1st Floor HRV Duct Positioning

rampantbunny

893bet wrote: »

Looks like we have the same architect and engineer.

...and me...we could have got a package deal.

Andrew_Doran

BarneyMc wrote: »

Ignoring the penetration of the air-tightness membrane, does it really make much of a difference having it here compared with having it in a service area below the joists?

For a 80m2 two story, with 1st floor served by ducts covered in 300mm mineral wool I think I worked it out at around a 1% overall loss in efficiency. Adding more insulation wasn't worth it (diminishing returns). Good workmanship and not over-sizing the ducts is more important (more duct surface area = more heat loss). Ductwork is one thing but unless you have no other good options putting the HRV unit itself in the attic is nuts.

http://www.wbdg.org/design/midg_design_tdcad.php

BarneyMc

Andrew_Doran wrote: »

For a 80m2 two story, with 1st floor served by ducts covered in 300mm mineral wool I think I worked it out at around a 1% overall loss in efficiency. Adding more insulation wasn't worth it (diminishing returns). Good workmanship and not over-sizing the ducts is more important (more duct surface area = more heat loss). Ductwork is one thing but unless you have no other good options putting the HRV unit itself in the attic is nuts.

http://www.wbdg.org/design/midg_design_tdcad.php

Thanks very much Andrew for sharing this. Interesting you mention the duct sizing. I was always aware that for ease of air flow a bigger duct is better but never thought about the conflicting fact that for heat loss it makes sense that a narrower duct is better. :rolleyes:

Rabbo

BarneyMc wrote: »

Thanks very much Andrew for sharing this. Interesting you mention the duct sizing. I was always aware that for ease of air flow a bigger duct is better but never thought about the conflicting fact that for heat loss it makes sense that a narrower duct is better. :rolleyes:

I would have thought the loss in efficiency due to friction in a narrow duct would be more of a factor than heat loss in a larger one. It's also worth bearing in mind that a narrower duct will be noisier

BarneyMc

Rabbo wrote: »

I would have thought the loss in efficiency due to friction in a narrow duct would be more of a factor than heat loss in a larger one. It's also worth bearing in mind that a narrower duct will be noisier

Yes indeed, there is always a trade off. I think a wide duct tucked under a warm blanket is the way to go!

Charlie Charolais

Is there any preference for solid or flexible ducting re MHRV ?

Calahonda52

Charlie Charolais wrote: »

Is there any preference for solid or flexible ducting re MHRV ?

Ther is some good quality flexible ducting which is smooth on inside and will bend 180 without kinking.
The downside is two fold, 1: limited to 32 cubic metres flow at 2.5m/s at I think 150 pascals pressure at the unit.
2: cost, but is easy to install.
The other issue is that diameter wise there is not a lot of choice

BarneyMc

BarneyMc wrote: »

I was speaking to a friend about this at the weekend and his builder suggests creating a 2 inch service void in the bedroom ceilings. Would this really be enough for wires, water pipes, etc?

If this is not done then does that mean the wiring for every wall socket, light switch and light will penetrate the membrane?

Went around to see this recently and am not sure now whether this IS the way to go. For one thing there is certainly a significant cost - material (lot of timber used) and labour. Also, it will mean the electrician having to drill through the batons (extra work).

Can I just ask, will there be a penetration for every light, light switch, electrical socket, HRV point in the airtight membrane? I suppose this all adds up but surely the membrane can be taped where these penetrations occur?

ht9zni1gs28crp

Hang the ceiling AFTER the first fix for all services. A few timber runners isn't going to break the bank as opposed to a couple of airtests to find leaks is!

If all your services penetrate the Air Tight line then Yes you have to deal with them accordingly.

rampantbunny

Is it not possible for those that are insulating at joist level, to just 'lift' the insulation and airtight membrane above the joists entirely, and then have their services underneath in the space between the joists. No penetration of the membrane.

You lose height in the attic but does it matter if the attic is essentially cold and unused.

[Edit]
On reflection, the airtight membrane would be under the joists and taped/stuck directly to the blockwork normally.
If you raised the membrane above the joists you have a lot of work to do bringing it down through the joists to the wall...but is it the lesser of two evils.

ht9zni1gs28crp

I think the original idea is that the services are all inbound of the membrane. By using the U/S of joist as Airtight line you are ensuring the best fit possible for air tightness because no penetrations and NO attempting to seal a membrane in and around joists.

rampantbunny

miller_63 wrote: »

I think the original idea is that the services are all inbound of the membrane. By using the U/S of joist as Airtight line you are ensuring the best fit possible for air tightness because no penetrations and NO attempting to seal a membrane in and around joists.

Comes down to what you're prepared to lose in ceiling heights versus the expense of extra labor and taping around joists I suppose.

Taping around joists must work though. Not aware of any other way of getting airtightness continuity if taking the airtight line from first floor walls up along the rafters (this is my case i.e. whole attic is inside the airtightness envelope).

BarneyMc

rampantbunny wrote: »

Comes down to what you're prepared to lose in ceiling heights versus the expense of extra labor and taping around joists I suppose.

Taping around joists must work though. Not aware of any other way of getting airtightness continuity if taking the airtight line from first floor walls up along the rafters (this is my case i.e. whole attic is inside the airtightness envelope).

My airtight membrane will be fixed to the underside of the joists and come down the walls and then taped, i.e. my attic is outside the airtight zone.

I've generous enough height on the 1st floor so could afford to go the batons route (service void). Would like to hear what folks are doing with theirs.

gooseygander

BarneyMc wrote: »

My airtight membrane will be fixed to the underside of the joists and come down the walls and then taped, i.e. my attic is outside the airtight zone.

I've generous enough height on the 1st floor so could afford to go the batons route (service void). Would like to hear what folks are doing with theirs.

My airtight membrane exactly as above taped to block walls on underside of joists. Duct runs and mhrv unit in cold attic, runs are all covered with soft touch insulation and all penetrations of airtight membrane resealed. Not sure if this is very best option but Suits me.

893bet

What are people using to attach the membrane to the joists? What I mean is my supplier has supplied a double sided tape that you run along the joists. Is this enough to hold it in place for a week or two while you get the blower test done and repair anything required? Or is counter banton required?

BarneyMc

gooseygander wrote: »

My airtight membrane exactly as above taped to block walls on underside of joists. Duct runs and mhrv unit in cold attic, runs are all covered with soft touch insulation and all penetrations of airtight membrane resealed. Not sure if this is very best option but Suits me.

So no service void under joists?

gooseygander

BarneyMc wrote: »

So no service void under joists?

Nope, only electric wires and mhrv ducts coming from attic, no water coming from attic. I would not have had the room height anyway for such a void. Ground to floor height currently is 8ft1in. To be honest even if I had the height I don't think I would have installed void.

rampantbunny

BarneyMc wrote: »

I've generous enough height on the 1st floor so could afford to go the batons route (service void). Would like to hear what folks are doing with theirs.

I'm jealous of all those with generous ceiling heights. Every house granted planning within a 5 mile radius of mine has had a restriction on height imposed as a condition.
When you view section details of the houses it looks near impossible to install an MHRV duct more than 75mm and maintain an 8ft'ish ceiling height on the ground floor.
It gets even tougher if you want to increase your rafter depths to get more insulation in between, or counter-batten the roof as it all pushes the height of the house up.

Anyway, I digress...back on topic. Barney, if I were you I'd put in the service void. Don't think I'd notice high ceilings in bedrooms as they are not exactly open plan...plus a lower ceiling probably increases the 'cosy' factor. Less holes to tape up in the membrane the better even if the tapes are top notch.

galwaytt

...isn't this alone a good reason to change to Open Web joists ??

hexosan

galwaytt wrote: »

...isn't this alone a good reason to change to Open Web joists ??

How do these compare in price to a length of 9x2