New Build - Airtightness

mrsWhippy

Hi,

Having done loads of research here about airtightness and the benefits of HRV, we are planning to build our new house to as high a spec as possible, definitely below 3m3/hr/m2.

We will be (hopefully) going to tender in the coming months, and have an architect to manage that phase. I know Irish builders don't have the best reputation for building airtight houses, but I want to make sure I ask the right questions to ensure we end up with the right contractor. Other than getting references and calling previous clients, is there any paper work, documentation or similar I could request which would give me a good indication of their abilities to build to these specs?

We will more than likely get an energy consultant for their advice on heating, insulation etc, would the materials used for airtightness be under their remit, or would it generally be the architect? Who signs off on 'airtightness' as such? How can we be sure the right materials are being used?

Thanks a lot for allowing me pick your brains
Cheers

MicktheMan

Airtightness starts with good design so one of the criteria I would use to choose my architect is what air tightness levels his/her designs have achieved in the past and what has been learned.

The great thing about air tightness is that it can be measured. No grey area, it is what it is. So, with that in mind, I would require any prospective builder to prove what he has achieved in the past (i.e. produce certs) and explain how he achieved these.

Air tight materials used (as well as how they are used) is crucial to the result. There are a lot of, imo, inferior and cheaper tapes, membranes etc which should be avoided. There is no reason why specific brands cannot be specified. So you might inquire where the builder sources their air tightness materials.

I would recommend two tests. The 1st when the house is initially sealed but before finishes are applied. This is the key test as weaknesses can readily be addressed. The 2nd (final) test is when works are complete and this is the official test.

If you are getting an energy consultant on board, then I suggest the earlier the better.

Finally, I would set a target much tighter than 3 m3/hr/m2@50Pa. With proper planning and careful workmanship / management, sub 1 m3/hr/m2@50Pa should be readily achievable.

mrsWhippy

Thanks a lot for the comprehensive reply!

Our architect has already been selected, plans have been created and have been lodged with planning. So unfortunately we missed the boat there.

What aspects of a design might affect airtightness? Bear in mind we haven't created tender drawings, just the planning drawings.

Thanks

MicktheMan

mrsWhippy wrote: »

What aspects of a design might affect airtightness?

There are plenty.

For example, house type. 11/2 storey and dormers are more difficult to air tighten than straight 2 storey or bungalow. If the roof space is to be used for habitable space (1 1/2 storey or dormer) then a true warm roof might be considered (this will then affect ridge height).

Another example:
to minimise penetrations of the airtightness barrier, creation of a service cavity inside the thermal envelope.

As said earlier, get the energy consultant on board sooner rather than later.

galwaytt

3.0 is too low imho. I would set the bar for your contractor to 1.5, but aim higher. I see 1.0 regularly enough these days without killing themselves, sometimes better.

mrsWhippy

Thanks for the feedback so far.

Who's responsibility is it to ensure the contractor is using the correct materials, not cutting corners, using the correct sealing techniques etc? Would that be the architect or the energy consultant? I understand that both of these guys will only have a certain amount of site visits etc.

Can reaching a certain airtightness rating be included in the contract with the builder?

Thanks

ht9zni1gs28crp

mrsWhippy wrote: »

Can reaching a certain airtightness rating be included in the contract with the builder?

Thanks

Yes, as mentioned above. Set a target the Builder has to achieve, a decent builder up to current speed with techniques should easily hit 1.0. Reality you should be aiming for this or lower.

mrsWhippy

Is a block build an option when going for these airtightness ratings? Or is TF/SIP preferable?

Would there be any benefits of going down the block route?

MicktheMan

Block built is just as good as tf/sip once one knows what they are doing.

Block options are block on flat with ewi or wide insulated cavity. Both sealed on the inside with air tight plaster and junction detailing as usual. The advantage here can be the extra thermal mass allowing for easier control on internal temperatures especially with low grade heating solutions such as u/f.

mrsWhippy

That's interesting, thanks. I had kinda dismissed block as an option tbh. How does it (generally) compare cost wise, compared to a standard TF construction?

galwaytt

MicktheMan wrote: »

Block built is just as good as tf/sip once one knows what they are doing.

Block options are block on flat with ewi or wide insulated cavity. Both sealed on the inside with air tight plaster and junction detailing as usual. The advantage here can be the extra thermal mass allowing for easier control on internal temperatures especially with low grade heating solutions such as u/f.

Your first line is spot on. But also illustrates the primary advantage of (any) offsite system: objective quality of build.

Thermal mass is sufficiently delivered in an offsite system through screeds etc. A big mass in a temperate climate like ours is actually a disadvantage not an advantage.

One advantage of a factory system - any factory one - is that it's thermal/airtightness properties are not down to the subjective efforts of a system built onsite because it's built in a factory environment.

Currently factory built does not carry any premium over masonry say, but due to shorter time onsite can actually work out cheaper.

ht9zni1gs28crp

Again its subjective to quality of build on both sides. Ive seen some shocking blockwork and some shocking TF erections, on the other side I've seen shining light examples of both.

galwaytt wrote: »

A big mass in a temperate climate like ours is actually a disadvantage not an advantage.

Interesting concept, can you expand on your reasons?

mrsWhippy

Not to put words in galwaytt's mouth, but I've actually read this before on the subject of underfloor heating. Because our weather is so changeable, having a high thermal mass in the house means it's more difficult to heat and cool the house as fast as the weather is changing. You'll end up roasting hot if the we have a warmer spell after a cooler one, and conversely freezing if the temperature suddenly drops as it takes hours (maybe days) to get back up to the required temperature.

I can see how UFH along with a block build would be really advantageous in climates where there are actual predictable seasons.

On that note, we're planning to install UFH but haven't decided whether we want it upstairs or not .....am I correct in saying that we can only use concrete screed upstairs in a SIP or block build, but not a standard TF build?

Jees, I really need a book on how to build a house or something .....

BryanF

mrsWhippy wrote: »

Not to put words in galwaytt's mouth, but I've actually read this before on the subject of underfloor heating. Because our weather is so changeable, having a high thermal mass in the house means it's more difficult to heat and cool the house as fast as the weather is changing. You'll end up roasting hot if the we have a warmer spell after a cooler one, and conversely freezing if the temperature suddenly drops as it takes hours (maybe days) to get back up to the required temperature.

I can see how UFH along with a block build would be really advantageous in climates where there are actual predictable seasons.

On that note, we're planning to install UFH but haven't decided whether we want it upstairs or not .....am I correct in saying that we can only use concrete screed upstairs in a SIP or block build, but not a standard TF build?

Jees, I really need a book on how to build a house or something .....

But if you build the fabric envelope correctly the external temp shouldn't have an immedaite impact in the internal temp..

mrsWhippy

Don't quite get what you're saying BrianF.... thanks

ht9zni1gs28crp

Too much focussing on certain elements rather than the whole picture on how your house will react. Every house is different and at the moment your intimations are all guesswork and based on hearsay and others views which may or may not be correct.

Whoever wrote that it takes ages to heat up or ages to cool down must be over emphasizing it or currently lives in a house with a very poor envelope....

mrsWhippy

Which is why I'm looking into getting an energy consultant on board .....!

Calahonda52

mrsWhippy wrote: »

Don't quite get what you're saying BrianF.... thanks

What is at issue here is that if the building fabric is designed properly u should be able to maintain say 22 degrees c 24/7/365 if required, regardless of outside weather.

One example from the cheap and cheerful converted dormer world.

If you don't include considering decrement delay in the insulation build up in the roof, some insulation will not slow the intense heat radiated from the roof tiles heated by the summer sun as well as others.