Cold unventilated loft - condensation risk?

[G and J]

The plan is to have an airtight membrane above our upstairs ceilings, so minimal water vapour gets from the house into the loft.

 

That will have oodles of insulation just above the ceilings.

 

I really like the idea of NOT ventilating the loft, (like in the diagram), instead using breathable (water vapour permeable) membrane under the tiles to control/limit humidity in the loft space.  
 

Our architect has expressed concerns that condensation will form when the roofing membrane freezes.  I think he’s not factored in the airtightness preventing steam filling the loft.  
 

Am I likely to have a problem?

[ProDave]

Seriously consider running the insulation and air tightness following the roof line.  It won't use much more insulation but it will be a lot easier to do and will give you a warm loft.

 

This traditional way of having a cold loft, takes a lot of detailing to get it right.  EVERY penetration through your ceiling level vapour membrane needs sealing properly.  There will be penetrations for cables to light switches, light fittings (heaven help you if you want downlights in the bathroom) and pipes e.g stack pipe and any ventilation fans or even MVHR.  Get ONE of those detailed badly and there is your route for warm vapour to get to the loft.  And that is before you even think about the loft hatch.

[JohnMo]

The other way and in my option better, is make the loft space warm. Insulation is then done at roof level.

Edited September 28 by JohnMo

[Kelvin]

Agreed. It is much easier to detail and reduces your condensation risk to near zero. You then have the option of a vaulted ceiling should you so wish. You also don’t need to worry too much about ceiling penetrations etc 
 

Why are thinking of doing it this way anyway? 

[JohnMo]

7 minutes ago, Kelvin said:

You then have the option of a vaulted ceiling should you so wish.

Just what we did, vaulted everywhere except one area, that became plant room come store room. Big loft hatch with integral ladder.

[Kelvin]

2 minutes ago, JohnMo said:

Just what we did, vaulted everywhere except one area, that became plant room come store room. Big loft hatch with integral ladder.

Our main living room is vaulted as is the upstairs sitting room and we created a high coombed ceiling (2.7m) in the master bedroom. It’s meant the rooms are all quite different which has added a bit of character I think. 

[SteamyTea]

Warm roof and put an MVHR extractor vent in.

I think that is what @joe90 did and his loft was very comfortable to be in.

[G and J]

So, thank you for these suggestions.  Clearly I’m no further forward on whether my plan will cause me a problem with condensation, but let’s go with the idea of a warm roof and see where that leads….

 

Some background: 

Don’t want vaulted ceilings, just a personal style choice.  And the increase in build cost and complexity of a non fink truss roof isn’t attractive.

There will be a small number of downlighters upstairs, the plan was to put each under it’s own plastic flowerpot (AKA Loftlid).

There will be a maximum of 6 no. 90mm flexible ducts up there breaching the airtight membrane.  All emerge in one area and then they connect to each ceiling vent.

Plan A also has a 4’ boarded PIR walkway down the middle and lots of easy to lay soft insulationy stuff down the sides.  We are determined not to store stuff in the loft, like ever, but one does need access from time to time.

Theres few rooms upstairs and so not too many switches and light fittings.

Ugly but sealed loft access hatches are available and that’s the default, though I’m devising a better looking homemade one.

 

Noted the shag of sealing up each of these but it made everything else so easy that it felt like it was worth it. 
 

I have chosen a cold roof because of the following issues: (I’m opened to be corrected/enlightened)

 

If I go warm roof, I think my airtight layer becomes the vapour barrier under the insulation within the fink trusses.  That means I’m taping up round many, many truss penetrations.  I think this alone would be more time consuming than sealing round me wires and me ducts.
 

I increase the amount of area to be insulated by a factor of circa 1.4.  I think that means a lot more than 1.4 times the cost as it will be different insulation, which will require fitting, and it will require a lower u value to achieve the same heat loss as there’s more area to loose the heat.  I think again this is more work than my ceiling sealing.

 

I think part of the issue is that we are going for champagne spec on beer money.  Airtight and fink trusses aren’t happy bedfellows.  But that’s why I need the buildhub mind….

[joe90]

49 minutes ago, SteamyTea said:

Warm roof and put an MVHR extractor vent in.

I think that is what @joe90 did and his loft was very comfortable to be in.

Yes I did, I had one extract vent left over so piped it to the top of the warm loft (as that is where any heat would rise too).

[G and J]

13 minutes ago, joe90 said:

Yes I did, I had one extract vent left over so piped it to the top of the warm loft (as that is where any heat would rise too).

I think that’s an inspired idea for warm lofts.  Brilliant collection of the rising hot air to recover the heat from.  

 

If I knew of a sensible way to airtight a fink truss roof I’d want to replicate that.

[JohnMo]

24 minutes ago, G and J said:

There will be a small number of downlighters upstairs, the plan was to put each under it’s own plastic flowerpot (AKA Loftlid).

I would airtight layer under ceiling joists, counter batten to form service cavity all wiring in service cavity, down lights in plasterboard. 50mm cavity will leave room for led transformer also. Plan to have no wires in the loft.

 

27 minutes ago, G and J said:

Ugly but sealed loft access hatches are available

Not ugly, airtight and insulated ones are also available.

 

29 minutes ago, G and J said:

There will be a maximum of 6 no. 90mm flexible ducts up there breaching the airtight membrane

Can you do it so the plenum for each room penetrates as easier to seal-up.

 

Tyvek Supro BBA certificate mentions a drying out procedure to be followed during first heating season. So perhaps dig that out also.

[G and J]

13 minutes ago, JohnMo said:

I would airtight layer under ceiling joists, counter batten to form service cavity all wiring in service cavity, down lights in plasterboard. 50mm cavity will leave room for led transformer also. Plan to have no wires in the loft.

I’ve mulled that over sooo many times.  We are restricted on ridge height thence headroom (long and not entirely happy story).   So to do that would mean raised tie trusses, adding between £500 and £1k to the build.

 

I’ve toyed with the idea of doing that anyway to gain circa 3.5” of head height which can be done without chamfered ceilings (which I dislike), so it might be part of the answer.

 

But I’m put off by the idea that I have a membrane above the ceiling that can billow in the wind.  Can you imagine how irritating that might be?  So it would need boarding under the membrane I think, and that would make future alterations really difficult and push the cost and effort up too.
 

 

[G and J]

20 minutes ago, JohnMo said:

Not ugly, airtight and insulated ones are also available.

Noted. Will keep looking but I like my ‘Thunderbirds’ type Heath-Robinson type mini projects.

[G and J]

21 minutes ago, JohnMo said:

Can you do it so the plenum for each room penetrates as easier to seal-up.

Good call.  We’re intending Ergovent vents for aesthetic reasons, so I think the airtight seal will be to them above each room (I know that’s not what I wrote though, so it’s a very valid point).  

 

24 minutes ago, JohnMo said:

Tyvek Supro BBA certificate mentions a drying out procedure to be followed during first heating season. So perhaps dig that out also.

Very good point.  Thank you.  

[Mike]

4 hours ago, G and J said:

I really like the idea of NOT ventilating the loft, (like in the diagram), instead using breathable (water vapour permeable) membrane under the tiles to control/limit humidity in the loft space.  Our architect has expressed concerns that condensation will form when the roofing membrane freezes.  I think he’s not factored in the airtightness preventing steam filling the loft.

 

The principle is good - the idea is that it cuts convection within the loft and reduces heat loss (by around 2% to 4%*), although that's only achievable with near-perfect air sealing. In addition to the issues raised above about airtightness at the ceiling level, airtightness below the tiles (and the junction at the eaves and gables) is also vital.

 

The good new is that, also from the study below*, provided you use membranes with very low vapour diffusion resistance then moisture seems very unlikely to be a problem even if there is leakage - you just miss out on the thermal benefits.

 

*Thermal and Moisture Performance of a Sealed Cold-Roof System with a Vapor-Permeable Underlay, by Tuomo T. Ojanen

 

[G and J]

11 minutes ago, Mike said:

 

The principle is good - the idea is that it cuts convection within the loft and reduces heat loss (by around 2% to 4%*), although that's only achievable with near-perfect air sealing. In addition to the issues raised above about airtightness at the ceiling level, airtightness below the tiles (and the junction at the eaves and gables) is also vital.

 

The good new is that, also from the study below*, provided you use membranes with very low vapour diffusion resistance then moisture seems very unlikely to be a problem even if there is leakage - you just miss out on the thermal benefits.

 

*Thermal and Moisture Performance of a Sealed Cold-Roof System with a Vapor-Permeable Underlay, by Tuomo T. Ojanen

 

If I’ve understood you correctly then, it should work without significant condensation risk and if I seal completely with the right taped up tile underlay, etc.then I will see a tiny thermal improvement.   If my periphery sealing isn’t perfect the loft will still be ok but I’ll not get the tiny thermal benefit.  I like the sound of that, thank you.

 

I’m surprised, given how often peeps have mentioned ‘air washing’ undermining insulation, that it isn’t more of an issue in the loft.  But I also like the idea of a substantially bug free loft too.

 

Our trusses will stop with their outside edge coincident with the outside face of our walls, so I’m thinking that I can do a truss version of a Tony tray on them to provide effectively a continuous air barrier from the VCL under the plasterboard to the tile underlay.  
 

I’m also hoping it will only ever rain at night lol

[Iceverge]

Skipping most of the above.....

 

Sealed attic cold loft is completely fine of you make a good job of the ceiling level membrane. 

 

If you don't, it's isn't and you'll have issues. 

 

We did it. Raised heal fink trusses in a hip roof. 450mm blown cellulose. Several inspections in all weathers have shown no issues. 

 

I was mega careful about the detailing though. 

 

5 wires in the attic, no ducts or pipes. Service cavity below trusses. 

 

Got 0.31ACH50 on the blowerdoor. 

 

What shape is your roof? 

 

If it's a simple pitched roof hung from a ridge beam then a warm loft might be a good option. For anything complex with no vaulted ceilings it's cheaper to do a cold loft. 

 

[Sparrowhawk]

9 hours ago, G and J said:

to do that would mean raised tie trusses, adding between £500 and £1k to the build.

 

That's nothing in the scheme of things. 3 days of a tradesman (if you're lucky), less than half the cost of a cooker/oven, for somethign that can change the spaciousness of all upstairs rooms.

[Iceverge]

10 hours ago, G and J said:

There will be a small number of downlighters upstairs, the plan was to put each under it’s own plastic flowerpot (AKA Loftlid).

 

Hmmmmmmmmm..... Would prefer slimline down lighters in a service cavity. 

 

10 hours ago, G and J said:

There will be a maximum of 6 no. 90mm flexible ducts up there breaching the airtight membrane.  All emerge in one area and then they connect to each ceiling vent.

 

Fine. Would be happy with this if I was going it myself. Fix ply where the outlets/inlets and use a hole saw to cut an exact hole size. Push the vent through. Seal with tape/sealant.

 

Same for ducts. One hole let one duct. 

 

Staple and tape the airtight membrane to the ply. 

 

Ensure the ducts are well covered with insulation. 

 

10 hours ago, G and J said:

Plan A also has a 4’ boarded PIR walkway down the middle and lots of easy to lay soft insulationy stuff down the sides.  We are determined not to store stuff in the loft, like ever, but one does need access from time to time.

 

Get a raised tie between some of the Webs of the trusses to put a boarded walkway on or even a few planks. Forget the PIR. 

 

10 hours ago, G and J said:

Ugly but sealed loft access hatches are available and that’s the default, though I’m devising a better looking homemade one.

 

I bought an official airtight one. I had to use extra sealant as it was leaking with my DIY blower door. 

 

Consider a second hand window double glazed and use that above a bog standard loft hatch. It'd be cheaper than the official hatch. 

 

10 hours ago, G and J said:

If I knew of a sensible way to airtight a fink truss roof I’d want to replicate that.

 

The only workable way is over the top. Taped OSB sarking is probably the easiest and then move to a warm roof with insulation over the sarking. 

 

9 hours ago, G and J said:

But I’m put off by the idea that I have a membrane above the ceiling that can billow in the wind.  

 

We have this with our tyvek membrane under the slates. I think some sarking wouldn't be a bad idea. That and double boarded ceilings for noise mitigation. 

 

6 hours ago, G and J said:

Our trusses will stop with their outside edge coincident with the outside face of our walls, so I’m thinking that I can do a truss version of a Tony tray on them to provide effectively a continuous air barrier from the VCL under the plasterboard to the tile underlay.  

 

That layer should really join the wild tight external layer of the building. 

 

What is your wall construction you settled on in the end? 

[G and J]

4 hours ago, Iceverge said:

What shape is your roof? 

Simple pitched roof on fink trusses unless we go raised tie trusses for a little extra headroom in the house.  No ridge beam, no vaulted ceilings. 
 

 

1 hour ago, Iceverge said:

Hmmmmmmmmm..... Would prefer slimline down lighters in a service cavity. 

I see the attraction but it costs us headroom which we’ve not enough of and I think the time I spend doing a service void well will be greater than making it airtight - given that we’ve a handful (maybe 5 in total) downlighters and less than a dozen cable entries. 
 

1 hour ago, Iceverge said:

Get a raised tie between some of the Webs of the trusses to put a boarded walkway on or even a few planks.

That was my first thought.  I still like the fact that it will be easier to get to below the walkway this way but (subject to cost) a PIR/board sandwich felt like the easiest to live with.  
 

1 hour ago, Iceverge said:

Consider a second hand window double glazed and use that above a bog standard loft hatch. It'd be cheaper than the official hatch. 

Inspired!  And we’ve a selection of windows to choose from sitting in the bungalow that will be demolished. 
 

 

[G and J]

1 hour ago, Iceverge said:

The only workable way is over the top. Taped OSB sarking is probably the easiest and then move to a warm roof with insulation over the sarking. 

That would push our ridge height up which means lower ceilings which we are looking at ways to improve.  
 

1 hour ago, Iceverge said:

That layer should really join the wild tight external layer of the building. 

 

What is your wall construction you settled on in the end? 

Noted.  I think I need to play on paper with this to get my head round it. 
 

Upstairs is timber cladding on counterbattens on fire board on frame with 140mm PIR in frame and 25mm PIR layer inboard.  Then service void and OSB inside fireline plasterboard.  
 

My ceiling VCL will be taped to the wall VCL, so if my tile underlay is taped to my wall breather membrane then the frame will be connected to the loft from a water vapour flow/airflow point of view.  I guess that might be a good thing. Plus I won’t need to get a membrane under the trusses.  

[G and J]

2 hours ago, Sparrowhawk said:

That's nothing in the scheme of things. 3 days of a tradesman (if you're lucky), less than half the cost of a cooker/oven, for somethign that can change the spaciousness of all upstairs rooms.

And I am erring towards going that way, but trouble is I’ve soooo many other ‘just another few hundred £’ things I’m looking at and they add up. 
 

However raised tie trusses will be made from 100mm timber whereas fink will be 72mm so that’s nearly half again as much weight and that will all be manhandled up there.  

[G and J]

2 hours ago, Iceverge said:

Fine. Would be happy with this if I was going it myself. Fix ply where the outlets/inlets and use a hole saw to cut an exact hole size. Push the vent through. Seal with tape/sealant.

 

Same for ducts. One hole let one duct. 

 

Staple and tape the airtight membrane to the ply. 

 

Ensure the ducts are well covered with insulation. 

It will be me and I like the ply/holesaw thing for the ducts.  I was intending to seek a proprietary grommet/gland. 
 

 

[Iceverge]

A quick sketch. Couple of things. 

 

I'Ve assumed a 35 deg roof pitch. 

 

PIR in the walls......................between timbers........................groan..............Anyhou.

 

 

Here's a sketch of my idea.  A fink truss with extended 300mm tails to allow for increased ceiling height. 

 

 

 

 

 

 

 

 

 

 

 

[ETC]

Read the BBA Certificate for the underlay. It will say if you need roof ventilation and under what circumstances you can omit eaves and or ridge ventilation. Some underlays can be installed without roof ventilation subject to a well sealed ceiling being installed.

 

On reflection I think Tyvec might be one of those that do allow an unvented roof but I think down lighters are not allowed. Check it out.

Edited September 29 by ETC