper wall, so in that area, definitely no reason for doubling up, unless you've some serious load to bear on the floor?

[Super_Paulie]

id whack 100mm of PIR flush to the flooring and be done with it. If you can sacrifice any height from the room then you could overboard it with PIR as well but i probably wouldnt bother. 100mm should be a decent improvement if you can get it all air tight.

[jayc89]

Best example I have found - https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors

[Roger440]

32 minutes ago, jayc89 said:

Best example I have found - https://www.ecologicalbuildingsystems.com/post/best-practice-approach-insulating-suspended-timber-floors

 

Good link.

 

I love this bit at the end:

 

Alternatively, you could make this a DIY project. Often the best installations are completed by homeowners who have taken time to install materials correctly. The most important skill needed is care and attention to the intricate details. You must also follow the appropriate health and safety measures at all times. We would be pleased to support you with further installation information if DIY is an option for you. 

 

If ever there was a vieled reference to crap workmanship by builders, this is it. Still at least they know what actually goes on in the real world.

 

[joe90]

17 hours ago, Bruce said:

This means that we will have loads of thermal bridging

But will you? Timber has an insulating value in its self (someone will quote it shortly), yes not as good as “insulation” but not as bad as “fresh air”.

[IGP]

I’ve completed the ecological building systems method on my ground floor. 
 

Results are amazing, can walk on it bare foot when it’s -5 outside and be very comfortable. 

[ectoplasmosis]

10 hours ago, IGP said:

I’ve completed the ecological building systems method on my ground floor. 
 

Results are amazing, can walk on it bare foot when it’s -5 outside and be very comfortable. 

 

Looks great! Would you mind sharing the total cost of materials?

[Neverfinished]

Hi, IGP. I came across your recent post & photo just now.

I've just joined Buildhub with a question 'Laying hardwood floor over suspended floor joists; would any underlayment help?'

Your posts & responders have helped answer my question to an extent, so thanks. 

I've paid £1700 for new floor planks to cover a room similar size as in yr photo & don't want them to rot, want optimum insulation too. Found the article in the thread - 'Best Practice Approach To Insulating Suspended Timber Floors' really helpful & plan to follow it. Sounds expensive though!

I attach a photo showing what I'm doing. Will wrap it with the appropriate membrane(s) but leave the insulation @ current 80mm as concerned to keep the joists breathing in part at least. Got the fireplace & hearth to upgrade too.

Cheers, Steve/Neverfinished

[MikeGrahamT21]

10 hours ago, Neverfinished said:

Hi, IGP. I came across your recent post & photo just now.

I've just joined Buildhub with a question 'Laying hardwood floor over suspended floor joists; would any underlayment help?'

Your posts & responders have helped answer my question to an extent, so thanks. 

I've paid £1700 for new floor planks to cover a room similar size as in yr photo & don't want them to rot, want optimum insulation too. Found the article in the thread - 'Best Practice Approach To Insulating Suspended Timber Floors' really helpful & plan to follow it. Sounds expensive though!

I attach a photo showing what I'm doing. Will wrap it with the appropriate membrane(s) but leave the insulation @ current 80mm as concerned to keep the joists breathing in part at least. Got the fireplace & hearth to upgrade too.

Cheers, Steve/Neverfinished

 

I'd recommend you fully fill the depth you have available to you with insulation, and ensure you have approx 150mm air gap underneath the timbers. 100mm isn't a great deal which is all i had space for (and looks same for you)

[IGP]

Yeah I’d fully fill the depth of joists where you can, otherwise do what you can. In terms of breathing, using a breather membrane and breathable insulation like mineral wool should be fine. Any moisture can flow over time through the mineral wool and out through the breather membrane. 
 

I have around 0.8m from the bottom of the joists to the ground below. 

 

On 06/12/2023 at 07:45, ectoplasmosis said:

Looks great! Would you mind sharing the total cost of materials?

 

In terms of cost, it was around £700 excluding insulation (entire ground floor of my house). I bought the materials from Latzel in Germany (there’s a thread about it somewhere) and it’s massively cheaper for the Pro-Clima stuff. Knowing what I know now, I’d probably skip the Pro-Clima membranes for this job, and get the equivalent but cheaper elsewhere however the tapes and primers etc I would still absolutely recommend. 

 

I hope that helps

Edited December 9, 2023 by IGP

[OwenF]

@IGP May I ask. Did you tape the breather membrane to the brick (looks like you did but only part way through in image)

 

did you then also insulate walls internally? If so, presumably that just got applied over that breather lapped onto the wall?

[marshian]

I did the same in the Autumn of 2022

 

I purchased 57.6 m2 of Kingspan foil backed PIR in 75mm because the pack size worked for the area I needed to do and the cost was £300 cheaper than 80mm, 70mm was more expensive by £100 and I'd only save £30 if I went down to 60mm

 

Cost was £1024

 

House has a suspended ground floor with a 2 to 3 ft crawl space (concreted) - There is an air brick every 5 bricks all round the house so very good circulation under there.

 

I have an access point in one room - most rooms are either tiled or overboarded in 18mm Oak so ripping up the floors and doing from above wasn't an option - and there are gaps in supporting walls under doorways throughout the crawl space.

 

It took me about 3 months to do the whole ground floor, I'd go under and measure up an area numbering the gaps between the joists and which was the narrow end (N) and wide end (W) - none of them were perfectly parallel which actually made life easier so my measurements & corresponding notes had to be good - come up for air and cut the boards to suit the area and then slide them down into the crawl space on crawler board - push them round and start to fit them as long as you used the taper to your advantage you could get a really snug fit

 

I held the boards up with 25mm treated tile batten - I did have some expanding foam for any gaps but really didn't use much at all

 

Not a job I want to do again but made a massive difference to floor temps, room comfort and heating bills

[IGP]

On 24/01/2024 at 19:19, OwenF said:

@IGP May I ask. Did you tape the breather membrane to the brick (looks like you did but only part way through in image)

 

did you then also insulate walls internally? If so, presumably that just got applied over that breather lapped onto the wall?

Yes I taped both membranes to the brick. I didn’t insulate the walls internally, got cavity wall insulation.