Green Building Forum - Insulating under a timber floor and Condensation worries.

Insulating under a timber floor and Condensation worries.

Radioman — Sun, 02 Nov 2008 22:17:41 +0000

I am insulating my suspended timber floor at present and am a little worried about condensation and ventilation. I am using 120mm glasswool between the 125mm deep joists and then overboarding the whole area with 50mm Kingspan screwed (with penny washers) to the underside of the joists.

I have fitted the Kingspan very snugly and intend to fill any small gaps with foam and then tape all the joints with aluminium foil tape essentially creating an air tight insulating skin below the joists.

My worry is the woodwork will be sealed in and will not be able to breathe which may cause damp? Am i right in thinking the timber floor will be warmed to temperatures similar to the inside of the hose and should be OK and that any condensation would form on the underside of the kingspan?

Another factor is i already have (wet) Underfloor heating in the kitchen and will be fitting UFH pipes in the front room and lounge before insulating, so this might keep things toasty?

What about cold bridges like where the now warm joists meet the cold walls could this cause damp?

My question is should i seal the kingspan layer air tight or leave the small gaps for some ventilation?

Insulating under a timber floor and Condensation worries.

CWatters — Sun, 02 Nov 2008 23:08:15 +0000

Technically you should get a condensation risk analysis done.

The bottom of the joists will be nearer the cold side (50mm) than the warm side (125mm) so if the insulation was uniform you might expect the temperature down there to be approx 50/125 ths of the way to the cold side temperature (eg if cold side is zero and warm side is 20C then about 20 x 50/125 =8C). Add a vapour barrier above the joists somewhere?

Insulating under a timber floor and Condensation worries.

Radioman — Sun, 02 Nov 2008 23:55:51 +0000

Hi thanks for the response, it is the vapour barrier thing i am as confused about, where does it need to go to prevent problems?

Also would the foil backing with taped joints on the Kingspan be regarded as a vapour barrier and if so is it in the correct place to help?

From what i have read (could be wrong) 50mm of kingspan is roughly equivalent to 200mm of glasswool? So should the calcs be done as if it where 320mm (200 +120) rockwool in total?

The set up i have been installing is like figure 6 on the link below, just to make it clear.
http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces?full_view=1

any advice is much appreciated thanks.

Insulating under a timber floor and Condensation worries.

tony — Mon, 03 Nov 2008 07:31:17 +0000

I would fit a vapour barrier on top of the joists, either above or below the floor boards. Most likely you wont have a problem but no the foil and sheet insulation are in the wrong place to be the vapour barrier. I would also fill the void formed by the joists fully.

Insulating under a timber floor and Condensation worries.

CWatters — Mon, 03 Nov 2008 10:16:21 +0000

What Tony said. Water vapour is created inside the house by washing, bathing etc. Barrier stops it going through the floor to the cold(er) timbers.

Insulating under a timber floor and Condensation worries.

Timber — Mon, 03 Nov 2008 11:00:22 +0000

Well hang on a second.

You already have a vapour barrier in the from of the rigid insulation!

You only ever want one vapour barrier, and you already have it, so don't go sticking another one in ontop of the joists, as that will cause moisture to be tapped in the void between the joists.

If you already have the rigid insulation installed, go round and make sure the floor void is air tight at the perimeter with some expanding foam and mastic, then leave it at that with NO additional barrier.

In theory the vapour barrier must always go on the WARM side of the structure. So in the case of a floor you would idealy have have floor boards, then vcl, then mineral wool insulation.

If you go for a rigid insulation solution, it must either go under the joists with nothing between them and no additional barrier, or between the joists with nothing below. By putting rigid below the joists then filling with mineral wool between you end up with a 'hybrid' system that is not a cold or warm design.

With a warm design, all the structure is on the warm side of the rigid insulation, and as such no condensation will form in the structure because there is no temperature differential.

With a cold design the insulation is within the structure, so moist air will condense as it passes though, but with a cold design, ventillation is provided which allows that moist air to be taken away without causing issue. With a hybrid, you get moist air passing though the structure and cooling, but there is not ventillation to take it away, and that is where problems begin. Although with the rigid insulation below the joists, the tempurature on the warm face of that insulation is unliekly to be cold enough to cause condensation to form, it can still cause issues.

If you put a vcl over the top of the joists it will stop that moist air getting into the floor void in the first place, but the timber joists will not be able to breathe and so could be subject to elevated moisture contents, with mould growth and even decay a possibility (quite likely!).

Do one thing, or the other, NOT both!

Timber

Insulating under a timber floor and Condensation worries.

tony — Mon, 03 Nov 2008 11:10:49 +0000

Try and understand the original post and make up -- with foil layer not on warm side.

Insulating under a timber floor and Condensation worries.

Timber — Mon, 03 Nov 2008 11:30:57 +0000

Posted By: tonyTry and understand the original post and make up -- with foil layer not on warm side.

Say what now?

Insulating under a timber floor and Condensation worries.

Radioman — Mon, 03 Nov 2008 23:51:40 +0000

So Basically i've made a rite Pigs ear of it

Luckily there is only about a days work to be undone, glad i asked now thanks for the advice so far looks like i need some more advice on the best way to resolve this.

SO what would be the best configuration bearing in mind i am installing from below (in my cellar) an existing (finished floor) with UFH installed and i have 50mm kingspan and 120mm and some 100mm glasswool available.

The kingspan (50mm) only in a single layer below the joists option looks good to me because i can seal it nicely to make the floor airtight and it deals with any thermal bridging of the joists.

I could also go for 50mm kingspan or 120mm glasswool between the joists?

or what about 50mm Kingspan between the joists acting as a barrier with 70mm of glasswool below it?

Any opinions on what would give me the best results and U value?

Again thanks to everybody for the advice this insulation stuff is actually fairly complicated and dificult to do correctly

Insulating under a timber floor and Condensation worries.

CWatters — Tue, 04 Nov 2008 00:09:37 +0000

Is the cellar heated?

Insulating under a timber floor and Condensation worries.

tony — Tue, 04 Nov 2008 00:43:18 +0000

is so no point insulating the floor! so no, it must be ventilated.

What you were going to do would very likely work OK

There were risks which are not worth taking -- cut sheet insulation between joists and then do quilt below that.

It would be nice to get rid of thermal bridge through joists but not essential.

Insulating under a timber floor and Condensation worries.

Timber — Tue, 04 Nov 2008 09:50:09 +0000

Well, for my money, I would go with the 50 mm cut between the joists just below the floor boards and sealed tight with foam, with the rest of the void packed with mineral wool.

that way you get to use your stuff you already have, you have a vapour barrier on the warm side, and breathability to the bottom of the joists.

Timber

Insulating under a timber floor and Condensation worries.

tony — Tue, 04 Nov 2008 12:57:46 +0000

We agreed Timber!

Insulating under a timber floor and Condensation worries.

Timber — Tue, 04 Nov 2008 13:03:00 +0000

Indeed!

Insulating under a timber floor and Condensation worries.

tony — Tue, 04 Nov 2008 13:40:33 +0000

Insulating under a timber floor and Condensation worries.

ludite — Tue, 04 Nov 2008 20:36:41 +0000

Could I just ask???

Hypothetically speaking.

Suppose you did get the vapour barrier wrong - or didn't bother with one.

How bad would it be?

So we have moisture in the house, from living. We also have moisture outside the house.

If we assume the moisture from either of these areas get trapped, how long would it take before the house rots/we get overcome by fumes/mould/mildew ruins out emulsion/ the joists/beams rot away and the house falls down??

Are we talking 6 months, 6 years or 60 years?

Insulating under a timber floor and Condensation worries.

Timber — Tue, 04 Nov 2008 23:04:39 +0000

Hard to say, but there is that potential risk.

Personally (and i don't mean to sound patronising) I would want to do it right once and once only!

Mould can start quite quickly. Once timber has been over 20% moisture content and not allowed to breathe, mould and fungus can start quite quickly. The breathability part is key really. Think of it like damp laundery that is put in a bin liner (we have all been there in our student days) mould and nastyness starts quite quickly.

Timber

Insulating under a timber floor and Condensation worries.

Brendan — Thu, 06 Nov 2008 19:02:17 +0000

I too have a cold suspended timber floor which needs sorting out. I've been following the multifoil thread so have a head full of radiation etc. I've concluded that there is little benefit to be gained for the price/hassle in having a radiant barrier in the loft as the main means of transfer here is conduction and convection. But conversely, under the floor boards i suggest most heat loss downwards is from radiation, with very little from convection, some from conduction out through the joists and not much from conduction through the air. Thus the resistance to conduction provided by the rockwool and kingspan are probably overkill in this application. Using the heat loss to price(or hassle) ratio, i reckon you may as well line the underside of the floorboards with newspaper to stop dust and wafts but allow breathability, then pin a load of foil mylar to hang from the joists (maybe put a strip of polystyrene between the foil and wood to stop any condensation touching wood, and reduce conductive losses). This is not advice, merely a thought. Mad? I 'm sure it wouldnt be as effective as putting rockwool between the joists and kingspan beneath, but would it be significantly less effective, or that much more dangerous for condensation?

Insulating under a timber floor and Condensation worries.

Radioman — Thu, 06 Nov 2008 23:17:04 +0000

Thanks for the post Brendan i had never really thought about the losses in terms of convection, however as you point out for a floor it is probably more important to prevent draughts rather than masses of insulation?

It may be slightly different in this case as i have Under Floor Heating pipes set in to the floor boards so i need fairly good insulation to prevent all the heat dissapearing in to my cellar.

Still can't decide if the way i have started it will be OK? bearing in mind the cellar is not likely to be as cold as outside ever? I am tempted to leave this until we get a really cold spell and them blast the UFH to create a big temperature difference for a day or so then take off the boards and see if any moisture has formed?

Might take some temperature readings just below the boards and in the cellar see what the potential temperature drop is? off to bed tired must rest brain.

Insulating under a timber floor and Condensation worries.

Radioman — Thu, 06 Nov 2008 23:26:36 +0000

"It would be nice to get rid of thermal bridge through joists but not essential. "

To reduce any thermal bridging could I fit 50mm kingspan between the joists to act as a vapour barrier then fit 75mm glasswool between and also extra under the joists held up with netting of some sort? or does the bottom of the joists need to be open to the air for ventilation?

Insulating under a timber floor and Condensation worries.

JackyR — Fri, 07 Nov 2008 18:32:51 +0000

Posted By: Radiomanbearing in mind the cellar is not likely to be as cold as outside ever?

Apparently it is in Washington DC http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces?full_view=1
(Mike George's link from http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=2813&page=1#Item_8 )

Just to confuse you more...

Insulating under a timber floor and Condensation worries.

Viking House — Tue, 25 Nov 2008 00:17:51 +0000

Hi Radiohead

I would forget about Kingspan in your floor. Better to use a Vapour open breathable insulation below the joists like Wood fibre board and pump the space between the joists with Cellulose to a high density, this will give you a good level of Decrement Delay, Windtightness and Breathability. 9mm OSB directly under the floorboards will act as a vapour control layer when the joints are taped

Another option would be to fill the void under the floor with Light Weight Aggregate and block up the vents, this option gives you a warm floor and improved Airtightness in one go.

Insulating under a timber floor and Condensation worries.

tony — Tue, 25 Nov 2008 00:38:37 +0000

Posted By: Viking HouseHi Radiohead

Another option would be to fill the void under the floor with Light Weight Aggregate and block up the vents, this option gives you a warm floor and improved Airtightness in one go.

If you try this in the UK you will finish up with dry rot and smells, falling through the floor problems etc.

Insulating under a timber floor and Condensation worries.

Mike George — Tue, 25 Nov 2008 00:52:41 +0000

Agree

Insulating under a timber floor and Condensation worries.

Viking House — Tue, 25 Nov 2008 00:52:41 +0000

You only get dry rot and smells if the moisture from inside the house condenses on the Cold underside of the joists.
This LWA warms up the joists so they have a similar temperature to the timber in your doors so no condensation or rot!
The LWA is anti capillary so no moisture is drawn up!
A similar system was used on the floors of 500 year old Scandinavian churches where the joists were packed with Kiln dried sand. No rot problems!

Insulating under a timber floor and Condensation worries.

Mike George — Tue, 25 Nov 2008 00:54:51 +0000

May work in Scandanavia, but in the UK we have lots of penetrating moisture into sub-floor walls - Its more than condensation thats the risk here

Insulating under a timber floor and Condensation worries.

jezza22 — Tue, 25 Nov 2008 10:44:58 +0000

I've used some Rockwool FLEXI semi-rigid batt type insulation which is 400mm wide x 12mm long x 100mm thick and is easily push-fit up (from below) between the joists. I was going to use some mulit-foil to hold it all snuggly up tight against the underside of the floorboards but as this will not breath I'm now using that black garden weed stopping material rather than netting. Its cheap, very strong and breathable - I just staple gun it to either side of the joists and it holds the Rockwool tight up to the underside of the floorboards.

I'll also lay 10mm of insulation board ontop of the floorboards before laying new flooring. This will be fully taped and sealed so will act as the vapour barrier.

No point storing up damp problems / worries with some of these untested techniques.

PS I have installed from the void below as I dont want to take up 100m2 of sound flooring and skirting etc.

Insulating under a timber floor and Condensation worries.

Viking House — Thu, 27 Nov 2008 00:01:42 +0000

Hi Mike and Tony

Any moisture in the walls will be given off to the LWA due to its strong hygroscopic power and the timber-joists will be wrapped in LWA which has an extremly low moisture content so the timbers will give off any moisture to the LWA.
The close proximity of the timbers and the LWA keeps the timbers in an extremly dry envoirnment similar to the timbers in German Fokwork houses (clay/timber) walls where the timbers are still healthy in 500 year old houses.
The complete floor can then breathe like a Limecrete floor and the Anti capillary action of the LWA prevents rising damp.
Due to the insulation effect of the LWA the sub-floor walls will now be at least 2 degrees warmer at the top than at the bottom eliminating any capillary effect.

I've tried this in a few renovations, even in a house for my sister with dampish sub-floor walls, I often return to check the joists with my moisture meter but they are as dry as a bone!

Insulating under a timber floor and Condensation worries.

Mike George — Thu, 27 Nov 2008 07:38:29 +0000

Still sounds risky to me - I used to work for a dry rot specialist and its not very nice.

There must be a limit to how much moistue LWA can take in.

It also seems like a time consuming and expensive method.

I'm about to do my timber floor and I am going to use 9mm Aerogel as carpet underlay. The sub-floor void to remain well ventilated

Insulating under a timber floor and Condensation worries.

JackyR — Thu, 27 Nov 2008 15:06:52 +0000

Ooh, does this mean you're happy Aerogel doesn't disintegrate under foot traffic - or are you taking a v expensive punt? Will be very interesting to see this at the 5 year point...