First Floor Service Cavity?

BarneyMc

For a cold attic roof construction on a masonry build, is it advisable to allow for a service cavity on the first floor ceiling below the airtight membrane?

BryanF

BarneyMc wrote: »

For a cold attic roof construction on a masonry build, is it advisable to allow for a service cavity on the first floor ceiling below the airtight membrane?

yes very advisable assuming your going for AirTight construction etc

BarneyMc

BryanF wrote: »

yes very advisable assuming your going for AirTight construction etc

Thanks BryanF. So the HRV, electrics, etc. slot in here and don't need to sit in the attic space and subsequently penetrate the airtight membrane?

BryanF

BarneyMc wrote: »

Thanks BryanF. So the HRV, electrics, etc. slot in here and don't need to sit in the attic space and subsequently penetrate the airtight membrane?

exactly
electrics definately yes, hrv dependent on space - have you got the space to play with?

the hrv in a cold space is not ideal as you need to consider insulating the pipework

BarneyMc

Will ask my PM what space we have.

If the HRV pipes are buried under the attic insulation then surely this would be fine?

BryanF

BarneyMc wrote: »

Will ask my PM what space we have.

If the HRV pipes are buried under the attic insulation then surely this would be fine?

yep. but you do have more penetrations, they are often in inaccessible places, the insulation often hides problems such as sagging pipework etc, you then have to find them if there is a problem down the line. + is your insulation non combustible? is this a requirement of the hrv manufacturer?

BarneyMc

BryanF wrote: »

yep. but you do have more penetrations, they are often in inaccessible places, the insulation often hides problems such as sagging pipework etc, you then have to find them if there is a problem down the line. + is your insulation non combustible? is this a requirement of the hrv manufacturer?

Thanks BryanF, will look into all of the above, much appreciated!

BarneyMc

Looks like the HRV ducting might have to sit in the attic but will know for sure later. Regarding the void what height would need to be given to it and apart from electrical and HRV ducting what else might go here?

BryanF

BarneyMc wrote: »

Looks like the HRV ducting might have to sit in the attic but will know for sure later. Regarding the void what height would need to be given to it and apart from electrical and HRV ducting what else might go here?

are you a fan of spot lights? their spec & covering will often dictate depth of service cavity. id say 100mm min to suit the sparkie pulling cable

BarneyMc

BryanF wrote: »

are you a fan of spot lights? their spec & covering will often dictate depth of service cavity. id say 100mm min to suit the sparkie pulling cable

If not using these spotlights then what would be reasonable?

BryanF

BarneyMc wrote: »

If not using these spotlights then what would be reasonable?

100mm - think fist size

BarneyMc

So my reading on this is (moving from cold attic downwards):

Cellulose/insulated 'rolls' between joists
Breathable membrane (attached to joists?)
Service cavity with batons/counterbatons
Plaster Board (insulated?)

mfceiling

BarneyMc wrote: »

So my reading on this is (moving from cold attic downwards):

Cellulose/insulated 'rolls' between joists
Breathable membrane (attached to joists?)
Service cavity with batons/counterbatons
Plaster Board (insulated?)

You probably don't need insulated slabs tbh barney.

Bryan is right with the 4 inches though.

I'm finding now that when pricing houses for suspended ceilings i'm also asked to price for a service cavity upstairs - this is for HRV, spots, cables etc.
I presume then you'll have your main unit of the HRV in the attic? Just make sure you have easy access to it - filters that need changed would break your heart if you had to crawl in the roofspace and then find they are on the other side of the unit and next to a wall/roof joist!!

BarneyMc

mfceiling wrote: »

You probably don't need insulated slabs tbh barney.

So just plasterboard?

mfceiling wrote: »

I presume then you'll have your main unit of the HRV in the attic? Just make sure you have easy access to it - filters that need changed would break your heart if you had to crawl in the roofspace and then find they are on the other side of the unit and next to a wall/roof joist!!

My supplier suggested having the unit in the atic but I wanted to have it in the hotpress area (it is large so could accommodate it). If it was located in the attic then would it not be better to keep the ductwork buried under the rolled insulation in the attic, bring it down where necessary through the ceiling and forget about the service void?

mfceiling

I remember reading somewhere that the effects of the insulated plasterboard were minimal in comparison to the price.

Wouldn't be as sure in keeping the pipework under the insulation. On the electrical forum there is conflicting views on keeping the electric cables under the insulation....some say yeah, others nay!!

john_cappa

BarneyMc wrote: »

So my reading on this is (moving from cold attic downwards):

Cellulose/insulated 'rolls' between joists
Breathable membrane (attached to joists?)
Service cavity with batons/counterbatons
Plaster Board (insulated?)

Where is your airtight layer going?

BarneyMc

mfceiling wrote: »

I remember reading somewhere that the effects of the insulated plasterboard were minimal in comparison to the price.

My thinking was that with 2 layers of insulation there could be a condensation problem in the void.

mfceiling wrote: »

Wouldn't be as sure in keeping the pipework under the insulation. On the electrical forum there is conflicting views on keeping the electric cables under the insulation....some say yeah, others nay!!

The HRV ducts just contain air so I assume it would be ok. The electrics could stay above the insulation.

Anyway I suppose I'm trying to determine whether it worth adding a void. On the plus side it reduces the amount of penetrations in the membrane (all services sit in the void). If anyone has done this then please let me know how it worked out.

BarneyMc

john_cappa wrote: »

Where is your airtight layer going?

I would have thought that was:
Breathable membrane (attached to joists?)

john_cappa

If it's breathable then it can't be airtight surely?

BryanF

john_cappa wrote: »

Where is your airtight layer going?

air-tight layer under joists

john_cappa wrote: »

If it's breathable then it can't be airtight surely?

the air-tight (AT)layer also acts as a 'vapour control layer' or (VCL) which often discussed as a 'breathable layer' (in the context of vapour permeable) there are fancy products on the market that claim to be vapour permeable (or 'vapour open') so in this context 'breathable does mean air-tight

BarneyMc wrote: »

My thinking was that with 2 layers of insulation there could be a condensation problem in the void.

yes - forget the insulated plasterboard, you can pack the void with mineral wool once the electrics are properly specified - but its best to have the insulation above the VCL/AT layer

BarneyMc

Where the airtight layer gets stapled/screwed into the joists above is there a need to cover with airtight tape.

Also, is there any measure to take where the batons are fixed to the airtight membrane above?

BryanF

Barney
some do, some don't

speak to your air-tightness specialist installer & tester

id be confirming what the performance standard is and how good measures will be throughout the building before covering staples or adding expanding tape where battens are being screwed through membrane

BarneyMc

Would be interested in hearing from anyone who has used this service cavity on the first floor ceiling.

fredgalway

I personally would not be in favour of a service cavity in this situation. I would keep insulated MHRV pipes in attic, insulate between joists with roll insulation then fix solid insulation to bottom of joists tape all joints with metallic tape seal edges adjoining walls with sealant or tape, and slab over. You should of course discuss this option with your engineer before doing

BarneyMc

fredgalway wrote: »

I personally would not be in favour of a service cavity in this situation. I would keep insulated MHRV pipes in attic, insulate between joists with roll insulation then fix solid insulation to bottom of joists tape all joints with metallic tape seal edges adjoining walls with sealant or tape, and slab over. You should of course discuss this option with your engineer before doing

Thanks Fred. Can I ask why you're not in favour?

One reason I can see why it may indeed not be a good idea is that it would be VERY disruptive if I ever wanted to get at any of the HRV ducts, electrical cables, etc. in future. If they're in the attic then they would always be accessible.

Corkblowin

fredgalway wrote: »

I personally would not be in favour of a service cavity in this situation. I would keep insulated MHRV pipes in attic, insulate between joists with roll insulation then fix solid insulation to bottom of joists tape all joints with metallic tape seal edges adjoining walls with sealant or tape, and slab over. You should of course discuss this option with your engineer before doing

The main issue with this method is that each light, vent, cable drop etc is a new penetration of the airtight layer - more penetrations more chance of leaks. By using the service cavity you can bring all your items to one or 2 points and punch through once only.

BarneyMc

Corkblowin wrote: »

The main issue with this method is that each light, vent, cable drop etc is a new penetration of the airtight layer - more penetrations more chance of leaks. By using the service cavity you can bring all your items to one or 2 points and punch through once only.

That's the massive plus I see with this method. For the HRV, for example, if the box was located in the hotpress upstairs then the ducting could work it's way up and spread throughout the void without penetrating the membrane. The only penetration here would be the in/out ducts to the outside world.

fredgalway

Hi The reason that I would be against a service void is that I would like to have access to pipework cables etc. In most first floor situations you are talking of bedrooms and bathrooms which would mean one light and one mhrv point not exactly difficult to seal. If there are recessed lights it is something else .

BarneyMc

fredgalway wrote: »

Hi The reason that I would be against a service void is that I would like to have access to pipework cables etc. In most first floor situations you are talking of bedrooms and bathrooms which would mean one light and one mhrv point not exactly difficult to seal. If there are recessed lights it is something else .

Think you hit the nail on the head. I don't think I'll be using recessed lights so I think I'll avoid the void! Thanks all for your input.

BryanF

fredgalway wrote: »

Hi The reason that I would be against a service void is that I would like to have access to pipework cables etc. In most first floor situations you are talking of bedrooms and bathrooms which would mean one light and one mhrv point not exactly difficult to seal. If there are recessed lights it is something else .

just confirm to us how you achieve air-tightness thanks?

BarneyMc

BryanF wrote: »

just confirm to us how you achieve air-tightness thanks?

My understanding is...

The air tightness membrane is fixed below the joists, brought down a bit onto the walls and taped
The walls are plastered overlapping the tape
The insulated plasterboard is fixed to the under side of the membrane

fredgalway

We do not use insulated slab. We use 80mm foil back insulation taped with air tightness tape, this acts as air tightness layer we then slab over. This is what we have being doing for a number of years. I think it still be okay going forward but not 100% sure.

fclauson

Barney - from the top down

• boards supported on timber verticals to joists (for supported loft storage space)
• 500mm of blown cellulose (it "flows" around joists which rolls of stuff never will)
• membrane stapled to underside of joists and taped at all joins and perforations - attached down walls and glued
• 180mm service voice using metal suspended ceiling
• plasterboard

in one central place have (from bottom up) for you cables

• plaster board
• service cavity
• membrane
• plywood square - say 2ft by 2ft - through which you run your cables - one hole per cable - and seal around each cable - glue board to membrane as well
• insulation

at outer perimeter of build see attached drawing - all rights reserved by its author

fclauson

fredgalway wrote: »

We do not use insulated slab. We use 80mm foil back insulation taped with air tightness tape, this acts as air tightness layer we then slab over. This is what we have being doing for a number of years. I think it still be okay going forward but not 100% sure.

just 80mm ??

BryanF

fredgalway wrote: »

I think it still be okay going forward but not 100% sure.

you seemed sure enough that a service was not required?

fredgalway

Yes Bryan I am sure a service void is not required now, what I said was that method used by us is fine now but not sure if this will be case going forward.As regards the 80mm insulation to underside of joists this is in addition to insulation between joists and is above what is presently required.

BryanF

fredgalway wrote: »

We do not use insulated slab. We use 80mm foil back insulation taped with air tightness tape, this acts as air tightness layer we then slab over. This is what we have being doing for a number of years. I think it still be okay going forward but not 100% sure.

fredgalway wrote: »

Yes Bryan I am sure a service void is not required now, what I said was that method used by us is fine now but not sure if this will be case going forward.As regards the 80mm insulation to underside of joists this is in addition to insulation between joists and is above what is presently required.

just tot be clear, are you the general contractor, the ceiling subbie or the air-tightness subbie?

fredgalway

Bryan I find your attitude a little puzzling to say the least. The question asked by the op was asking if a service cavity was advisable. you have given your opinion as have I. Why do you feel the need to question me as regards my occupation, surely we are allowed to have opinions regardless of what we do. I am disappointed by your attitude.

fclauson

fredgalway wrote: »

Yes Bryan I am sure a service void is not required now, what I said was that method used by us is fine now but not sure if this will be case going forward.As regards the 80mm insulation to underside of joists this is in addition to insulation between joists and is above what is presently required.

How air tight had this approach prooved to be ?

893bet

fclauson wrote: »

• 180mm service voice using metal suspended ceiling

Any more info on the above or perhaps a link or PM about the product you used for the suspended ceiling? Images?

• plywood square - say 2ft by 2ft - through which you run your cables - one hole per cable - and seal around each cable - glue board to membrane as well

Makes alot of sense!

fclauson

check out the following for the suspended roof

https://www.facebook.com/photo.php?fbid=165904420281337&set=pb.154344008104045.-2207520000.1387842905.&type=3&src=https%3A%2F%2Fscontent-b-ams.xx.fbcdn.net%2Fhphotos-ash3%2F574662_165904420281337_1202211607_n.jpg&size=960%2C720