Boarding the loft - tradesman has left the edges?

WIAWSNB

Grandad2b said:

WIAWSNB said:

Even if the insulation thickness is now limited - squished down - to that of the joist height, it'll be considerably more effective than an equivalent thickness of 'open' loft insulation. It's almost SIP

I'm not sure I understand this comment

Sorry - as Stuart suggested.

Loft insulation is a loose puffy thing, sits lightly in each joist gap, kinda gets into the edges and corners betwixt t'p'board and the joists - but likely not - and it often held adrift by cables and stuff. And in a normal, well-ventilated, loft, you can assume the top couple of inches at least will be thoroughly ventilated by the cross-draft, which will be nigh-on zero degrees in winter. 

That draft is a good thing in general, but it'll also mean that the first couple of inches will be much less effective than even the rest. Loft insulation just ain't a high-level or high-performance insulation material, which is why it relies on a typical thickness of nearly 300mm to be effective.

So, even if this ~300mm were squished down to, say, 150mm by floorboards (the thickness of a 6" joist), I would hazard that any reduction in overall insulating effectiveness would be minimal, if any. 

The insulation material will be much more condensed, will properly fill the voids it is placed in - right up to the edges and corners - and it will be effectively sealed against the worst of the cross-drafts. It'll be a much more effective insulation layer, inch for inch. 

And, the chipboard layer itself is also a good insulator... 

Grandad2b

You're putting an awful lot of faith in that being >150mm loose insulation. I'd suggest more likely 100mm between 100mm joists. I recently took up the boards which I laid across the top of the joists about 20 years ago and did the job properly with loft legs and an extra 100mm of rockwool. 

Depending when the property was built there may be no vapour barrier between the ceiling and insulation so condensation should also be a consideration.

FreeBear

WIAWSNB said: So, even if this ~300mm were squished down to, say, 150mm by floorboards (the thickness of a 6" joist), I would hazard that any reduction in overall insulating effectiveness would be minimal, if any.

Insulation relies on pockets of air trapped inside the material. You take 300mm of fibreglass and squash it down to 150mm, the thermal resistance drops from 7.5m²K/W down to 3.75m²K/W or less. If you want to maximise insulation, you either have to use a higher performance material (say PUR/PIR) or increase the thickness. Sometimes, that means having to use taller legs and accepting there will be marginally less headroom.

https://www.loftleg.com/post/2019/01/28/are-you-accidentally-halving-your-loft-insulations-effectiveness

WIAWSNB

FreeBear said:

WIAWSNB said: So, even if this ~300mm were squished down to, say, 150mm by floorboards (the thickness of a 6" joist), I would hazard that any reduction in overall insulating effectiveness would be minimal, if any.

Insulation relies on pockets of air trapped inside the material. You take 300mm of fibreglass and squash it down to 150mm, the thermal resistance drops from 7.5m²K/W down to 3.75m²K/W or less. If you want to maximise insulation, you either have to use a higher performance material (say PUR/PIR) or increase the thickness. Sometimes, that means having to use taller legs and accepting there will be marginally less headroom.

https://www.loftleg.com/post/2019/01/28/are-you-accidentally-halving-your-loft-insulations-effectiveness

Yeah, I don't believe it. :-)

grumpy_codger

FreeBear said:

WIAWSNB said: So, even if this ~300mm were squished down to, say, 150mm by floorboards (the thickness of a 6" joist), I would hazard that any reduction in overall insulating effectiveness would be minimal, if any.

Insulation relies on pockets of air trapped inside the material. You take 300mm of fibreglass and squash it down to 150mm, the thermal resistance drops from 7.5m²K/W down to 3.75m²K/W or less. If you want to maximise insulation, you either have to use a higher performance material (say PUR/PIR) or increase the thickness. Sometimes, that means having to use taller legs and accepting there will be marginally less headroom.

https://www.loftleg.com/post/2019/01/28/are-you-accidentally-halving-your-loft-insulations-effectiveness

I think this is very simplified estimation, probably not backed by any reliable experiments, otherwise a plastic bag with air inside would have the same insulating properties as the same bag filled with insulation. They give a link to  case studies, but I don't see anything there even  if I search for 'loft leg' or 'insulation'.

The 'pockets' part is very important and shouldn't be ignored. Pockets restrict air circulation and convection. The smaller the pockets, the better. Compressed insulation restricts convection more efficiently, so it's not exactly 50% loss. Boards stop air exchange between the loft and the insulation under the boards, so add more than their own thermal resistance (unless the insulation is PIR or some soft Polythene Wrapped Insulation).

WIAWSNB

grumpy_codger said:

FreeBear said:

WIAWSNB said: So, even if this ~300mm were squished down to, say, 150mm by floorboards (the thickness of a 6" joist), I would hazard that any reduction in overall insulating effectiveness would be minimal, if any.

Insulation relies on pockets of air trapped inside the material. You take 300mm of fibreglass and squash it down to 150mm, the thermal resistance drops from 7.5m²K/W down to 3.75m²K/W or less. If you want to maximise insulation, you either have to use a higher performance material (say PUR/PIR) or increase the thickness. Sometimes, that means having to use taller legs and accepting there will be marginally less headroom.

https://www.loftleg.com/post/2019/01/28/are-you-accidentally-halving-your-loft-insulations-effectiveness

I think this is very simplified estimation, probably not backed by any reliable experiments, otherwise a plastic bag with air inside would have the same insulating properties as the same bag filled with insulation. They give a link to  case studies, but I don't see anything there even  if I search for 'loft leg' or 'insulation'.

The 'pockets' part is very important and shouldn't be ignored. Pockets restrict air circulation and convection. The smaller the pockets, the better. Compressed insulation restricts convection more efficiently, so it's not exactly 50% loss. Boards stop air exchange between the loft and the insulation under the boards, so add more than their own thermal resistance (unless the insulation is PIR or some soft Polythene Wrapped Insulation).

Totally agree.
The example in that link isn't comparable either. I was comparing loose-laid insulation, exposed to the ventilated loft air, with squished-and-overboarded. I'd hazard there would be little or now't between them.

youth_leader

I believe my son is also having a positive input ventilation unit installed, I have one here at the bungalow and it has reduced the condensation here to almost nothing. I also had two 'ventilation tiles' in the east facing side of my concrete tiled roof. Should I recommend this to my son too?  I'll probably have to pay for it!

WIAWSNB

youth_leader said:

I believe my son is also having a positive input ventilation unit installed, I have one here at the bungalow and it has reduced the condensation here to almost nothing. I also had two 'ventilation tiles' in the east facing side of my concrete tiled roof. Should I recommend this to my son too?  I'll probably have to pay for it!

Ha-ha! 
I have no personal experience if PIV, but pretty much every report I've read - usually on here - have been good.
However, my personal take on the condensation malarkey is that the vast majority of cases can be controlled by normal proper room ventilation. For instance, overnight condie can be pretty much eliminated by having windows cracked open all night, and using a good duvet to keep warm - an all-night leccy blanket if needed.
PIV must surely cost in energy loss, as it's a catch-all, with house air being expelled regardless of whether the room is badly effected. And then there's the installation cost.
I'd only fit one if all other approaches failed.

grumpy_codger

WIAWSNB said:

youth_leader said:

I believe my son is also having a positive input ventilation unit installed, I have one here at the bungalow and it has reduced the condensation here to almost nothing. I also had two 'ventilation tiles' in the east facing side of my concrete tiled roof. Should I recommend this to my son too?  I'll probably have to pay for it!

....
PIV must surely cost in energy loss, as it's a catch-all, with house air being expelled regardless of whether the room is badly effected. And then there's the installation cost....

Installation - yes, but in a good system with heat recovery energy loss can be minimal because of the heat exchange between the  air pumped in and going out. As a result, with a good heat exchanger, the incoming air can theoretically have the same temperature as the air leaving the house. 

ETA: the above picture doesn't show well how heat recovery works. In fact directions of two flows it the heat exchanger are opposite:

youth_leader

I did have a L3 on this bungalow but six months later discovered all the floors were rotten.  Combination of a high water table (apparently built on a 'muddy hill') causing much condensation, the base of the house is/was wet when the floors were removed.  I can't remember what this is called at this moment.  No pooling when it rained.  The front of the bungalow has had some form of cavity insulation, there are 'round' holes in the brickwork. 

The previous vendor had unwittingly had cowboys in 2016, 4 years before I purchased, who had replaced the floors with weyroc and the joists were non tannelised wood with a carrier bag stapled over the ends.  The one room with wooden floors had wet/dry rot and woodworm - and a repair had been attached to woodwormed joists. 

I came from a single glazed house with open fires and was surprised at the condensation here on the double glazed windows, I thought I'd left that behind.  I spoke to my neighbour who said to get a Karcher, which I did, but it took about 40 minutes every morning.   I only have tiny air vents in front bedrooms and did have them open, there are air bricks and I make sure they are clear.  Both of the chimneys have been capped and only one has a grille.  The extension which goes across the back to make the bedroom, bathroom and lounge bigger is concrete floors and the air bricks are fake at the back of the property.  I had to have all of the floors replaced and the independent damp/timber specialist did say I should go infill and concrete, but it would have been £40K.  I only had £20K equity from my house sale, so had to go timber.  I do have a dehumidifier which I run when I am drying laundry inside.  Last winter, after the work had been done,  I discovered the carpet under the bed was wet underneath the plastic bags I store my linen in.  I moved the carpet and only have scatter rugs now, and I move everything regularly.

As there is no storage I thought of having the loft boarded, and when I opened the loft hatch, water droplets poured out.  My surveyor said it was condensation.  I had the positive input ventilation unit put in first, then the two roof ventilation tiles, then on a snowy day the roofer knocked to say he could do the additional loft insulation.  26 rolls of insulation later I'm spent up, this bungalow has been a very expensive buy.