Insulation And Airtightness Queries.

iamlegend2008

Hi

I had a specialist in today to quote me for making the roof airtight in my 1800 sqft dormer and will get a quote in a few days. he suggested using 100mm rockwool in the rafters as the preferred insulation for the membrane.

He also explained that since the house was not designed with airtighness in mind from the start that there would always be leaks from around whether the first floor joists are anchored into the walls. He suggested that I get the plasters to put a quick skim on the blocks before fitting the insulated slabs (38+12.5) which would help make the walls airtight and allow an area to tape the membrane too (rather than just the block).

My questions are:

(a) Is airtightness all or nothing, ie will i achieve anything by installing the membrane if there are still some leaky areas in the walls ? (I would not go scuba diving in a wetsuit with a leak )

(b) The rockwool people have informed me that even 150mm of rockwool (which would leave no air breather space) but not achieve regs and so are recommending insulated slabs underneath the rafters (if using rockwool). Is this a wise recommendation or am I looking at the wrong material here (given that my rafters are only 150mm) ?

(c) Can the breathable membrane be used with a solution such as xtratherm rafterloc (or alike) which would allow me to meet the 0.2 regs for pitched roofs and still have 25-50mm of air space (depending on whether U used the 100 or 125mm version of rafterloc)

Any pointers would be welcome - Cheers

Retro-Fit

You might get a shock at the quote. many people are using taped OSB board as a cheaper alternative to breathable vapour barrier. Was your expert independant or linked to a supplier?

On the insulation side, you need to build a eaves vent box to allow the air ventilate from the eaves to eaves over the insulationbut also dampen storm pressure. 100 mineral wool is not suppicient, you need rockwool board inside the rafters. I normally use osb inside the rafters and then create a 50mm services zone inside that with plasterboard on 2x2 battens, (run the cables through, tape, then cut kingspan to fit, then apply the plasterboard witha skim.) when this is too pricey, i just use the 50mm composite drylining board. When you get your airtightness under [EMAIL="3ach@50"]3ach@50[/EMAIL] pascals yove achieved a good result. Going any further you have diminishing returns for your investment.

You have a good solution with the air barrier, it will stop smelly air from the cooked felt and roof timbers entering the habitable space in summer, this can make dormers air quality quite nasty and will allow you to use heat recovery ventilation. I'd guess if you are a self builder you might find this option a little expensive. Let us know how you get on. Very interesting post.

Chimpster

I will try to quantify air tightness for you. The regulation for new houses is that house should have a permeability of 10m3/hr/m2 or less at a pressure difference of 50 pascals between inside and outside the house. If you can imagine 10m3 of air, well thats the amount of air per square metre of floors space that your house will leak over an hour.

Now this is not a accepted level of air tightness in my opinion. The best practive figure for permeability is to get the house down to less than 3 m3/hr/m2. Passive certification requires that you get this down to below 1 m3/hr/m2.

If you choose not to fit an air tightness membrance then it is very difficult to achieve anything below 6 m3/hr/m2. Especially if its a dormer build. To get down to the 3 level or below, all rwindows should be taped using a suitable air tight tape (Siga, Pro Clima, Vario), you should have no openfires, you should use room sealed stoves, you should use a membrane across the roof section from wall plate to wall plate and ideally the first floor wall junction should be sealed also, no letter box on door. You should also get a provisional air tightness test carried out before you plaster your house on the inside. This will allow you to see are you achieving the required 3 m3/hr/m2 and if not it will allow to rectify where the leaks are.

Is it worth it? Well it seems to be €15 per m2 to supply & fit these membranes from what I can see + extra to tapes windows. A provisonal and final air tightness test will set you back up to €1000. Put it this way, i'd invest in your air tightness before i'd consider a heat pump or a set of solar panels. Insulation is your main priority, once you have got all exposed surfaces to the required U Value then air tightness is your next priority. REDUCE THE HEAT DEMAND OF YOUR HOUSE.

Carlow52

Op: useful post here:)

[One thing that is not clear is whether yours is a new build or a finished house.]

The revised Part L published in March '08, show 2 different types of make-ups for the above roof type. Both have insulation between the rafters.

One obvious difference between the 2 types is that in one, the insulation that spans the rafters is outside (above) the rafters and in the other is inside (below) the rafters.

Regardless of which one you go for, both require the use of a vapour control layer (VCL) on the inner side.
This link gives more info on the purpose of the VCL, the main one being to reduce vapour laden air permeating into the insulation and condensing in the colder region.

http://www.visqueenbuilding.co.uk/product_detail.asp?id=58&sid=159

In your case the use of the insulated slab for the inside may provide you with an integrated VCL but the I would be happier to use a separate one between the inside across-rafter insulation and the plaster slab.

The inside across-rafter solution seems to be more popular for 2 reasons:
1: there seems to be some difficulty sourcing, at least in ROI, any outside across rafter insulation.
2: the inside solution allows the use of ordinary 'sarking' felt under the battens, as opposed to using a breathable roofing membrane.

Your point (c) refers to an air space with a breathable membrane. If you look at the Part L regs, u will see that the breathable membrane goes with the outside across-rafter solution and has no air gap requirement [UNDER the breathable membrane]
In contrast, the inside across-rafter solution uses a non-breathable sarking felt with an air gap between it and the insulation.

However this air gap must be ventilated at the eaves and apex so what some people do is use the breathable membrane instead of the sarking felt and dont ventilate the airspace at eaves and apex. Whether this is 'within regs', I dont know.

In relation to 1, RETRO-fit mentions rockwool board but this seems to be only available in large quantities and not generally available.
Have you had any success in sourcing it?

Chimpster refers to a membrane across the roof from wall plate to wall plate: I presume this is a VCL and not a breathable membrane.

What I don't know is if a product that meets the VCL requirements is sufficient to act as a airtight barrier if installing a MHCR unit or is there a higher spec required.

To answer ur question about partial airtightness, I would think that every little helps in terms of saving on heat loss, but that to achieve a certain level of airtightness a fairly complete solution is required.

iamlegend2008

Hi Carlow52

My house has stood at first fix stage for 3.5 years as I was working away during this time (waits for the sound of people going what!!!!!!)

Therefore Im not quite sure whether strickly speaking I need to abide by the regs at the time that I received planning permission or the current regs.

The problem I face now is if I try to adhere to the current regs, the current structure of the house limits me to what I can achieve.

Anyhow, Im back to finish it now one way or another:)

Cheers

mickdw

iamlegend2008 wrote: »

Hi Carlow52

My house has stood at first fix stage for 3.5 years as I was working away during this time (waits for the sound of people going what!!!!!!)

Therefore Im not quite sure whether strickly speaking I need to abide by the regs at the time that I received planning permission or the current regs.

The problem I face now is if I try to adhere to the current regs, the current structure of the house limits me to what I can achieve.

Anyhow, Im back to finish it now one way or another:)

Cheers

There are transitional arrangements relating to all changes to regs.
If works commence before 30th June 2008 or if planning was applied for before 30th June 2008 and substantial works complete before june 2009, then the latest conservation of fuel & energy reg need not apply although all boiler installations after march 2008 must meet the proposed efficiency.
Also of course these transitional arrangement are not there to allow people build the shell of a house and then complete it 10 years later to a totally out of date standard. So grey area there and your engineer might well be of the opinion that you should meet the latest regs where possible

sinnerboy

Agree with much of the above . I would aim to beat regs - it's not in your long term interest to "dodge" them

Getting the barrier around the timbers running at right angle to the wall is tricky - not impossible . Run noggins in between the timbers - about 300mm off the inner wall face . Then turn the membrane from the wall face and across the underside of the timbers over head and tape to the cross noggins .

Then use the topside of the noggings to "start gain" at the next storey up

For the roof - if you have the head space - look at the cost of running 100 x 50 timbers across your rafters - to allow 250mm minerals wool fill ( U value 0.18 ) . Then breather felt over counter battens and battens - to create a 50mm vent space over the felt

You then need soffit vents - 25mm continous and apex vents - vent tiles at 1750mm c/c are less expensive than dry vent ridges

iamlegend2008

Thanks Guys.

Wasnt suggesting that I dodge the regs (well not now anyway:)) just pointing out that there is currently a small window where house designs have not changed to take account of the new standards (in my case due to my own delay in completing the build) which adds to the general issues in trying to meet the regs.

I will ask the question about membrane installers/prices in a seperate thread.

Im learning lots thanks to this forum