Floor - insulation between joists.

Monaou2 · 31-10-2018 11:18pm #1

We are starting a house renovation and are trying to decide on insulation upgrades. We will dry-line internally (hollow block) and insulate the attic.

My query is regarding floor insulation. The house has suspended timber floors and we are trying to decide if it worth insulating between the joists. We have heard mixed views on this - those for it say the floors will never be right unless they are insulated and we’ll regret not doing it if we end up with draughty floors. On the other side would be the argument that heat rises, so floors aren’t the most important to insulate and a good underlay could suffice.

Cost is approx €4,500 so it would be a significant saving, but we want to do a proper job when doing it at all.

Any thoughts?

listermint · 31-10-2018 11:20pm

If I had the money available I'd insulate them . But make sure it's airtight .

The reviews saying that it's no good usually means the job wasn't good.

Lumen · 31-10-2018 11:32pm

Assuming your current floors are not airtight, i.e. planks, yes, do it. Don't conflate insulation and air tightness, you need both, and with neither you'll really feel it.

€4,500 seems a bit pricey. What are the details?

Per sqm you're looking at about €10 for 18mm T&G OSB3, €4 for 200mm earthwool, maybe €2 for membrane on the underside, tape to the walls, skip hire...? And that's all incl. full VAT.

enricoh · 31-10-2018 11:48pm

Just put earthwool between the joists, costs peanuts.

Gumbo · 31-10-2018 11:50pm

Insulate between the joists.
Air tight membrane below the joists.

There’s some details in SR54 and /or the acceptable construction details documents.

Lumen · 01-11-2018 12:12am

kceire wrote: »

Air tight membrane below the joists.

Airtight on the inside, windtight on the outside.

https://proclima.com/building-physics/why-windproof

Although since the windproofing membranes seem to be mostly designed to deal with driving rain, which you're not going to get under a floor, I'm not sure it makes much difference.

BryanF · 01-11-2018 8:23am

Monaou2 wrote: »

On the other side would be the argument that heat rises, so floors aren’t the most important to insulate and a good underlay could suffice.

Can we assume your only taking about ground floor? If so what heat are you talking about?

4.5k : What is the m2 area ?

Monaou2 · 01-11-2018 9:19am

Yes only talking about ground floor.

Price - is approx 90 sq mtr. We have got numerous quotes and all in this range ...horrific Dublin prices.

Are putting in a new gas heating system (previously storage heaters).

Lumen · 01-11-2018 9:39am

Monaou2 wrote: »

Yes only talking about ground floor.

Price - is approx 90 sq mtr. We have got numerous quotes and all in this range ...horrific Dublin prices.

Hmm. Well I guess if you want to save money you could do it yourself. It's a fairly easy DIY job that doesn't require very precise tolerances.

Calahonda52 · 01-11-2018 11:01am

kceire wrote: »

Insulate between the joists.
Air tight membrane below the joists.

There’s some details in SR54 and /or the acceptable construction details documents.

https://www.nsai.ie/about/news/publication-of-sr-542014-code-of-practice
&
http://www.environ.ie/search/archived/current/category/housing/type/publications?query=acceptable%20construction%20details

Calahonda52 · 01-11-2018 11:02am

enricoh wrote: »

Just put earthwool between the joists, costs peanuts.

Would you like to share with us exactly what you mean by "put" here please?

Lumen · 01-11-2018 11:20am

Calahonda52 wrote: »

Would you like to share with us exactly what you mean by "put" here please?

Oh, on this subject, I posted previously that I had problems with sagging of the membrane under the insulation, but since then I was told that the correct detail is to batten the membrane against the joist to keep it taught. I don't remember reading this in the "acceptable construction details".

Wartburg · 01-11-2018 12:48pm

Lumen wrote: »

Airtight on the inside, windtight on the outside.

Although since the windproofing membranes seem to be mostly designed to deal with driving rain, which you're not going to get under a floor, I'm not sure it makes much difference.

Wind tightness is important even on the floor level. It prevents that draughts can get access into your insulation layer. Mineral wool and all other fluffy insulation materials working on the principle of standing air in between the insulation to create the full insulation performance. If you have cold air streaming through that insulation layer, it definitely lowers the insulation performance.

Lumen · 01-11-2018 1:38pm

Wartburg wrote: »

Wind tightness is important even on the floor level. It prevents that draughts can get access into your insulation layer. Mineral wool and all other fluffy insulation materials working on the principle of standing air in between the insulation to create the full insulation performance. If you have cold air streaming through that insulation layer, it definitely lowers the insulation performance.

Of course. My question is whether the difference between airtightness membrane and windproof membrane matters for the application of detailing the underneath of suspended timber floor insulation.

loyatemu · 01-11-2018 1:47pm

could this be achieved without lifting the floor? Some sort of insulated underlay taped to the walls and then a floating timber floor (or carpets) on top?

We have a similar setup - there's actually a fairly big gap under the floor (it's nearly 5 ft in places) and the boards though T&G have quite a few gaps, resulting in drafts.

Wartburg · 01-11-2018 1:53pm

Lumen wrote: »

Of course. My question is whether the difference between airtightness membrane and windproof membrane matters for the application of detailing the underneath of suspended timber floor insulation.

Why not? It´s the same principle as in all other parts of the house. Breathable as possible to the outside and air tight on the inside. You need to allow the travel of vapour in a controlled manner out of your building structure. If you put a tight material like a VCL to the cold site, you stop the vapour and create condensation. I would threat the wind tight membrane under the timber floor under another aspect as well - as a barrier to prevent access of pest into the insulation layer.
Unfortunately I didn´t come across that much with suspended timber floors at ground floor level that I can advise how to install the wind tight layer in an appropriate way. If I would have this floor type in my own house and consider a refurbishment, I would probably think about a complete replacement with rigid insulation, underfloor heating and screed on top. More expense for the moment but future proof for myself and my descendants.

Lumen · 01-11-2018 2:40pm

loyatemu wrote: »

could this be achieved without lifting the floor? Some sort of insulated underlay taped to the walls and then a floating timber floor (or carpets) on top?

We have a similar setup - there's actually a fairly big gap under the floor (it's nearly 5 ft in places) and the boards though T&G have quite a few gaps, resulting in drafts.

You can fix air tightness by taping those gaps with appropriate product (they vary in the tape material but the permanently sticky, flexible glue is the important component), but any significant insulation (even relatively dense like PIR or high density XPS) is going to add a couple of inches of depth and screw up things like doors and first step height on stairs.

BryanF · 01-11-2018 2:40pm

VB warm side of insulation
Breathable felt cold side of insulation

.

Monaou2 · 02-11-2018 10:39pm

Thanks for all your comments.

I guess my query really is how significant a benefit would it be....in that I know what is possible, and the cost of it, but I’m trying to assess the benefit vs cost of it. For example, if I was trying to decide on changing from old windows to new to make a house warmer, I would assess the cost benefit here as very high - in that you get a massive benefit in comfort levels moving from old windows to new. Similarly for attic insulation - it’s a big benefit from a relatively low cost. However, I struggle to assess the cost vs benefit analysis for insulation between the joists vs a good underlay....so this is really the question I’m trying to answer.

Has anyone been through this? your thoughts would be much appreciated.

Lumen · 03-11-2018 7:00am

Underlay has almost zero insulation value, it only helps with draughts.

UK based and irritatingly free from workings of calculations, but....

http://www.energysavingtrust.org.uk/home-insulation/floor

Judging by that the payback is long.

It's not just about joist insulation though. When you rip the boards up you can insulate pipes which will make your heating work better and fix air tightness at the wall floor junction.

I wouldn't bother unless you're replacing floor coverings anyway.

BryanF · 03-11-2018 9:05am

Monaou2 wrote: »

Thanks for all your comments.

I guess my query really is how significant a benefit would it be....in that I know what is possible, and the cost of it, but I’m trying to assess the benefit vs cost of it. For example, if I was trying to decide on changing from old windows to new to make a house warmer, I would assess the cost benefit here as very high - in that you get a massive benefit in comfort levels moving from old windows to new. Similarly for attic insulation - it’s a big benefit from a relatively low cost. However, I struggle to assess the cost vs benefit analysis for insulation between the joists vs a good underlay....so this is really the question I’m trying to answer.

Has anyone been through this? your thoughts would be much appreciated.

I did it
Breathable felt under joist, insulation between, perimeter PIR, vapour barrier over joists, taped & sealed everywhere, relayed & sand floors.
Best thing I could have done to reduce drafts and keep the heat in the place.

dathi · 03-11-2018 11:02am

Monaou2 wrote: »

Thanks for all your comments.

Has anyone been through this? your thoughts would be much appreciated.

i did it ten year ago with pir insulation fully filling between the joists added 400mm insulation to the attic insulation mannaged to knock 1000 liters of oil of the heating a year by doing that

Bonzo Delaney · 03-11-2018 11:11am

BryanF wrote: »

I did it
Breathable felt under joist, insulation between, perimeter PIR, vapour barrier over joists, taped & sealed everywhere, relayed & sand floors.
Best thing I could have done to reduce drafts and keep the heat in the place.

Hi Bryan what insulation did you use between the joists

BryanF · 03-11-2018 11:13am

Bonzo Delaney wrote: »

Hi Bryan what insulation did you use between the joists

Mineral wool

BoardsMember · 05-11-2018 5:55pm

Interesting discussion. Have been putting off doing a job on the house for ages, including dealing with very draughty suspended floor boards.

Conversation currently has become underfloor heating or not, if not then we do the rockwool type approach. Can't decide if we want to do underfloor or not. Extension of circa 40sq metres plus existing 40sq metres. Very tempted by underfloor but would be mixing with standard roads upstairs. Anyone have any thoughts or recommendations as to how to proceed? I know it'll be more costly to go underfloor, but are running costs way higher? I've seen mixed views.

BryanF · 05-11-2018 11:14pm

BoardsMember wrote: »

Interesting discussion. Have been putting off doing a job on the house for ages, including dealing with very draughty suspended floor boards.

Conversation currently has become underfloor heating or not, if not then we do the rockwool type approach. Can't decide if we want to do underfloor or not. Extension of circa 40sq metres plus existing 40sq metres. Very tempted by underfloor but would be mixing with standard roads upstairs. Anyone have any thoughts or recommendations as to how to proceed? I know it'll be more costly to go underfloor, but are running costs way higher? I've seen mixed views.

1920’s house? don’t do it. Unless the place is really well insulated/thermal bridges addressed, and has thermal mass (ie not dry-lined)

zl1whqvjs75cdy · 06-11-2018 12:22am

Underfloor heat in a suspended floor? You'd be mad

BoardsMember · 06-11-2018 9:46am

Yes, a 1920s house. Understood re thermal mass requirement and concern about thermal bridges.

No, not underfloor in suspended. If we go underfloor it would be back fill cavity, insulate and screed.

BryanF · 06-11-2018 2:12pm

BoardsMember wrote: »

Yes, a 1920s house. Understood re thermal mass requirement and concern about thermal bridges.

No, not underfloor in suspended. If we go underfloor it would be back fill cavity, insulate and screed.

Down to the detailing and supervision - most people can’t stomach paying for design/detailing & site supervision - assuming this don’t bother installing ufh

BoardsMember · 06-11-2018 2:58pm

BryanF wrote: »

Down to the detailing and supervision - most people can’t stomach paying for design/detailing & site supervision - assuming this don’t bother installing ufh

Noted. We'll be using the services of an architect for the renovation/extension. Is this something that would allay the concerns, assuming our builder is a reliable sort? i.e. does that bring back onto the table the extra cost.

If you have the time....you mention needing thermal mass, and say not dry lining. It's a part red brick, so external insulation is not an option except on the new extension and back of the house. Is the issue with dry lined that the walls are not acting as thermal mass, or are there bigger issues?

fmlarnapairce · 09-11-2018 3:57pm

interested in this topic also. 1930s house renovation, suspended floor as well. We're thinking of putting UFH downstairs with air to water hearing, but advice we've received that it would not work very well with rads upstairs. i have looked a bit into putting UFH upstairs as well but am now veering towards rads in the whole house, with oil heating, all of which are in situ already.
we would be insulating the sh1t out of the place, with woodfibre insulation on external walls.
Any thoughts anyone?
should we go UFH downstairs and rads upstairs?

BryanF · 09-11-2018 6:04pm

BoardsMember wrote: »

Noted. We'll be using the services of an architect for the renovation/extension. Is this something that would allay the concerns, assuming our builder is a reliable sort? i.e. does that bring back onto the table the extra cost.

If you have the time....you mention needing thermal mass, and say not dry lining. It's a part red brick, so external insulation is not an option except on the new extension and back of the house. Is the issue with dry lined that the walls are not acting as thermal mass, or are there bigger issues?

Not necessarily. You need a site assessment

fmlarnapairce · 14-11-2018 1:11pm

bumpity bump

BryanF · 14-11-2018 4:50pm

BryanF wrote: »

1920’s house? don’t do it. Unless the place is really well insulated/thermal bridges addressed, and has thermal mass (ie not dry-lined)

BryanF wrote: »

Down to the detailing and supervision - most people can’t stomach paying for design/detailing & site supervision - assuming this don’t bother installing ufh

fmlarnapairce wrote: »

bumpity bump

.

BoardsMember · 15-11-2018 9:10am

If that is directed at me....

Yes, 1920s house. I've already had a thermal survey done so I know where improvements need to be made

The renovation will be done under the direction of an architect who has good knowledge of insulation considerations, so I am happy that the design and detailing will be appropriate.

I'll probably engage an expert third party to do a site visit to get their thoughts on insulate versus back fill, underfloor, thermal mass considerations, ventillation, dry line everywhere versus partial ewi (red brick to front so can't to that competely).

fmlarnapairce · 15-11-2018 10:10am

hi, nope not directed specifically at you but at anyone who can advise, but thanks for above, i had not considered getting a third party expert in to see what should be done, i will do that. all the best

caesarthechimp · 20-11-2018 4:30pm

Sprayfoam between the floor joists will take care of insulation and draughtproofing, and also stiffen and strengthen up the floor if its a bit creaky. Deals with everything except the rats underneath.

Mind you, a basic uninsulated suspended floor can sometimes be an advantage because its relatively cheap to rip out if you were looking at installing a really well insulated concrete floor with underfloor heating combined with an air source heat pump. Taking out a crappy concrete floor and then excavating the hardcore to get enough extra depth for the insulation is a builders nightmare.

fmlarnapairce · 21-11-2018 10:21am

caesarthechimp wrote: »

Sprayfoam between the floor joists will take care of insulation and draughtproofing, and also stiffen and strengthen up the floor if its a bit creaky. Deals with everything except the rats underneath.

Mind you, a basic uninsulated suspended floor can sometimes be an advantage because its relatively cheap to rip out if you were looking at installing a really well insulated concrete floor with underfloor heating combined with an air source heat pump. Taking out a crappy concrete floor and then excavating the hardcore to get enough extra depth for the insulation is a builders nightmare.

thanks, yes this is the decision we have been trying to make for months. our problem is that while the air source and new floor would be a no brainer if the house was 1 story, the installation of UFH in the second floor is the issue. Seems mixing ufh and rads on the one air source HP is not ideal due to the different optimal temps for ufh and rads. that is why we're leaning towards just going with rads and oil all over, system is in place and working fine. insulation in the house is brutal though as was built in 1932

GreeBo · 21-11-2018 4:45pm

fmlarnapairce wrote: »

thanks, yes this is the decision we have been trying to make for months. our problem is that while the air source and new floor would be a no brainer if the house was 1 story, the installation of UFH in the second floor is the issue. Seems mixing ufh and rads on the one air source HP is not ideal due to the different optimal temps for ufh and rads. that is why we're leaning towards just going with rads and oil all over, system is in place and working fine. insulation in the house is brutal though as was built in 1932

Are you hoping to be boiler free or would you consider a smaller boiler just for upstairs rads?

dathi · 22-11-2018 11:13am

fmlarnapairce wrote: »

thanks, yes this is the decision we have been trying to make for months. our problem is that while the air source and new floor would be a no brainer if the house was 1 story, the installation of UFH in the second floor is the issue.

google aluminium underfloor heating trays

fmlarnapairce · 22-11-2018 3:17pm

GreeBo wrote: »

Are you hoping to be boiler free or would you consider a smaller boiler just for upstairs rads?

hadn't considered this, i suppose we just want a simple one source system. but what you say could work.

fmlarnapairce · 22-11-2018 3:18pm

dathi wrote: »

google aluminium underfloor heating trays

this is interesting, you've put our build back a few weeks!

BoardsMember · 22-11-2018 4:16pm

Is it possible to get Air Source Heat Pump to do rads in any sort of an efficient/effective way? Thinking UFH downstairs, but rads upstairs, and not having to have to run a small boiler as well as the Air Source Heat Pump.

Would also be looking to use the heat pump to do the hot water, so maybe expecting too much of it. Planning to install Solar PV as part of a deep retrofit.

GreeBo · 22-11-2018 6:10pm

^
You can, but you will need to look at the BTU of your rads for the temps your HP will be giving them.

You will probably need to double the amount or size of the rads.

fmlarnapairce · 27-11-2018 11:15pm

It seems it is possible with aluminium rads upstairs but an extra buffer (I think) tank needed. I will know more shortly

Calahonda52 · 29-11-2018 1:52pm

https://www.ecologicalbuildingshop.ie/news_detail.asp?newsId=7869
for ideas only

caomhino · 11-03-2019 12:55pm

Just wondering has anyone ever used PIR insulation boards between floor joists ?
I was considering using these in and sit them on batons and tape any gaps but reading in a few places that mineral wool might be a better option. Problem with the boards seems to be that they are rigid and inflexible. Any thoughts ?
Thx.

Polar wizard adventure · 11-03-2019 6:34pm

I used board and batons as you have described. I was able to do the work from the underside so didn't need to raise the floorboards (a good thing) but had to work in a crawlspace (a bad thing). You do need to be reasonable precise in the cutting of the PIR but any gaps on the edges or between the boards are pretty easily sealed with a good expanding foam gun. This also helps to ensure the boards stay in position. The downside with mineral wool is that it is air permeable so if there are gaps between the floor boards then it will only slow the drafts from below but wont stop them.

A lot of work but it worked like a charm.

BoardsMember · 11-03-2019 6:41pm

Polar wizard adventure wrote: »

I used board and batons as you have described. I was able to do the work from the underside so didn't need to raise the floorboards (a good thing) but had to work in a crawlspace (a bad thing). You do need to be reasonable precise in the cutting of the PIR but any gaps on the edges or between the boards are pretty easily sealed with a good expanding foam gun. This also helps to ensure the boards stay in position. The downside with mineral wool is that it is air permeable so if there are gaps between the floor boards then it will only slow the drafts from below but wont stop them.

A lot of work but it worked like a charm.

Presume you didn't use an airtight membrane so?

BryanF · 11-03-2019 10:24pm

Polar wizard adventure wrote: »

The downside with mineral wool is that it is air permeable so if there are gaps between the floor boards then it will only slow the drafts from below but wont stop them.

I’d rather have mineral wool & and vapor barrier on warm side, than timbers that can’t breathe due to PIR & foam.

Polar wizard adventure · 12-03-2019 11:26am

Presume you didn't use an airtight membrane so?

No, the PIR and foam sealing is enough.

I’d rather have mineral wool & and vapor barrier on warm side, than timbers that can’t breathe due to PIR & foam.

The floorboards can breathe to the inside without issue. The joists can breath both to the inside and the outside.

My understanding of how a vapor barrier or airtight membrane would be installed properly is that it would require the floor boards to be lifted to get the full span of the floor.

Floor - insulation between joists.

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