Major condensation in loft space

[Busamav]

I have a 1930's house with 300mm of loft insulation , with breaks at the loft hatch and near the water tanks.

It has an extremely well vented eaves all round as it is hipped everywhere ,

plus we have no felt ( and the roof tiles havent been sucked away by the wind )

The loft is dry throughout , and it is a 42 degree pitch.

Edited by Busamav on Wednesday 16th December 21:55

[Simpo Two]

Sam_68 said:

Simpo Two said:

Yes, if the loft is colder relative to the fresh air.

No, it's not; even with masses of insulation, the roof space is slightly warmer on a night than the outside air.

But the problem is just the same as the one that causes dew to fall on the ground: warm air that has been happily absorbing moisture all day cools down dramatically at night and can no longer hold that moisture, so it dumps it as condensation or 'dew'.

Ventilation helps a little, but its complex (for example, the air in the loft space remains cooler in the day time, so if left undisturbed would absorb less moisture in the first place... you could find that you're ducting in warm, moisture-laden air that then dumps part of its moisture out as it mixes with the cooler air in the loft part of the time, if you're not careful) and the net result is that you shouldn't expect ventilation, whether natural or fan-driven, to be sufficient to avoid a problem with a highly insulated 'cold roof' construction.

Fair points.

Clearly there is a market for a smart fan with humidity and temperature detectors!

[Soft Top]

jules_s said:

Soft Top said:

My thought is that personally you don't need to care what is causing this.

The house is 5 years old so by my reckoning is still covered by the NHBC guarantee. Get the original builder in or contact the NHBC if you can't find them or they have gone bust.

In otherwords get someone else to sort it out for free.

That's not the case I'm afraid, if the OP has added another 6" of insulation into the house at ceiling tie level in the roof then the developer will (IMO) laugh at the claim.

Personally (after a bottle of vino I might add) I think the OP has driven the dew point level up into the extra layer of insulation he has added in the roof.

The OP needs a vapour barrier on the warm side of the insulation methinks.

Edit: Sam said that ^^^^ in his longpost above that I didn't have the focus to read lol.

Incidentally Sam, I'm not sure increasing the air circulation in the loft will help much as the condensation may be interstitial....maybe replacing the ceilings with foil lined plasterboard would be cheaper than hacking about the eaves/ridges/tiles anyway?

Edited by jules_s on Wednesday 16th December 21:55

OK, fair enough I did wonder if the after market mods might cause a problem but as the issu was there before the extra 170mm was added and it is only to half the loft space, a few hours to remove it would be time well spent. The builder could then see the problem occuring to the oft space with the same specification as they left the house in.

[Sam_68]

SJobson said:

would it be feasible to use some other sort of loft insulation which is less absorbent?

Yes, absolutely.

The problem (for volume housebuilders) is that it's very expensive compared to good, old-fashioned glass fibre.

This is the stuff we'll have to use if and when we are forced to go over to warm roof construction, and it's one of the main reasons we're reluctant to make the move.

[Sam_68]

Busamav said:

Sam_68 said:

Busamav said:

I suspect it is due to a lack of ridge ventilation , particularly if you have a steepish roof pitch .

What leads you to this conclusion?

Eaves to eaves venting is fine for a shallow pitch , but there will be no air movement or change of air at high level in a steep pitched roof

But the insulation is at low level, so it's at low level that you need air circulation to clear any condensation that has settled on the insulation quilt. Air circulation at high level is relatively unimportant.

[mrmaggit]

Busamav said:

mrmaggit said:

Sam_68 said:

Busamav said:

I suspect it is due to a lack of ridge ventilation , particularly if you have a steepish roof pitch .

What leads you to this conclusion?

I was thinking this too, putting continuous ridge ventilation (will require removing and then replacing the ridge tiles), will help through-flow of air (given that the eaves are clear), as hot air rises (you don't say!). My guess (my parents house had the same problem) is the condensation is worst at the ridge, so something like marley Ridgefast will aid airflow.

Make sure there is access for air to enter the roofspace at the eaves.

you could just fit a couple of vented tiles into the pitch near to the ridge line , rather than replace the whole ridge system

True, But,

They look bloody awful, and they cost the same as 6 linear metres of dry vent ridge, that you can fit and forget, no buggering about with re-pointing the mortar.

[Busamav]

Sam_68 said:

[ Air circulation at high level is relatively unimportant.

I think you dismiss this too easily , plus I do not have the in depth research experience you have ,to discuss it , other than my many years experience .

[Sam_68]

jules_s said:

Incidentally Sam, I'm not sure increasing the air circulation in the loft will help much as the condensation may be interstitial...

I agree that increasing air circulation will not help much - that's what I've been saying all along!

Lots of the things being suggested (including increased ventilation) will help to a certain extent, but my experience has been that if you have a major problem, then it will take fairly fundamental changes to eradicate it completely.

It's unlikely to be interstitial condensation, though; if you read my earlier posts, you'll see that I explained that the envelope of most modern houses is so air tight (and the internal moisture relatively well managed by extract fans and background ventilation) that very little internal air finds its way into the insulation that way.

Unless there are significant flaws/weak points (leaking or badly seated loft hatch, holes for downlighters, noticeable cracks in the ceiling plasterboard) combined with fairly high levels of internal humidity (extracts and background ventilation not working or an unusual pattern of lifestyle/occupancy), it is unlikely that enough water vapour will be migrating from the internal space, through the ceiling and into the loft insulation for interstitial condensation to be the big problem.

[jules_s]

Sam_68 said:

Busamav said:

Sam_68 said:

Busamav said:

I suspect it is due to a lack of ridge ventilation , particularly if you have a steepish roof pitch .

What leads you to this conclusion?

Eaves to eaves venting is fine for a shallow pitch , but there will be no air movement or change of air at high level in a steep pitched roof

But the insulation is at low level, so it's at low level that you need air circulation to clear any condensation that has settled on the insulation quilt. Air circulation at high level is relatively unimportant.

You missed my point earlier Sam.

If the dew point temperature is within or High up near the surface of the roof insulation no amount of ventilation will help. Regardless of high/low level ventilation rates.

Also FYI, I'm having major problems getting models to work these days with warm pitched roof as effectively we have to consider heating the roof void as well.

Food for thought....

[jules_s]

Cross posted again....sorry Sam

[Sam_68]

Busamav said:

Sam_68 said:

[ Air circulation at high level is relatively unimportant.

I think you dismiss this too easily, plus I do not have the in depth research experience you have, to discuss it, other than my many years experience .

It's because I have the in-depth research that I dismiss it.

Been there, done that, fitted the ridge tiles ...not experienced the improvement.

Permeable roofing felt makes bugger all real difference either, to be honest (it makes a small but worthwhile improvement, but not enough to consider it a 'cure').

Which is why I'm convinced that these issues are NOT primarily a ventilation problem, and NOT primarily a problem with interstitial condensation...

...there is enough of an issue with 'dew' forming from the moisture held in the air within the airspace of the loft itself as it cools overnight or in cold weather, and the inability of the insulation to ever thoroughly dry out due to its great thickness these days, to cause a problem regardless of how well ventilated the loft is, or how well sealed it is from the dwelling below.

As to the many years of experience, I have that too, but as I explained in my detailed post above, the balance has been tipped in completely the opposite direction in recent years by the changes to the regulations (vastly increased insulation, much better management of internal moisture, much better air-tightness). The old solutions were solutions to a quite different problem.

Edited by Sam_68 on Wednesday 16th December 23:08

[Sam_68]

jules_s said:

Also FYI, I'm having major problems getting models to work these days with warm pitched roof as effectively we have to consider heating the roof void as well

Yes, obviously; sorry I should have made myself clearer - if and when we make the move to warm roof construction, the assumption would be that the roof space would be used as habitable (hence heated) accommodation. It makes no sense either technically or economically to use an expensive insulation arrangement that encloses a large volume of space within the insulated envelope, then leave that space unused.

I think were fairly close to the tipping point on a lot of conventional building technology and that we'll see some fairly major changes to developer housing coming out of the increaseing demands of Building Regulations and Code in coming years.

But then I could be wrong... it's only 3 or 4 years since we were drawing pictures of 'Code 3' housing that looked like an Architecture student's wet dream and was festooned with solar panels and micro-wind turbines, yet today we're building Code 3 houses that look exactly like the houses we've been building for the last 20 years, and we can get to Code 4 with no more than an air source heat pump and MVHR!

Edited by Sam_68 on Wednesday 16th December 23:11

[jules_s]

Sam_68 said:

Busamav said:

Sam_68 said:

[ Air circulation at high level is relatively unimportant.

I think you dismiss this too easily, plus I do not have the in depth research experience you have, to discuss it, other than my many years experience .

It's because I have the in-depth research that I dismiss it.

Been there, done that, fitted the ridge tiles ...not experienced the improvement.

Permeable roofing felt makes bugger all real difference either, to be honest (it makes a small but worthwhile improvement, but not enough to consider it a 'cure').

Which is why I'm convinced that these issues are NOT primarily a ventilation problem, and NOT primarily a problem with interstitial condensation...

...there is enough of an issue with 'dew' forming from the moisture held in the air within the airspace of the loft itself as it cools overnight or in cold weather, and the inability of the insulation to ever thoroughly dry out due to its great thickness these days, to cause a problme regardless of how well ventilated the loft is, or how well sealed it is from the dwelling below.

As to the many years of experience, I have that too, but as I explained in my detailed post above, the balance has been tipped in completely the opposite direction in recent years by the changes to the regulations (vastly increased insulation, much better management of internal moisture, much better air-tightness). The old solutions were solutions to a quite different problem.

Hmmmmmm,

I agree with quite a bit of what you are saying but disagree with some points too.

We've had loft insulation for ages...so why condensation problems these days? when the insulation values were upped circa the 80's I can specifically recall rockwool etc specifying a vapour barrier, at the same time 'they' claimed cavity fill wall insulation was a great idea

As for better management of internal moisture and air-tightness management...thats a new age myth.

[Sam_68]

jules_s said:

We've had loft insulation for ages...so why condensation problems these days?

Because the thickness has increased dramatically in recent years. When I began my career, 2" was acceptable under B.Regs. We're now using between 11" and 18".

I explained the consequences of this above; primarily that the 'semi-exposed' loft space above the insulation is at much closer to external air temperature, so is much more likely to cool to the point where dew begins to form, and the increased insulation thickness takes a lot longer to dry out once it becomes wet.

jules_s said:

...at the same time 'they' claimed cavity fill wall insulation was a great idea

FWIIW, we're still using cavity fill wall insulation (both blown fibre and ecobead) quite happily and with negligible incidence of defects.

jules_s said:

...As for better management of internal moisture and air-tightness management...thats a new age myth.

I disagree and, particularly for the air tightness management, I have the air pressure testing certificates to prove it. When we first started experimentally air testing plots about 3-4 years ago, we were getting results of around 9-10 if we were lucky, and some plots were as low as 11-12. These days we're consistently getting under 5.0 and frequently get down into the mid 3's. We use a design figure of 6.0 in the SAP calcs unless we're going for higher levels of Code, but any result above 5 would now raise my eyebrows.

[M005]

Wow - great discussion.

Sam 68 - seems I got my companies mixed up, Bloor built the house, persimmon did some of the others a few streets away. Ours is the only house of this design.

X 7usc - by header tank I mean very small additional cold water tank, definitely not hot water.

Jules S - you may be right with regard to the dew point, however what hasn't helped is the number of variables that have changed in a short space of time; extra insulation added, weather got colder, stuff in loft moved well away from the eves to allow better ventilation.

I went up this morning (07h00) to get the NHBC stuff, and my feeling was that it felt a lot colder up there (better insulation & increased ventilation) but that there was also more water dripping everywhere.

Will give Bloor and NHBC a call today and see where I get to.

[motco]

How can extractor fans - not MVHC - work in an airtight house? Air-out must be balanced by air-in. In my kitchen I try to educate the users of the cooker hood to crack the door to an adjacent room which has a (largely unused but open) flue so that the air can be replaced. It's like the old days when fresh air heaters in cars were trying to pump warmed air into a closed box. Then Ford had the brillaint wheeze of putting exit vents in the rear of the car! I'd been cracking open a rear side window for this purpose on my old cars for years.

[Dave_ST220]

Martin Keene said:

I found this hard to believe as it had been fine for 12 years. When I sat down and though it through, I realised it started after swmbo moved in and a lot of stuff got put in the loft. Being terreced the vents where on the front and rear eves and we had the contents of the loft on a boarded section across the middle.

A damn good clear out, and rearranging what was left against the party Walls in the loft removed the condensation. We had blocked the airflow in the loft with all the st in there.

Exact same deal here, loft got so full the airflow...err...didn't!! Good clear out & all sorted.

[M005]

Dave_ST220 said:

Martin Keene said:

I found this hard to believe as it had been fine for 12 years. When I sat down and though it through, I realised it started after swmbo moved in and a lot of stuff got put in the loft. Being terreced the vents where on the front and rear eves and we had the contents of the loft on a boarded section across the middle.

A damn good clear out, and rearranging what was left against the party Walls in the loft removed the condensation. We had blocked the airflow in the loft with all the st in there.

Exact same deal here, loft got so full the airflow...err...didn't!! Good clear out & all sorted.

Interesting distinction I hadn't picked up on.

All our stuff is now in the centre boarded section, whereas some had been along the edges close to the eves.

The eves are front & rear of our property as well, and I assumed that the airflow would go over the top of anything stacked in the middle?

[Sam_68]

motco said:

How can extractor fans - not MVHC - work in an airtight house? Air-out must be balanced by air-in.

I should explain that when airtightness is measured on a house, we temporarily seal up the background trickle vents and extract fans (basically, you then attach a bloody great fan to the front door opening, pressurise the house and measure how fast the air leaks out; the figures I gave above are in cubic metres of air leakage per hour, per square metre of floor area, with the house pressurised to 50 Pascals).

Obviously in operation, the air out from the extractor fans is balanced by the air in through the trickle vents.
Also, the Building Regulations demand that we leave a gap under the internal doors to allow air to be drawn through the dwelling.

That's why - in response to your ealier post - I said that excessive air tightness was a problem if the ventilation wasn't managed correctly; the ventilation systems should (in theory) maintain a managed amount of air flow that prevents both health and internal condensation problems.

Edited by Sam_68 on Thursday 17th December 12:53

[Deva Link]

Maybe, as there's no requirement for water tanks in lofts these days so no essential requirement for access, the roof space should be sealed, as well as the house!

Perhaps build them in a factory somewhere and the drop them in place on site.