Air tightness in attic

[cwr]

Hi folks

I'm debating if I am going about the air tightness at my first floor ceiling the best way. We are to have attic trusses to allow future conversion of the space in the attic, and the build is to have a high level of air tightness. The provisional plan was to have the air tight layer in the first floor ceiling (shown in green below), and use air tight light fittings . Future conversion would then have the air tight layer follow the blue line in the sketch. Seems a bit convoluted though, and possibly not the best approach? 

Any thoughts on what best to do?

Thanks

Chris

 

 

[Onoff]

You could counter batten the ceiling to avoid puncturing the airtight layer then use conventional low profile LED fittings. 

[cwr]

Yes did consider that, would be good for now may not help that much if/when we convert the attic space.

 

If the air tight layer ultimately followed the yellow line in the sketch below, it would be ideal for services etc, but probably a real pain to get round the rafters... Lots of taping to rough swan wood presumably. Would that even work?

 

[ProDave]

The joists of attic trusses are not usually that rough, but agree it would be a lot of detailed taping to get right.

[cwr]

Certainly sounds tedious to get a gd airtight layer...

Would a sprayed in foam insulation along the yellow line help, either in addition to a membrane or work on its own?

[joe90]

Well my build has a warm roof (yellow line) but I did not use trusses, I used I beam ridge to wall plate to give me 350mm of insulation depth so the span did not need extra support. I lined the inside with osb3 and glued and screwed all the joins. The detail at ceiling/loft floor was the fiddly one but I foamed it all and my air test proved that it worked well. This also meant my MVHR, wiring etc did not have to penetrate the air tight layer (which in my house is wet plastering). The loft is boarded and used fir storage to save on cupboards full of crap in the house ?

[Moggaman]

On 18/09/2019 at 09:00, joe90 said:

Well my build has a warm roof (yellow line) but I did not use trusses, I used I beam ridge to wall plate to give me 350mm of insulation depth so the span did not need extra support. I lined the inside with osb3 and glued and screwed all the joins. The detail at ceiling/loft floor was the fiddly one but I foamed it all and my air test proved that it worked well. This also meant my MVHR, wiring etc did not have to penetrate the air tight layer (which in my house is wet plastering). The loft is boarded and used fir storage to save on cupboards full of crap in the house ?

Hi Joe90,  i am interested in your roof post. I have simple proposed roof attached. I was hoping to keep the airtight membrane/thermal envelope along the red line and then the small space under the ridge could be used as storage etc.The storage area would be uninhabited basically. Is that what you achieved? Do you ventilate that space?  because if i have to put 300mm of insulation over the ceiling, it means the space cannot be used.

 

Thanks

 

 

 

[joe90]

Hi @Moggaman, my whole loft is fir storage only and has no “ridge storage” like your drawing, yes all my insulation (350mm) is between I beams in the actual structure and works very well. @SteamyTea recently posted that the loft was very stable temperature wise (he left some data recording gubbins up there).

[SteamyTea]

18 minutes ago, joe90 said:

@SteamyTea recently posted that the loft was very stable temperature wise (he left some data recording gubbins up there)

Yes it is, goes to show that airtightness and good insulation is all that is needed.

I think an MVHR extract up there is a good idea as that can scavenge a bit of warms when the sun is just right, but also control humidity, which is the important thing.

Joe90 also has a good loft ladder, that helps.

[Moggaman]

7 hours ago, joe90 said:

Hi @Moggaman, my whole loft is fir storage only and has no “ridge storage” like your drawing, yes all my insulation (350mm) is between I beams in the actual structure and works very well. @SteamyTea recently posted that the loft was very stable temperature wise (he left some data recording gubbins up there).

Thanks, i presume that where ceilings are insulated with rolls etc, it’s a no no to put an attic door in ... it’s basically a hole in your thermal envelope ?

[joe90]

1 hour ago, Moggaman said:

it’s a no no to put an attic door in ... it’s basically a hole in your thermal envelope ?

You can get insulated loft hatches with rubber seals but I don’t know how good they are.

[Thorfun]

8 hours ago, SteamyTea said:

Yes it is, goes to show that airtightness and good insulation is all that is needed.

I think an MVHR extract up there is a good idea as that can scavenge a bit of warms when the sun is just right, but also control humidity, which is the important thing.

Joe90 also has a good loft ladder, that helps.

I'm going for the same setup that yourself and @joe90 have then (following the yellow line approach of the OP). interesting about the MVHR extract, I hadn't considered that a requirement and the MVHR designer hasn't designed one in either. if there isn't an MVHR extract how would the moisture be extracted? what sort of a problem could it be? just wondering if I can make sure I have a spare port on my MVHR manifold for the upstairs that I can take an extract to the loft or if it's a little excessive and worrying about little.

[joe90]

Frankly the loft extract was an afterthought, I had a spare extract (to make the number of supply and extract the same) and the unit is in the loft and thought any spare heat would rise to the highest point (ridge of the loft) so thought I may as well pull it back down to the heat exchanger.

 

on another tact, I hung my unit (on rubber mounts) from the roof timbers so it was not sat on the bedroom/bathroom ceiling so no possibility of noise/vibration in the bedrooms etc. Works very well.

[Thorfun]

3 hours ago, joe90 said:

Frankly the loft extract was an afterthought, I had a spare extract (to make the number of supply and extract the same) and the unit is in the loft and thought any spare heat would rise to the highest point (ridge of the loft) so thought I may as well pull it back down to the heat exchanger.

 

on another tact, I hung my unit (on rubber mounts) from the roof timbers so it was not sat on the bedroom/bathroom ceiling so no possibility of noise/vibration in the bedrooms etc. Works very well.

Ok, cool. So not really something to worry about if a spare extract is not available!

 

our unit is going in the plant room in the basement (assuming we can afford a basement that is!)

[SteamyTea]

10 hours ago, Thorfun said:

our unit is going in the plant room in the basement (assuming we can afford a basement that is!)

Does that mean you are going to have some extra long duct work?

Edited January 29, 2021 by SteamyTea

[Thorfun]

44 minutes ago, SteamyTea said:

Does that mean you are going to have some extra long duct work?

I wouldn’t say extra long, no. The plant room is front and centre in the property so the ducts go straight up from the unit to a manifold on each floor for distribution.