Insulating under suspended floor - worth the effort ?

[KidKruger]

I'm ripping-up a suspended floor covering (board) and while I've got the timbers exposed before I lay the new floor I realise it's a chance to insulate underneath. I'm thinking of either stapling mesh between the joists and filling with loft-type rolls of insulation or pinning polystyrene sheeting in long thin sheets cut to fit the joists, in both cases only about 40mm deep.

Anyone done this / can testify to the benefits or whether it's a waste of time because heat rises anyway ?

It's a kitchen forming the rear end of a Victorian house, so 2x external walls and normally a bit chiller than rest of house.

[Tradesman32]

This will definitely help and when we do loft conversions it is a requirement to satisfy building control. The important thing is to ensure it remains within the suspended floor and doesn't drop as to allow it to do so would hinder airflow. We prevent the insulation dropping by stapling chicken wire in between the joists and laying loft roll (rock wool) within.

Its not rocket science but I have attached a photo of how we do it which I hope is of help.

[KidKruger]

Trade - this is a ground floor with 2ft space under.

I believe I need a membrane laid across the joists (above the insulation) to avoid condensation - any tips on that ?

ie. should I use a damp-proof membrane or other material ?

Thanks so far.

[Tradesman32]

Shouldn't need a dpm on top of the joists.

A suspended timber floor is designed to have sufficient air flow underneath to ensure condensation does not form. I wouldn't advise a full membrane across the joists as if moisture exists it may condensate on this membrane and eventually cause damage (rot) to the timber floor joists. If you have an issue with condensation you need to vent the floor correctly to resolve. The insulation will prevent heat loss and also if installed correctly draft.

The only places on a traditional suspended timber floor there should be a dpm is between the wall plate and the dwarf wall (sleeper wall). If timber sits on an outer wall ledge there should also be DPM there. Take a look at the link I posted below. Look at the floors from the 1920's. If your house was built beforehand it may not have any DPM. If you have the floorboards up now all this should be visible.

http://fet.uwe.ac.uk/conweb/house_ages/elements/section3.htm

[red devil]

KK, are you worried about condensation forming between the newly installed insulation and the floor finish?...

[MrBen]

Two excellent answers from Tradesman. Both are my understanding also having just done it (chicken wire and rock wool on joists of suspended ground ground floor. Def makes room warmer.

[red devil]

This article gives some info on costs/energy savings...http://www.energysavingtrust.org.uk/Insulation/Floor-insulation

I think the 'condensation' that KK is referring to might be what is known as interstitial condensation, and the 'membrane' a breathable type membrane, common in roof construction to prevent this happening, rather than a damp proof membrane (DPM) which is a separate issue. This article gives info on both...http://www.greenbuildingstore.co.uk/page--insulating-suspended-timber-floors.html

KK, what's the reason why you're limited to 40mm depth of insulation? Personally I think it would be a waste of time putting a mineral wool type insulation down at such a shallow depth, a rigid foam board would give you much better thermal insulation properties @ 40mm. Be careful not to 'lag' anything electrical that could overheat.

As an aside...I'm all for stopping noticeable cold draughts, but old Victorian buildings weren't designed to be hermetically sealed boxes, they need to 'breathe', and natural background ventilation in rooms is actually good for them, especially when the weather is too inclement to open windows. If you block up a fireplace you have to put in an air vent to prevent condensation forming inside the chimney leading to damp. If you have an internal kitchen/bathroom (i.e. windowless), Regs ask for a 10mm ventilation gap under the door to act as an air inlet. It's good for the building to have these 'changes of air'...

[MrBen]

RD - you may have a point if depth below joists to ground is short.

I had a lot of subspace to play with - approx 75cm depth between bottom of my ground floor joists and hardcore at the foundation level. So a lot of ventilation in that cavity. And room for the rockwool to work. Our floors are a lot warmer in that room versus another gf room that didnt have it done (which is always 0.5 -1C cooler).

[Too Good To Be True]

We made sure everything was well ventilated, that there was dampproof protection between the joists/sleeper walls and we used Kingspan.

Seems to work well.

So basically what Tradesman said.

Slathering absolutely everything in damp proof membrane in a Victorian house will definitely cause lots of condensation and rot.

[KidKruger]

RD - 40mm was probably a miscalculation, my joists are about 80mm deep so I can go to that depth. I'm not sure how I could go deeper (say 150mm) unless I let the mesh hang down below the level of the joists. But if I do that I suspect limited value in it because there will be gaps in the insulation directly under the joists which would presumably allow the cold air in the underfloor void to diminish the advantage of insulation deeper than the joists.

Damp-proof membrane - my understanding was that you need a membrane and it should be at the 'warm side' of the insulation. If the membrane is on the 'cold side' of the insulation, condensation may occur causing dampness/mould to the insulation. This is all what I've read, not what I know to be fact - hence my post asking for advice.

I'm not trying to hermetically seal the house (a weird conclusion to draw), I'm trying to make one room (the kitchen) warmer seeing as I'm replacing the floor boarding anyway.

[red devil]

KidKruger Wrote:

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> Damp-proof membrane - my understanding was that

> you need a membrane and it should be at the 'warm

> side' of the insulation. If the membrane is on the

> 'cold side' of the insulation, condensation may

> occur causing dampness/mould to the insulation.

> This is all what I've read, not what I know to be

> fact - hence my post asking for advice.

You've got me on this one, I've no idea why a DPM would be used in such an application.

The attached link shows a diagram (No. 4) of a typical retro fit, here they've used a breather membrane which allows the insulation to breathe and avoid interstitial condensation...http://www.greenspec.co.uk/building-design/ground-floor-insulation/

> I'm not trying to hermetically seal the house (a

> weird conclusion to draw)

I didn't say you were, I was speaking in general terms about how some general background natural ventilation is actually good for a building...

[KidKruger]

This is exactly why I posted originally - there seems to be lots of views on it and I want to divine the best for my needs, before winter hopefully !

Thanks.

[GrahamP]

Hi, KidKruger,

I fitted solid kingspan/celotex sheets between the joists - wedged tightly and with nails to prevent them dropping. I was able to prevent cold draughts coming up between the floorboards. The space below is well ventilated.

The materials are more expensive but I believe the insulation value is better.

As usuual my jobs are not 100% complete, with some boards not fixed down. You are welcome to visit in Dunstans Road if you would like a closer look.

Graham

[KidKruger]

Thanks GrahamP, very kind, may take you up on that (if boards still up), I'm round corner on CP Rd.

The reason (one reason) I mentioned damp proof membrane is this:

Does this guy have it wrong ?

[KidKruger]

Tradesman - "The only places on a traditional suspended timber floor there should be a dpm is between the wall plate and the dwarf wall (sleeper wall). If timber sits on an outer wall ledge there should also be DPM there. Take a look at the link I posted below. Look at the floors from the 1920's. If your house was built beforehand it may not have any DPM. If you have the floorboards up now all this should be visible.

[fet.uwe.ac.uk]"

I just checked that link and I have a sleeper wall as per the diagram, although it's 3 or 4 brick pillars on footings which I built myself years ago, with a sleeper timber laid on top. It seems to (hurrah !) have held up solidly and being separate pillars, the main diference being that with only a small area (top of each brick pillar) holing up the timber I used slate as the DPM.

[red devil]

KidKruger Wrote:

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> Thanks GrahamP, very kind, may take you up on that

> (if boards still up), I'm round corner on CP Rd.

>

> The reason (one reason) I mentioned damp proof

> membrane is this:

>

> Does this guy have it wrong ?

Some text appears on the vid just as he's about to put the DPM down...

We used a thick black damp proof membrane as our vapour layer however regular vapour barrier is fine.

A DPM isn't a vapour control layer at all as it's non breathable. The whole point of using a vapour control layer is that it is breathable, so he's actually contradicting himself. IMO the DPM used in this situation is more likely to make things sweat. Here's a link explaining vapour control layers...http://www.dupont.co.uk/products-and-services/construction-materials/tyvek-building-envelope/brands/airguard-vapour-control-layer.html

[KidKruger]

RD this is the bit that I didn't get, either.

A non-porous barrier being used as a breathable layer !

Clearly his underfllor is well-ventilated (as is mine), so you'd need to assume the room above the floor is also well ventilated. As long as underneath is ventilated my thinking is he won't actually get condensation underneath (even if he's not used an appropriate barrier material).

[red devil]

KidKruger Wrote:

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> RD this is the bit that I didn't get, either.

> A non-porous barrier being used as a breathable

> layer !

Don't worry, by the time he nailed all the floor boards back down it would've resembled a sieve!

> Clearly his underfllor is well-ventilated (as is

> mine), so you'd need to assume the room above the

> floor is also well ventilated. As long as

> underneath is ventilated my thinking is he won't

> actually get condensation underneath (even if he's

> not used an appropriate barrier material).

Exactly. Also worth noting that a company like Dupont aren't advertising it as being used in such a situation, potentially it could be a massive market for them, but they don't, which suggests it's just not needed...

[KidKruger]

Just spoke to builder working on next door and I asked how he'd insulate under a floor like mine, he said use solid insulation / celotex and no need for a sheet of anything for a 'barrier '.

[Jeremy]

Yeah I didn't really understand why you thought you'd need a membrane... I would have thought that a little airflow is a good thing, as long as you're keeping most of the heat in.

[KidKruger]

I was approaching this with almost no previous knowledge, so after checking www and finding conflicting info I escalated to EDF, sorter of all things unclear !

[Jeremy]

KidKruger Wrote:

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> almost no previous knowledge

come come now...

[KidKruger]

OK, OK, imperious levels of knowledge - none of which helped with a definitive answer !

[EGL]

We did it ourselves using solid insulation. It was a huge faff, as our floorboards were mostly in place and we basically had to lie on our backs in the cavity under the boards and lodge the insulating material between the joists (having pre-cut it to size). It took ages but it was worth it. Very effective. With hindsight, though, we probably should've paid someone else to do it.

[ryelane]

Hi

about to insulated my suspended timber floor

does anyone have any recommended suppliers of natural insulation ? plenty of places selling it online.

ideally sheep's wool

thanks