Air tightness questions

[davejura]

I have a few questions about achieving an acceptable level of airtightness in our new build. Bear with me while I explain my issues!

1 - We have attic trusses and Velux windows, but our loft space is currently "storage". We will convert it in the future, but at the moment the priority is getting in. We are having all the walls and ceilings skimmed, so was wondering if the upstairs ceiling being skimmed would be airtight enough to pass? The builder seems to think so. All the walls have airtight membrane taped, so would just be the upstairs (living area) ceiling. Ii realise there will be light cables and spot lights. Is there a way to seal around these?

2 - My builder is old school and thinks the air tightness is a lot of nonsense (I agree with him to a certain extent) but I keep telling him you have to play the game. One of the joists is hard against the internal wall down the one edge creating a void that I cannot get insulation into (the other side had a gap that I stuffed wool into). Not sure whether to drill it and fill with foam (will take a good few cans) or just put a layer of 30mm PIR and the membrane over it?

3- We have a porch on steel stilts. The architect didn't specify how we are to deal with this regarding airtightness. Obviously it will be insulated, but am wondering if we could get a door that is well sealed between the porch and the hallway which would deal with this? Or do we need to wrap the whole porch with membrane.

 

We have upped the insulation from what was required (from 100m to 140mm between and 25mm to 30mm on the inside) so will this help if we fail the airtight test? We just want to get in, even if it is on a temporary habitation certificate. I will then have the time to deal with the attic space and get it insulated and sealed properly. 

Any advice appreciated!

[ProDave]

You really want to aim for an air tight layer behind the final plasterboard, with enough of a gap for a service void for cables.

 

With a room in roof, it is unlikely you want sockets or lights on the sloping bit of ceiling so no need for a service void there, just put the plasterboard straight on the air tight membrane.

 

With a room in roof it is far easier to run the insulation and air tight layer following the roof line from eaves to ridge and back down again.  This means any eaves storage space is warm and inside the air tight layer and you usually vet a very small "loft" space at the top of the room in roof.  This means you can fit light fittings in that, and any part of the downstairs ceiling without worrying about the air tightness anywhere.

 

Not only will this make it a better building for just the cost of the air tight membrane and tape, it is also a lot easier to do that trying to make the insulation and air tight layer follow the habitable space.

 

You won't need an air test for a temporary habitation.

[joth]

10 hours ago, davejura said:

We have upped the insulation from what was required (from 100m to 140mm between and 25mm to 30mm on the inside) so will this help if we fail the airtight test?

It's almost the opposite: beyond a certain level (your levels aren't quite there) adding more insulation is pointless is you don't have sufficient airtightness.

Also, are you installing mechanical ventilation with heat recovery (MVHR) or using trickle vents on windows for passive ventilation? If MVHR, again you need good airtightness for that to work efficiently. (But given you are inclined to think of draftproofing as pointless I would guess you're going trickle vents.. )

 

[Mike]

15 hours ago, davejura said:

My builder is old school and thinks the air tightness is a lot of nonsense (I agree with him to a certain extent)

Why would you agree with him? You'd rather pay unnecessarily high heating bills than require him to do a good job? What other corners is he cutting?

[Iceverge]

16 hours ago, davejura said:

My builder is old school and thinks the air tightness is a lot of nonsense (I agree with him to a certain extent)

 

Never mind the energy savings or comfort. 

 

Airtightness is a big issue with building fabric decay as moist internal air gets carried out on air paths and causes damage to the structure. 

[SteamyTea]

20 hours ago, davejura said:

My builder is old school and thinks the air tightness is a lot of nonsense (I agree with him to a certain extent) but I keep telling him you have to play the game

I have a boat I can sell you, or to your builder.

 

[davejura]

6 hours ago, Mike said:

Why would you agree with him? You'd rather pay unnecessarily high heating bills than require him to do a good job? What other corners is he cutting?

I don't, but I do think things have gone a bit OTT regarding airtightness! I am doing all the airtight membrane and taping, it's just some things he has done has made it a bit more awkward than it could have been. He is a very good builder and very thorough in other aspects. 

I'm using class O tape from Toolstation which I am finding fine. I see some people are spending £20-£30 per roll on fancy tapes. Are they really that much better? Will you ever recoup the extra money in savings on bills?

 

Prodave, thanks for the advice. I will eventually do the loft as you have suggested but it's good to know we won't need the airtightness test for temporary habitation. That will help us get in a lot quicker as we can leave the loft for later.

[davejura]

The main advice I need is regarding the porch which is on steel columns. Do i try and get that airtight or have an airtight door into the main house?

 

[joth]

34 minutes ago, davejura said:

The main advice I need is regarding the porch which is on steel columns. Do i try and get that airtight or have an airtight door into the main house?

 

Where is the thermal envelope?

[davejura]

I could probably have the airtight seal up to the internal door as long as the door had a good seal. I guess it could be taped for the airtightness test? Just think I'm going to struggle to make the porch airtight.

 

[Jenki]

54 minutes ago, joth said:

Where is the thermal envelope?

Is the porch insulated, are the wall connecting the porch to the house well insulated or are these deemed internal.  Answer the above as @joth asked and you will have your answer.

 

As an aside, you mention air tightness is a bit OTT, if you use the heat loss calculator, and play with the ACH figures you soon see the benefits in running costs. And these are yearly savings.

[davejura]

The porch will be insulated. The main wall connecting the porch to the house is insulated and will have the airtight membrane right to the door.