applying two types of roof insulation simultaneously, but in which order? woodfibre & mineral wool

[mgo]

I am starting to insulate the roof (consisting of slates and felt) of an old stone cottage that currently has no insulation.

.

One layer of either woodfibre or rock/mineral wool placed between the rafters (12cm deep) is not going to be enough in terms of insulation value, and while the rafters are only 12 cm deep, i will have to install two layers in a crossed manner.

I am now contemplating about a different second layer of insulation on the inside, because of the cost of woodfibre, i thought of rockwool (dampopen).

 

Is it ok to use two materials back to back and then finish with a vpl layer?

 

If so, what would be the best order: rockwool between the rafters (on the outside), then wood fibre on the inside, followed by a vpl layer, or the other way around?

assuming both materials can be installed directly against each other without any cavity.

 

Are there any negative effects i am missing with using two damp-open materials on top of each other?

 

the finishing layer is either going to be the original wooden planks (if we can restore them) or lime plaster.

 

Thanks

[Iceverge]

Any pics, especially of the eves area and the backside of the felt.

 

Ideally you need some ventilation of the rafters so that any vapour that escapes from your house can dry harmlessly and condensate on the timbers causing decay. 

 

Normal practice is to ventilate above the roofing insulation but below the felt. However if you have a semi permeable roof felt you may not need any ventilation space. A well sealed VCL will be critical then however. 

 

Pics would help. 

 

 

 

 

 

 

 

 

 

 

[Roger440]

34 minutes ago, mgo said:

I am starting to insulate the roof (consisting of slates and felt) of an old stone cottage that currently has no insulation.

.

One layer of either woodfibre or rock/mineral wool placed between the rafters (12cm deep) is not going to be enough in terms of insulation value, and while the rafters are only 12 cm deep, i will have to install two layers in a crossed manner.

I am now contemplating about a different second layer of insulation on the inside, because of the cost of woodfibre, i thought of rockwool (dampopen).

 

Is it ok to use two materials back to back and then finish with a vpl layer?

 

If so, what would be the best order: rockwool between the rafters (on the outside), then wood fibre on the inside, followed by a vpl layer, or the other way around?

assuming both materials can be installed directly against each other without any cavity.

 

Are there any negative effects i am missing with using two damp-open materials on top of each other?

 

the finishing layer is either going to be the original wooden planks (if we can restore them) or lime plaster.

 

Thanks

 

Im not quite sure i follow what you are doing? Why are you using woodfibre specifically? Why not just rockwool?

 

Rule of thumb, most impermeable layer inside (Vapour control layer) most permeable furthest outside.

 

As iceverge say, dont block the ventilation path.

[SteamyTea]

10 hours ago, Roger440 said:

Rule of thumb, most impermeable layer inside (Vapour control layer) most permeable furthest outside.

All depends which side the warm, wet, air is.

Trouble with a loft is it can quickly swing between both.

 

So, is this a warm or a cold roof?

[mgo]

Thank you all for your quick reply,

 

i have a few pictures, we took down the skim plasterboard walls and ceilings, which showed back the old ceiling still in place

 

I dont know how to check what type of felt has been used, i would asume a vapour open, but am not 100% sure, no brand name is visible.

The roof continues at an 45 degree angle, so there is +/- 120cm above the middle part of the old ceiling. We will open this up this afternoon hopefully.

The roof exists further of slates on wooden battens, no solid layer is in between this and the outside as far as i can see.

 

Wood fibre i am attracted to because of its moisture control characteristics.

Now that we can see the wall situation, i am also wondering to install woodfibre against / between the wooden frame that has become visible.

[mgo]

here you can see the wall / ceiling section

[mgo]

here is a detail of the roof felt, would anybody have an idea what type this could be? I dont see any brandname anywhere

 

outside of this there is 3-4 cm cavity and then the back of the slates

 

 

[mgo]

Here a quick update of the work so far

 

we used 50mm woodfibre between the rafters, and a second 100mm layer (to be installed).

 

The rafters are 110mm deep, giving about 50-60 mm ventilation between the woodfibre and felt. I will need to open up the eaves, as i dont see any particular ventilation system in place. Yet we are in one of the windiest locations in Ireland, so there should be a bit of a flow at al times. I left a gap at the top of the rafters above the collar ties, to provide ventilation between both sides, and insulated below this.