Roof Membrane Questions

Lemlin

Just about to start the rafters on the roof. Was wondering about the membrane -
I rang one insulation and membrane provider and they stated that I should go ahead with the roof and that they'd then pump insulation into the rafters and then put on the membrane when the house is watertight. They said there was no reason for them to call out now. This company were recommended to me by a SIGA rep.
I thought however that the membrane should be put on when the roofers are putting up the rafters. This installer is telling me to wait until the roof is on and house is watertight and they will then come and do the roof and all other membrane when the tradesmen are finished. I thought however that the roof was done now as the roofers are completing it and the other membrane would be done when all the first fixing is complete.
The roofers reckon they have seen the membrane done before and could wrap it onto the rafters. Should I just be buying rolls of normal SIGA membrane or what? That's if I should do the membrane now.

tred

Lemlin wrote: »

Just about to start the rafters on the roof. Was wondering about the membrane -
I rang one insulation and membrane provider and they stated that I should go ahead with the roof and that they'd then pump insulation into the rafters and then put on the membrane when the house is watertight. They said there was no reason for them to call out now. This company were recommended to me by a SIGA rep.
I thought however that the membrane should be put on when the roofers are putting up the rafters. This installer is telling me to wait until the roof is on and house is watertight and they will then come and do the roof and all other membrane when the tradesmen are finished. I thought however that the roof was done now as the roofers are completing it and the other membrane would be done when all the first fixing is complete.
The roofers reckon they have seen the membrane done before and could wrap it onto the rafters. Should I just be buying rolls of normal SIGA membrane or what? That's if I should do the membrane now.

are you talking about the air tight membrange internally or the actually breathable roof felt.

Lemlin

tred wrote: »

are you talking about the air tight membrange internally or the actually breathable roof felt.

Internal air tightness membrane.

gears

Sounds like the advise is correct to me. In most cases the external membrane (roofing felt in old money) would be put on first then insulate the interior and install your air-tight membrane as there would be no other way to install the insulation if it's a quilt or rigid board. On the other hand if your pumping something like cellulose you would certainly need both internal and external layers plus a 50mm air gap in place prior to pumping. Really it depends on how you plan to insulate.

tred

Lemlin wrote: »

Internal air tightness membrane.

Ok , and what about the wiring, wouldnt you want the wiring in place before you start putting in the membrane.

Lemlin

tred wrote: »

Ok , and what about the wiring, wouldnt you want the wiring in place before you start putting in the membrane.

Do you mean electrical wiring or what wiring?

tred

Lemlin wrote: »

Do you mean electrical wiring or what wiring?

well yeah, think of your electrcian coming along to wire the house and that membrane in place. I suppose it depends on ur implemenation as well. Once my wiring is in place, I am going to start tackling the membrane.

Lemlin

tred wrote: »

well yeah, think of your electrcian coming along to wire the house and that membrane in place. I suppose it depends on ur implemenation as well. Once my wiring is in place, I am going to start tackling the membrane.

I realise the walls can't be done but can the membrane of the roof not be completed as the roof is being done? Electrical wires up in the rafters will be minimal.

sydthebeat

Lemlin wrote: »

I realise the walls can't be done but can the membrane of the roof not be completed as the roof is being done? Electrical wires up in the rafters will be minimal.

the proper layout is to create a service cavity between the plasterboard and the membrane, by using say 2 x 2 battens under the rafters. no service tradesperson should be allowed puncture the membrane... all these issues should be designed out befoe they arrive on site

Lemlin

If I wasn't going with the sprayed insulation though and the roofers are installing rockwool or a similar type of insulation in the rafters, should the membrane not be installed before that? In other words, the rock wool goes over the membrane after the membrane has been installed to the rafters.

gears

Lemlin wrote: »

If I wasn't going with the sprayed insulation though and the roofers are installing rockwool or a similar type of insulation in the rafters, should the membrane not be installed before that? In other words, the rock wool goes over the membrane after the membrane has been installed to the rafters.

In a word, No. Air-tight membranes should be on the warm side of insulation to prevent condensation forming with-in the insulation.

sas

Lemlin wrote: »

If I wasn't going with the sprayed insulation though and the roofers are installing rockwool or a similar type of insulation in the rafters, should the membrane not be installed before that? In other words, the rock wool goes over the membrane after the membrane has been installed to the rafters.

Your descriptions are a tad confusing, are you suggesting that you want to do this?

1. The rafters are in place, nothing else.
2. The airtightness membrane is fitted below the rafters.
3. The rockwool is installed into the rafters from above i.e. supported by airtightness membrane.
4. The felt is installed and the roof slated etc.

If you are not suggesting this then how do plan on getting rockwool between the rafters once the membrane is installed?

To touch on other points raised here. You most certainly do not do any wiring before the membrane is installed. As Syd described, you create a service cavity on the room side (i.e. warm side) of the airtightness membrane
and run all wiring etc. in between timber battens. You could then choose to insulate also between the battens and then plaster slab over them.

The only time you install an airtightness membrane before insulation is when you are going to insulate with pumped cellulose. I know of no other situation when it makes sense to do this.

In a nutshell, you want your airtightness membrane exposed for the minimum amount of time with the minimum amount of construction work being done during this time. Otherwise it will get damaged and negate some of its benefit.

Hope this is clear\helpful.

Lemlin

sas wrote: »

Your descriptions are a tad confusing, are you suggesting that you want to do this?

1. The rafters are in place, nothing else.
2. The airtightness membrane is fitted below the rafters.
3. The rockwool is installed into the rafters from above i.e. supported by airtightness membrane.
4. The felt is installed and the roof slated etc.

If you are not suggesting this then how do plan on getting rockwool between the rafters once the membrane is installed?

To touch on other points raised here. You most certainly do not do any wiring before the membrane is installed. As Syd described, you create a service cavity on the room side (i.e. warm side) of the airtightness membrane
and run all wiring etc. in between timber battens. You could then choose to insulate also between the battens and then plaster slab over them.

The only time you install an airtightness membrane before insulation is when you are going to insulate with pumped cellulose. I know of no other situation when it makes sense to do this.

In a nutshell, you want your airtightness membrane exposed for the minimum amount of time with the minimum amount of construction work being done during this time. Otherwise it will get damaged and negate some of its benefit.

Hope this is clear\helpful.

I am talking about the inside of the attic where I plan to put the membrane. I'm now looking at going with foam insulation (if cost allows) but should this be put in first or should the membrane? One installer said to me the foam insulation is put on and then the membrane is put on after it.

From what the roofer was saying, his plan was to put in the membrane and then put the insulation on top of it. The plan was for 100mm of rockwool.

sas

Lemlin wrote: »

I am talking about the inside of the attic where I plan to put the membrane. I'm now looking at going with foam insulation (if cost allows) but should this be put in first or should the membrane? One installer said to me the foam insulation is put on and then the membrane is put on after it.

From what the roofer was saying, his plan was to put in the membrane and then put the insulation on top of it. The plan was for 100mm of rockwool.

I'm assuming you are planning on putting your insulation and airtightness on the slope of the rafters.

You can't install spray foam after membrane. The installer for the spray foam knows more about his system that your roofer in all likelihood.

You can't install rockwool between rafters if membrane is in place. How was your roofer planning on getting the insulation into place?

dathi

you are going to need a lot more than 100mm of rockwool to meet the building regs

Lemlin

dathi wrote: »

you are going to need a lot more than 100mm of rockwool to meet the building regs

What about 150mm and then an insulated slab?

sydthebeat

Lemlin wrote: »

What about 150mm and then an insulated slab?

lemlin, your just throwing specifications around willy-nilly now. do you not have a certifying architect / engineer on this build???

thats who these questions should be directed towards, as they will have to ultimately certify compliance. Any suggestions here are only that, suggested specifications.

it seems to me that you are only shooting in the dark hoping to hit on someone who'll give you an easy answer. You need someone who will do the actually calculations and tell you exactly what you need to include in your specification to comply with regulations, or better still, how to exceed min regs and future proof your home.

Lemlin

sydthebeat wrote: »

lemlin, your just throwing specifications around willy-nilly now. do you not have a certifying architect / engineer on this build???

thats who these questions should be directed towards, as they will have to ultimately certify compliance. Any suggestions here are only that, suggested specifications.

it seems to me that you are only shooting in the dark hoping to hit on someone who'll give you an easy answer. You need someone who will do the actually calculations and tell you exactly what you need to include in your specification to comply with regulations, or better still, how to exceed min regs and future proof your home.

I see your point and I've spoken to both the architect and builder.
The plan is to go with 120mm high density fibreglass insulation (which I've been told is better than rockwool).
This will then be covered with a 50mm insulated slab.

gears

Lemlin wrote: »

I see your point and I've spoken to both the architect and builder.
The plan is to go with 120mm high density fibreglass insulation (which I've been told is better than rockwool).
This will then be covered with a 50mm insulated slab.

I still think you've not got enought insulation to meet regs..

Lemlin

gears wrote: »

I still think you've not got enought insulation to meet regs..

There'll also be an airtightness membrane fitted.
120mm high denity is all I can fit into six inch rafters so would a thicker insulated slab be the recommendation then?

sydthebeat

Lemlin wrote: »

There'll also be an airtightness membrane fitted.
120mm high denity is all I can fit into six inch rafters so would a thicker insulated slab be the recommendation then?

you are required to have a 50mm gap between the felt and the insulation, this means 100mm is the most you can get into an 6" (150mm) rafter...

what does your architect suggest / require in order to certify?

Lemlin

sydthebeat wrote: »

you are required to have a 50mm gap between the felt and the insulation, this means 100mm is the most you can get into an 6" (150mm) rafter...

what does your architect suggest / require in order to certify?

Going to speak to him today. I might go with the spray foam. I've been told it costs about 80 euro a square metre. My attic area is 17 foot by 40 foot. Can't find a converter online but my architect has the square metre measurents so going to give him a call. Expecting a quote back on Monday anyway.

sas

How do you keep a 50mm air gap with spray foam?

Lemlin

sas wrote: »

How do you keep a 50mm air gap with spray foam?

Good point. I'm also a tad puzzled as an actual insulation expert told me I could get 150mm of rockwool into a 6 inch rafter. They never mentioned about any gap of 50mm being needed.

sydthebeat

Lemlin wrote: »

Good point. I'm also a tad puzzled as an actual insulation expert told me I could get 150mm of rockwool into a 6 inch rafter. They never mentioned about any gap of 50mm being needed.

'insulation expert'...???

Lemlin

sydthebeat wrote: »

'insulation expert'...???

Quick typing here in work. Anyway, the question stands that you were asked:

How do you keep a 50mm air gap with spray foam?
Also, why have no insulation company mentioned needing a 50mm air gap?

sydthebeat

Lemlin wrote: »

Quick typing here in work. Anyway, the question stands that you were asked:

How do you keep a 50mm air gap with spray foam?
Also, why have no insulation company mentioned needing a 50mm air gap?

#
maybe because insulation companies are not building regulation savy???

Lemlin

sydthebeat wrote: »

#
maybe because insulation companies are not building regulation savy???

So the three insulation companies I've spoken to are not regulation savy?

And you still haven't answered the question re the foam and the 50mm gap.

Just Bob

If you use beathable "felt" can the 50mm gap be left on between the slates and "felt"? eg use 2x2 battons to attach slates?

sydthebeat

Lemlin wrote: »

1. So the three insulation companies I've spoken to are not regulation savy?

2. And you still haven't answered the question re the foam and the 50mm gap.

1. from what you have posted....

2. you dont... but hey, i wouldnt go near the spray on foam anyway....

Lemlin

sydthebeat wrote: »

1. from what you have posted....

2. you dont... but hey, i wouldnt go near the spray on foam anyway....

And here's the million dollar question - what would you do?