starting to freak out

Para1

Hi everyone,

I havent been on boards that recently and I now find myself a bit freaked out about what iv been reading. I'm currently a bit into a new build in West Galway on a 3000 square foot two story house. The reason I'm a bit freaked out is because it's only now come to my attention that having an insulated cavity and an internal insulated board may cause condensation!?! Below is the spec agreed with the engineer and we are up to wall plate level already :

150mm cavity with 100mm PIR
200mm floor insulation
200mm roof insulation
37mm insulated board internally
hollow core
oil central heating zoned with radiators
heat recovery system
Airtightness of 2
triple glazed windows with insulated frames
Pv panels on southfacing roof

I was assured that this would be adequate to achieve a good BER cert and make the house fairly good to run.

Is that the case?
Is there anything I can change to improve it and reduce the risk of condensation problems down the line?

Thanks

BarneyMc

Can you not up the width of the insulation in your cavity or have you already started installing it? There's 150mm full fill I think but maybe West Galway is not the place to be doing this (driving rain)? Either way I think you're safe enough as the 37mm internal board is still considerably less than your 100mm cavity insulation.

sydthebeat

there is a rough "rule of thumb" which says that you shouldnt have more than a 33% insulation on the inner face... which would mean that youre 'probably' ok.... as you have 25% internal.
but the only way to know for sure is to have a dynamic condensation risk analysis carried out (or wait to see what happens)

by the way, id suggest it will be quite difficult to get a airtight result of 2 with that spec... but can you please post here to let us know how you get on with it?

Calahonda52

OP
Re this
Airtightness of 2
Is this written into the contract with the builder?
What is the detail for the A/T layer?

Barney, he is at wall plate already

Metric Tensor

A few thoughts:

1. The air-tightness detailing will be difficult with the dry-lining board. Check the spec with the engineer and builder. There needs to be an air-tightness layer behind the baords - this could be a membrane or a wet (i.e. plaster) proposal. You need to limit any penetrations through this plaster.

2. If you are buying the cavity insulation and the dry-lining board from the same supplier the technical dept. of that supplier might carry out an interstitial condensation analysis for you if you ask nicely.

Para1

Thanks for the replies.

We're already up to wall plate level so changing the cavity insulation is not really an option. We had initially looked at the 150mm full fill but we were told that it was best to leave an air break in the cavity, I think this was due to area and climate.

The plan is to do a scratch coat of plaster behind the insulated board however I'm not sure what the method of fixing is being used, any recommendations?

As regards to the Airtightness , we are a self build so we don't have a main building contractor. I was in touch with an airtightness company and he reckons 2 should be achievable? A friend recently did his house using the same company and achieved an airtightness of two which was certified by a separate company. Only difference in our house spec was he had 150mm full fill in the cavity. Maybe that would have a big bearing?

chillit

did you do a provisional BER? Some county councils are now asking for this when you submit your commencement notice

Para1

chillit wrote: »

did you do a provisional BER? Some county councils are now asking for this when you submit your commencement notice

No there was nothing said about that, we got the site with full planning and the building had already started some years back so the commencement notice was given before the new regulations. It only got as far as the raft before work stopped, we just took it up from there.

sydthebeat

chillit wrote: »

did you do a provisional BER? Some county councils are now asking for this when you submit your commencement notice

can you begin to call this by an alternative name... because a provisional BER is something different entirely

a preliminary DEAP assessment is more accurate

chillit

sydthebeat wrote: »

can you begin to call this by an alternative name... because a provisional BER is something different entirely

a preliminary DEAP assessment is more accurate

fair point

Metric Tensor

OP you need to get a preliminary DEAP assessment done as quickly as possible to help you alter any remaining items if necessary to make your house meet the EPC and CPC requirements of technical guidance document L. Ask your engineer ... there's a possibility he did that when he was helping you put the spec together at the start.

As an aside - have you gotten certification for the foundation that was already there when you bought the site?

MicktheMan

Did you wrap the hollowcore for airtightness continuity?

Para1

MicktheMan wrote: »

Did you wrap the hollowcore for airtightness continuity?

Yup that's covered. Was speaking to my airtightness fella today and he reckons we'll have no problems getting 2. Am I missing something?

Para1

Metric Tensor wrote: »

OP you need to get a preliminary DEAP assessment done as quickly as possible to help you alter any remaining items if necessary to make your house meet the EPC and CPC requirements of technical guidance document L. Ask your engineer ... there's a possibility he did that when he was helping you put the spec together at the start.

As an aside - have you gotten certification for the foundation that was already there when you bought the site?

Still waiting to hear back from him on this. As for the foundation it was done by father in law and was checked by engineer before we started building on it.

MicktheMan

Para1 wrote: »

Yup that's covered.

Good.

Para1 wrote: »

Was speaking to my airtightness fella today and he reckons we'll have no problems getting 2. Am I missing something?

Assuming your "airtightness fella" is who will be testing the house, then I would rely on his advice.