Eaves - soffit or what?

[eandg]

...and does an MVHR system located in the loft space get round those problems which you have described @Sensus? 

 

p.s. Forum politics is a depressingly boring subject but Prince Andrew is, indeed, a bastard. 

[joe90]

8 minutes ago, Sensus said:

I suppose it is incumbent upon me to now expand to some degree, much as I'll probably regret it:

 

@Sensus that is a very comprehensive and informative piece of information, this is the kind of knowledge we need on this forum, I see no reason why you will regret it, it is what you have discovered through your job and as you said no two situations are the same, but a trend with more insulation being demanded nowadays. ? one of the reasons I love this forum is information given is from people’s experience in the real world not sales hype.

[SteamyTea]

2 hours ago, jamiehamy said:

You're allowed to discuss anything off topic you like as long as it's not Brexit or His Randy Highness Andrew

Was that one of the 3 or 4 topics that got locked in a day.

So can I discuss the upcoming Grand Tour? Or is that going way too far.

@Onoff I have never had an official reprimand. I shall start to behaviour even more badly now.

Edited November 20, 2019 by SteamyTea

[Ed Davies]

@Sensus, good explanation, thanks. But something to be clear about: when you write “cold roof” do you really mean “cold loft”? The way I've seen these terms used before:

 

Warm roof: roof where (more than half of) the insulation is above the roof structure (rafters, etc). Intrinsically a warm loft.

 

Cold roof: where the roof structure is on the cold side of (more than half of) the insulation.

 

Warm loft: where the insulation is in the roof (might be either a cold or a warm roof).

 

Cold loft: where the insulation is directly above the ceiling. Intrinsically a cold roof though I doubt anybody would bother to call it that.

 

[Jeremy Harris]

A significant part of the problem may relate to the amount of energy available to allow condensed moisture to phase change to vapour and then move out of the structure by ventilation.  Increasing ventilation may increase the amount of heat energy available to enable the water to change phase, or it may not.  It's a variation on the dynamic interstitial condensation risk that may impact other areas of construction.

 

It doesn't need to be a new build, either.  I opted to improve the airtightness and loft insulation level in our old house.  I sealed up most of the tiny spaces that were leaking air into the loft space (made up a home made blower to find them), and, because I don't like handling fibreglass much, opted to lay an additional 200mm layer using the stuff that comes wrapped in a thin, perforated, plastic bag.  In theory, the perforated bag is vapour permeable, but in practice it is not liquid water permeable (the holes are too small, and surface tension stops water flowing through them).  After a while, I started seeing puddles of moisture on top of the insulation.  I thought it must have dripped from the underside of the sarking, but it was clear that it hadn't, the upper surface of the insulation had just dropped below dew point and water had condensed out.

 

After measuring the humidity and temperature up there I reckon that the problem was that I'd unwittingly created a very effective one way dew trap.  The period during which the top surface of the insulation was below the local dew point was  longer in winter than the period in which the same surface was above the dew point, for any 24 hour period.  The result was that more and more moisture collected.  To make things worse, as puddles formed they tended to stay cold for longer, because of the relatively high heat capacity of water, so they then needed more and more energy in order to evaporate.

 

I fixed it by adding loads more soffit vents, and by stuffing short lengths of 32mm waste pipe into the sarking felt overlaps between every truss to provide a bit more ventilation.  I'm not convinced that it was the increased ventilation alone that was the fix, but that increasing the ventilation allowed the loft space to warm up more quickly during the day, perhaps.  The problem may well have been exacerbated by the roof ridge running more or less North/South, so the roof pitches didn't get much energy from the sun during winter.

[Big Jimbo]

6 minutes ago, joe90 said:

 

@Sensus that is a very comprehensive and informative piece of information, this is the kind of knowledge we need on this forum, I see no reason why you will regret it, it is what you have discovered through your job and as you said no two situations are the same, but a trend with more insulation being demanded nowadays. ? one of the reasons I love this forum is information given is from people’s experience in the real world not sales hype.

That's the thing Joe90. Conversation makes you think. I would prefer to hear from anybody on here about real world "these screws, are great, always use em) When you go around any of the home build etc shows. Most of the people on the stands talk total Boll208s. Just telling you that they have the best product ever. A friend on another forum, some time ago had a big prob on his build. His engineer proposed a £30k get around. He put his problem to the forum, and one of the guys gave him an alternative. He put it to his engineer, who said yeah, you can do it like that. The fix cost £8k and the forum saved him £22k

[Bitpipe]

19 minutes ago, eandg said:

...and does an MVHR system located in the loft space get round those problems which you have described @Sensus? 

 

Given it's a sealed unit, I would not expect it to make any difference just by virtue of locating it in the loft.

 

If however it was actively extracting or supplying (not sure which one would be most effective) in the loft then it would certainly change the variables but whether enough to make a difference is moot.

[Bitpipe]

10 minutes ago, Big Jimbo said:

That's the thing Joe90. Conversation makes you think. I would prefer to hear from anybody on here about real world "these screws, are great, always use em) When you go around any of the home build etc shows. Most of the people on the stands talk total Boll208s. Just telling you that they have the best product ever. A friend on another forum, some time ago had a big prob on his build. His engineer proposed a £30k get around. He put his problem to the forum, and one of the guys gave him an alternative. He put it to his engineer, who said yeah, you can do it like that. The fix cost £8k and the forum saved him £22k

 

This forum and its predecessor have saved me more than that easily - plus other aesthetic considerations (like poking a soil vent through a flat roof). 

[SteamyTea]

15 minutes ago, Jeremy Harris said:

The result was that more and more moisture collected.  To make things worse, as puddles formed they tended to stay cold for longer, because of the relatively high heat capacity of water, so they then needed more and more energy in order to evaporate.

What that spreadsheet I did about 10 years ago was all about.

Just looked at the probability of condensation forming over a period of time.

May have to revisit this problem as adding in evaporation would make it more accurate.

Edited November 20, 2019 by SteamyTea

[Patrick]

OK. I thought I hold back on this topic since loads on here  are more clever then me. But maybe some readers here have a similar problem with the long explanations, just like me.

I always find a short YouTube video much more easy to understand than a 1/2 page written on the same topic. And sometimes I feel right stupid on here for not understanding a word, but then... Maybe some others are the same, so this might help understanding the ventilation issue :

 

 

 

Edited November 20, 2019 by Patrick

[Jeremy Harris]

8 minutes ago, SteamyTea said:

What that spreadsheet I did about 10 years ago was all about.

Just looked at the probability of condensation forming over a period of time.

May have to revisit this problem as adding in evaporation would make it more accurate.

 

Yes, IIRC I mentioned this over on the other place around 10 or more years ago, and I think we had a discussion there about it.  I did the work to improve the old house around 17 years ago now, which is when the condensation problem became apparent.  I suspect I only spotted it because I'd opted to use the perforated plastic bag wrapped fibreglass, if I'd laid unwrapped stuff the condensation would have ended up soaking into the insulation, and not being visible until it stained the ceiling, perhaps.

[Big Jimbo]

Patrick, i'm going to watch that later. But i see that in the first 30 seconds, he said the words, Science, and Physics. I am going to have to stock up on chrisps, choc, and fizzy pop for later berore i tackle it.

[Mr Punter]

15 minutes ago, Ed Davies said:

@Sensus, good explanation, thanks. But something to be clear about: when you write “cold roof” do you really mean “cold loft”? The way I've seen these terms used before:

 

Warm roof: roof where (more than half of) the insulation is above the roof structure (rafters, etc). Intrinsically a warm loft.

 

Cold roof: where the roof structure is on the cold side of (more than half of) the insulation.

 

Warm loft: where the insulation is in the roof (might be either a cold or a warm roof).

 

Cold loft: where the insulation is directly above the ceiling. Intrinsically a cold roof though I doubt anybody would bother to call it that.

 

 

It seems that the definitions vary but I believe the following are widely accepted:

 

With flat roofing, cold roof is insulation between rafters, ventilation gap, decking and waterproofing.  Warm roof is insulation above rafters.  Some have decking above but strictly the waterproofing should be straight onto the insulation.

 

With pitched roofing, cold roof has insulation at ceiling level with the loft ventilated, warm roof is insulation between the rafters (perhaps above / below as well), (sarking) membrane, vented gap and roof covering.

 

I never hear anyone refer to 'cold loft' construction even though it is more technically correct.

[Ed Davies]

3 minutes ago, Mr Punter said:

I never hear anyone refer to 'cold loft' construction even though it is more technically correct.

 

On another forum I've seen both “cold loft” and “cold roof” used for the same thing and confusion result, hence my suggested clarification.

[PeterW]

@Sensus appreciate the technical explanation of that - the million dollar question is ..... how to prevent it in the first place ..??? 
 

What would a “robust detail” to use a term look like ..? 

[joe90]

7 minutes ago, PeterW said:

What would a “robust detail” to use a term look like ..? 

 

Warm roof?

[Jeremy Harris]

From a simple physics perspective, prevention means:

 

1.  Preventing the temperature inside the void dropping to the local dew point, or:

 

2. Preventing the humidity of the air in a cold void rising to a level where moisture could condense out under the range of temperatures it might be exposed to, or:

 

3.  Having no void at all

 

1 is relatively easy to achieve, 2 is near impossible to practically achieve long term I suspect, 3 is pretty much the same as 1 in practice.

 

Hard to see how any other option than 1  is likely to work reliably under all anticipated UK environmental conditions, for the expected life of the building.

[SteamyTea]

38 minutes ago, Big Jimbo said:

I am going to have to stock up on chrisps, choc, and fizzy pop for later berore i tackle it.

I always fall asleep after a nutritious snack too.

[Jeremy Harris]

1 minute ago, Sensus said:

 

And it is difficult, in practice, to achieve 1 without expending energy (you've either got to raise the temperature of the space outside the insulated envelope, or reduce the relative humidity of that space).

 

And expending energy to overcome a problem that is caused by measures intended to conserve energy kinda defeats the object...

 

1 is a warm roof, though, where the void is sealed inside the heated envelope.

[joe90]

Well I have a warm  roof and it’s not “habitable”, cost of insulation for the roof is not extortionate nor was the osb making the airtight layer. It is a storage space to save cupboards in the house, somewhere fir the MVHR, I am not sure how much it costs me to heat as the roof insulation is greater than the walls, no windows. My wife stores out of season clothes up there . Also I did not have to worry about making wires etc airtight in the bedroom ceilings. Win win in my opinion. (and I don’t have a condensation problem).

[Mr Punter]

12 minutes ago, Sensus said:

No point in having a warm loft space unless you're going to use it for habitable accommodation (you're just wasting the heat it takes to keep it warm, otherwise)

 

The additional volume does not cause any heat to be wasted (although the insulation over the larger sloped surface area would need to be improved to compensate).  The insulated space is ideal for siting MVHR and ducting.

[Jeremy Harris]

8 minutes ago, joe90 said:

Well I have a warm  roof and it’s not “habitable”, cost of insulation for the roof is not extortionate nor was the osb making the airtight layer. It is a storage space to save cupboards in the house, somewhere fir the MVHR, I am not sure how much it costs me to heat as the roof insulation is greater than the walls, no windows. My wife stores out of season clothes up there . Also I did not have to worry about making wires etc airtight in the bedroom ceilings. Win win in my opinion. (and I don’t have a condensation problem).

 

 

In practice any additional, pretty small, potential heat loss is almost all due to the increase in volume of heated air, and results from the additional ventilation heat loss.  The fabric insulation can be fairly easily brought up to a level where the loss from the increased surface area is small enough to not worry about too much.  Ventilation heat loss can be mitigated by using MVHR, plus there's no need to heavily ventilated the warm loft void, anyway, as there's no moisture or CO2 generation up there, and diffusion should mean that the warm void ends up at equilibrium with the rest of the house.

[Mr Punter]

1 minute ago, Sensus said:

 

Yes it does: additional exposed area, and additional heat loss through air leakage?

 

Which is why I said 

4 minutes ago, Mr Punter said:

although the insulation over the larger sloped surface area would need to be improved to compensate

 

[joe90]

2 minutes ago, Sensus said:

 

Yes it does: additional exposed area, and additional heat loss through air leakage?

 

As I said above I have no heating figures to compare. Our heating has come on only briefly so far this year, the air test was very good so the whole house has very little ventilation losses including the loft space. I can only say that I am very pleased with my situation of incorporating a warm roof.

[Bitpipe]

I guess its a different consideration for the volume house builder who is trying to squeeze every penny out of the build cost to maximise the profit and the self builder who had a different set of motivations.

 

Warm roof insulation, airtightness detail and MVHR are not standard volume house build elements and if they're not contributing towards creating a habitable space that can be marketed and be reflected in the house price (like a usable room in roof, which then implies windows, stairs, meets regs etc) then little reason to do it - unless there is downstream cost of cold roof construction such as a penalty or fine on the house builder.