This is a retorical question, because I think it does.

We have built an extension on our 200 yr old cottage. Extension is thatched, with brick on the ground floor and weatherboarding on the first floor. The problem is on the 1st floor.

Behind the weatherboarding there is a flame-resistant tarpaulin material covering chipboard (I think it's chipboard, could be wrong). Walls are insulated with 4 - 10 inches of rock wool or glass wool.

There's an oak frame upstairs, visible on the inside but not the outside.

The builder is now in the process of putting plasterboard on the inside of the upstairs walls and sloping ceilings.

I have just noticed that when I pull out a handfull of rock wool just behind the eaves I can see daylight. There are up to 1 mm gaps around the ends of the rafters, especially around the eyebrow windows. There is no seal with eg polyurethane foam.

I would expect that when the wind blows on a cold winter night, cold air will come in the cracks and blow through the rock wool, drastically reducing the effectiveness of the insulation.

I can appreciate that is may be difficult to seal the irregular joins around the windows, but isn't this absolutely essential if we are to stay warm? It's like installing brilliant insulation in your house then opening the windows. If bulk air is moving through the insulation it is clearly not going to do much.

The builder is a prickly bastard so this will probably cause a row, but I've got to stand up to him haven't I?

I haven't yet spoken to building regs but I'm angry that they seem to be waving this kind of thing through. They are doing it right on paper, but not making it work in practice - which is completely pointless and stupid.

I'd appreciate any comments, in particular do you agree that sealing to the outside is essential?

This make up does not sound good. The chipboard you describe is mostly likely OSB or oriented strand board and is often used as a sheathing material that is cheaper than and not as good as ply but still relatively effective. This CAN be used to create an airtight seal if it is sealed during the build with a gunned silicon type material and sometimes tape, but most builders wouldn't even consider doing this. What colour is the 'tarp' material? Does it appear woven? This would be your rain screen/ breathable felt, mostly likely a 'Tyvek' material which most modern roofs use now to provide breathability to allow moisture out of a building in lieu of ventilation and there-by prevent interstitial condensation. Usually this would need to have an un-vented air gap beneath it so not be in contact with the insulation in order for it to work properly of around 50mm. Rockwool of between 4 and 10 inches sounds woefully inadequate not only in thickness but also in choice of material. It is likely positioned between the timber structure? Is this the same stuff you get in rolls for loft insulation? Is there going to be a vapour barrier internally or some 'board' insulation like Celotex which can act as a vapour barrier internally over the timber structure prior to the plasterboarding? Sorry for the load of questions. I am not sure what you mean about the eaves and the eyebrow windows I am afraid, although it would be normal for windows to be silicon sealed as a completion item with Building Control.

Thank you very much for your comments

I spoke to my builder this morning. He says the timber "can't" be sealed any better, and this is normal building practice.

> I am not sure what you mean about the eaves.

When you look from the outside you can see the cut-off ends of the rafters. most of the leaks are around these, especially where the thatch slopes down on either side of the windows.


> Usually this would need to have an un-vented air gap beneath it so not be in contact with the insulation
> in order for it to work properly of around 50mm.

What's an un-vented air gap - how long and wide is it? Can wind whistle through it on a cold night?

The tarp is woven.

My point is that 1 inch of rockwool that is sealed would be better than 12 inches where cold air blows through it. A vapor barrier inside the insulation will not help with that.

The rock wool comes in batts but is similar to the rolls.

We have multifoil over the inside of the sloping ceilings So that is a bit of a vapor barrier. But that does not cover the side-walls which are about 60 mm high. There is Celotex in places, but I believe that needs to be jammed in to work well in eg holes in the oak frame. It is a bit loose.

Is there some kind of insulation, that can fill a space like rock wool, that has closed cells, i.e. air can't move through it? I'm thinking eg broken up sponge mattresses.

David and others, thank you very much for your comments.

> I would be tempted to pull out the Rockwool, seal the existing timber based panel to rafters, wall plates, etc,

How would you seal it? With a polyurethane foam gun? By stapling a membrane to the timber, or with tape? (Last two hard to do at this stage.)

I would agree with what David has said but beg to differ with regard to the airgap below a breathable membrane. Building Regs AD C refers to BS 5250:2002 which requires a 50mm airgap below them, although most BBA certs will allow no gap what so ever. Practically BS 5250:2002 I think is due an update and the new one is likely to concentrate on Robust details specifically those items David has outlined above and allow the laying of the breathable membrane directly over another material such as your insulation or a sarking board. The long and short is... I agree with Nick.. sorry....David:)

When OSB is placed you can silicon edges to ensure a seal is made, and a form of silicon maybe better than your expanding foam as David has outlined above for your timber timber connections especially if they are possibly going to be viewable from outside.

If your Tri-Iso has been properly installed then it will form a very good vapour barrier. The low wall however should either have a similar treatment, Celotex or similar PUR/PIR board on the room side of the studwork, all joints taped or another vapour barrier of some kind. The whole idea as I am sure you are aware is to stop mositure laden air getting into the structure and to allow any that does to vent or 'breathe' to external, all the while stopping the free flow of air which will detract from your 'U'-Value depending upon where the insulation and the air is getting to.

The inclusion of vent tiles in roofs is still a good idea to avoid damage to rafters and roof structure from condensation but isnt so good for insulation purposes. The principles are the same in your roof. As long as any moisture has a path out of your building through the use of the correct materials then stopping unwanted air leakage will be a good thing.

Posted By: mattwpriceBuilding Regs AD C refers to BS 5250:2002 which requires a 50mm airgap below them, although most BBA certs will allow no gap what so ever.

What role is the air gap supposed to perform? Isn't it just there for historical reasons?

A number of breather membrane BBA certificates are very specific about avoiding unventilated voids beneath sarking boards. I think the reason for this is that the air in the unventilated void will be prone to huge variations in temperature between day & night.

In the day the roof will tend to get hot & atmospheric moisture will be driven into the unventilated void over many hours. When night falls, the temperature in the roof will drop very quickly & on a clear night sky it can drop well below outside air temperature. This will lead to atmospheric moisture condensing in the unventilated air gap.

Under certain weather conditions, this can become a continual source of condensation in the gap. This will clear as atmospheric conditions change, but is an unnecessary risk. Avoiding the gap & placing the insulation tight up against the underside of the breather membrane reduces or avoids the problem altogether.

I think the 50mm gap comes from a failure to understand breather membranes (that they are 2 way breathers & don't require air gaps), a reluctance to change long standing ventilation advice/practice & a concern for ensuring that the membrane can be draped, so that water can drain under tile battens.

Put it another way, why would an air gap be required under a breathable sarking membrane on a timber framed roof when it isn't required under a breathable sheathing membrane on a timber framed wall?

David