Suspended Timber Floor Insulation - Materials/Product Choices?

rob w · 22-10-2020 1:36pm #1

Hi,

I am going to start a job on replacing floorboards downstairs in my house. Will be insulating between joists before putting down the new boards and had a couple of questions. I've read lots of the threads on here and other sites and understand that the best job is to drape a breathable membrane over the joists, fill with insulation and then put an airtight (vapour control layer) over the top before installing new boards.

I will be doing something similar to this site [snip]

However, i am wondering if there are alternatives to the products used there, I have called some builders providers but they haven't been much help or just confused me/them!!

Is netting a viable alternative to the breathable membrane (They are recommending Solitex Plus) to support insulation or is it a waste of time?
Any alternatives to the Pro Clima DA for the top layer, which might be a bit easier on the pocket also?

I am planning to use mineral wool rather than rigid boards for the insulation.

Also form a practical point of view, I am concerned that there may be some boards I wont be able to get under to install the airtight membrane (Such as right under the first step of the stairs) Will this negate all OF my efforts elsewhere if i cant access this part of the floor or how should I go about sealing up that area from draughts?

Thanks!

BryanF · 22-10-2020 7:04pm

Hi, I used those two products above and below joists/ insulation

The breathable felt below, allows moisture out and stops wind getting in which the net will. There are cheaper products available, just check they have same properties.

Re The top layer, air-tightness membrane, I’ve used several, but found that product to be the best, and least likely to rip on you. Again there are cheaper products available, but IMO its not worth scrimping on this or the tapes. Again I’ve used several tapes and found the contega (white roll) the beat for internal use.

Regarding your last paragraph. You need to lift the floors and do it right. Pointless to lay any membranes unless you deal every last pin hole.

I say that as I sit in my cozy living room, delighted I took the time to get the floor right.

In terms of the stairs step. You should be able to remove the face of the step and possibly the first step to sort junctions

Calahonda52 · 22-10-2020 7:19pm

the devil is in the detail here, you need to plan how to deal with the porous block walls at each end, and how to deal with that pesky space between the wall and the first or last joist that runs // to the wall.
In most cases I have cut the joist and used wall hangers

Whats the aversion to rigid?

whats wrong with 1" by 1" , or 2 by 1 strips nailed to the bottom edge of the joists.
Fit the alu coated rigid on top of the strips
fill all gaps with x-foam.
tape all joints with alu tape or similar
- deal with the walls as you see fit

https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice

rob w · 24-10-2020 11:49am

BryanF wrote: »

Hi, I used those two products above and below joists/ insulation

The breathable felt below, allows moisture out and stops wind getting in which the net will. There are cheaper products available, just check they have same properties.

Re The top layer, air-tightness membrane, I’ve used several, but found that product to be the best, and least likely to rip on you. Again there are cheaper products available, but IMO its not worth scrimping on this or the tapes. Again I’ve used several tapes and found the contega (white roll) the beat for internal use.

Regarding your last paragraph. You need to lift the floors and do it right. Pointless to lay any membranes unless you deal every last pin hole.

I say that as I sit in my cozy living room, delighted I took the time to get the floor right.

In terms of the stairs step. You should be able to remove the face of the step and possibly the first step to sort junctions

Thanks for that advice.

Re the tape, that's another confusing part. Will that contega tape do everything I need? As in taping joints together for both layers? Taping to the wall surface where it turns up?

rob w · 24-10-2020 11:53am

Calahonda52 wrote: »

the devil is in the detail here, you need to plan how to deal with the porous block walls at each end, and how to deal with that pesky space between the wall and the first or last joist that runs // to the wall.
In most cases I have cut the joist and used wall hangers

Whats the aversion to rigid?

whats wrong with 1" by 1" , or 2 by 1 strips nailed to the bottom edge of the joists.
Fit the alu coated rigid on top of the strips
fill all gaps with x-foam.
tape all joints with alu tape or similar
- deal with the walls as you see fit

https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice

Thanks for the reply.

What do you mean when you say dealing with the porous block walls at each end? In relation to what? External walls only or internal too?

The only aversion to rigid is the cutting and fitting into the spaces which are not all necessarily the same size etc and all of the filling of gaps. I figure rolling out and stuffing with mineral wool would be easier. I know it's a messy itchy job but I don't mind that too much.

Calahonda52 · 24-10-2020 1:00pm

External & Internal as you are only doing it once

Block is porous if not rendered

I have done both and I prefer the rigid, bought semi perfect stuff direct from Kingspan
I just got with the task: promised myself a case of beer when it was done!.

if using X foam to fill any gaps, don't over cook it as you will need to cut it flush and tape over it
There is now a paint on air barrier which you could use on the block work, easier to apply

BryanF · 24-10-2020 8:27pm

rob w wrote: »

Thanks for that advice.

Re the tape, that's another confusing part. Will that contega tape do everything I need? As in taping joints together for both layers? Taping to the wall surface where it turns up?

Yes.

Calahonda52 · 25-10-2020 10:55am

you may need to think about priming the wall to make sure it is dust free 4 the tape

rob w · 26-10-2020 12:44pm

Thanks for those replies.

Have started lifting floor in and around the stairs today. Here is a photo of the area at the moment.

Wondering what would be the best way to address this? i.e getting the membranes in place where the footing of the stairs is sitting, as this will be staying in place during the works.

I can wrap the airtight membrane up to it, but the breather membrane underneath is what I'm confused about, as it can't be drape dover the joists as it would be normally.

rob w · 26-10-2020 12:46pm

Forgot to attach photos!

Calahonda52 · 26-10-2020 3:54pm

Hope it will be ready for Santa!
I would just add a series of tightly fitted noggins, stitched together, for the full width of the newel post and an inch either side. You can then just staple the membrane to it and the joists and keep going

phobia2011 · 02-11-2020 11:20pm

Following this with interest, have same floor, built in 1960’s, would appreciate a pm with snipped website!

My problem is I’m thinking to retrofit UFH into it.

rob w · 05-11-2020 12:58pm

phobia2011 wrote: »

Following this with interest, have same floor, built in 1960’s, would appreciate a pm with snipped website!

My problem is I’m thinking to retrofit UFH into it.

PM sent there!

rob w · 05-11-2020 1:02pm

Hi,

Been meaning to post back here but haven't had a chance.

Calahonda52 - another question on the comment regarding the porous blocks - what needs to be done here prior to fixing the tape on? What are the implications if i put the tape directly against the block?

Also, my house has cavity insulated external walls, so will this mean there is a gap in the insulation between the proposed floor insulation and the cavity insulation - is this going to cause any problems with condensation or thermal bridging?

rob w · 05-11-2020 1:06pm

Another question on fixing floorboards back down over the vapour control layer - can this be screwed directly through into the joists?

It doesn't seem to make sense puncturing it with screws but how else would it be done if this is not the way?

dathi · 06-11-2020 10:42am

is the fifth floor joist in your first picture sitting in the middle of the floor vent?

rob w · 06-11-2020 11:07am

dathi wrote: »

is the fifth floor joist in your first picture sitting in the middle of the floor vent?

Yes, it is. There is still a decent gap between the vent and sleeper wall though. I will open out the block infront of the vent to allow better airflow there too.

rob w · 10-11-2020 8:52am

rob w wrote: »

Hi,

Been meaning to post back here but haven't had a chance.

Calahonda52 - another question on the comment regarding the porous blocks - what needs to be done here prior to fixing the tape on? What are the implications if i put the tape directly against the block?

Also, my house has cavity insulated external walls, so will this mean there is a gap in the insulation between the proposed floor insulation and the cavity insulation - is this going to cause any problems with condensation or thermal bridging?

Can anyone shed any light on those questions and the fixing of floorboards?

Thanks

Calahonda52 · 10-11-2020 9:14am

what I was suggesting between the joists was to explore using the paint, easier than trying to cut and tape on a barrier around these tricky spots.
The tape is good if on clean dust free surfaces.

Re the TB, the insulation will be up to the u/s of the floor so thats the best you can do.

rob w · 11-11-2020 6:35pm

Calahonda52 wrote: »

what I was suggesting between the joists was to explore using the paint, easier than trying to cut and tape on a barrier around these tricky spots.
The tape is good if on clean dust free surfaces.

Re the TB, the insulation will be up to the u/s of the floor so thats the best you can do.

OK thanks for that. Ill look into that paint, but my initial thought was to bring the barrier up onto the plaster behind the skirting and tape on there.

rob w · 11-11-2020 7:32pm

Has anyone ever used the Fakro N35 Eurotop as a breather membrane under the floor?

https://www.fakro.com/roofing-accessories/roofing-underlays-and-membranes/membranes/

Would this be a suitable alternative to the Solitex Plus membrane, its characteristics look very similar on their data sheets but maybe I'm missing something?

rob w · 28-11-2020 11:33am

rob w wrote: »

Has anyone ever used the Fakro N35 Eurotop as a breather membrane under the floor?

https://www.fakro.com/roofing-accessories/roofing-underlays-and-membranes/membranes/

Would this be a suitable alternative to the Solitex Plus membrane, its characteristics look very similar on their data sheets but maybe I'm missing something?

I'm going ahead using the Fakro membrane above. Installation instructions says "printed side facing outwards in relation to roofing. When using on the floor, I presume the same goes, printed said facing down with insulation tucked in against the non printed backing?

Comparing thus to the installation instructions for Solitex plus, which says the printed side facing the installer. If doing this on a suspended floor from above then it would be printed side facing up?

Just want to make sure I get it down the right way! Confused! :-/

rob w · 05-12-2020 9:27pm

I have the first full room done on this job now.

Breathable membrane down to cradle the insulation and keep windtight. 100mm mineral wool in void. Pro Clima DA airtight membrane on top. Periscopic floor vent into void from airbrick to keep ventilation OK.

Just have to tape them up to the walls now and fix the boards down. Happy with it so far, can be a tricky job working on top of joists thougg once the membranes are down, hard to see where you are stepping etc!! Any practical advice on this from past experiences would be appreciated!

Also, is it the norm to just fix boards directly through the membranes into the joists with screws? Seems mad to puncture it like that!

GreeBo · 07-12-2020 10:33am

rob w wrote: »

Forgot to attach photos!

Random question, but what's holding up that newel post?!

rob w · 07-12-2020 12:33pm

GreeBo wrote: »

Random question, but what's holding up that newel post?!

It's resting on the floorboard. The stairs does a 180 degree turn and strings on both sides are fixed to wall all the way up. The newel post doesn't seem to be providing a massive amount of support, it's been there like that a long time before we moved in to the house.

GreeBo · 07-12-2020 1:31pm

rob w wrote: »

It's resting on the floorboard. The stairs does a 180 degree turn and strings on both sides are fixed to wall all the way up. The newel post doesn't seem to be providing a massive amount of support, it's been there like that a long time before we moved in to the house.

Yikes, typically it would be bolted to the joists, they can take a lot of abuse as people often swing from them as they go up/down the stairs.

But I guess if its still there years later (and isn't loose!) then not to worry!

rob w · 07-12-2020 5:48pm

GreeBo wrote: »

Yikes, typically it would be bolted to the joists, they can take a lot of abuse as people often swing from them as they go up/down the stairs.

But I guess if its still there years later (and isn't loose!) then not to worry!

Yeah, the stairs looks like it was an afterthought put in afterwards to convert it to a dormer. I've come across a few things done in a similar manner in the house since we started renovating but nothing major!

phobia2011 · 11-05-2021 9:36am

Finally I’m ready to start this process,

The breathable membrane that’s used as netting, is this the same as would be used in roofing?

Breathable membrane
Insulation - solid or rock wool?
Airtight membrane
Ufh
Osb

Any opinions?

Calahonda52 · 11-05-2021 1:13pm

https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice
page 150, with the lambda values on page 146, makes for me, rigid the go to solution
Cut the rigid, fill any gaps with foam, cut off surplus and tape over the foam with alu tape

BryanF · 11-05-2021 1:52pm

phobia2011 wrote: »

Finally I’m ready to start this process,

The breathable membrane that’s used as netting, is this the same as would be used in roofing?

Breathable membrane
Insulation - solid or rock wool?
Airtight membrane
Ufh
Osb

Any opinions?

Are you lowering the joists?

phobia2011 · 11-05-2021 3:02pm

Have no intention of lowering joists which are only 4”, I’m thinking of using 4” rigid insulation but letting in hang below joists via notches in it or fit a breathable membrane as a netting. Do I need to fit airtight membrane above the insulation? I will be fitting a aluminium spreader plates between joists with UFH pipework in it.

Suspended Timber Floor Insulation - Materials/Product Choices?

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