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Floor - insulation between joists.

Monaou2

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?

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listermint

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

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

Just put earthwool between the joists, costs peanuts.

Gumbo

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

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

Lumen

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

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

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

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

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

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

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

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

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

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

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

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

VB warm side of insulation
Breathable felt cold side of insulation

.

Monaou2

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

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

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

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

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

Bonzo Delaney wrote: »

Hi Bryan what insulation did you use between the joists

Mineral wool

BoardsMember

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

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

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

BoardsMember

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

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

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

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?