New build - Loft boarding and roof insulation

[RegPrescott1]

Hi all,

We are having a house built at the moment and I am about to start boarding out my loft and have a couple of questions. I am a complete novice so some of the questions may be very obvious to some!

I have searched through other posts on these subjects but can't find the exact info I need.

Here goes:

1. I aim to use 22mm thick boards, but how do I go about selecting which screws I need? ie what diameter and length should they be? The joists are 14cm deep so there is no problem regarding selecting a screw that is too long, but what sort of length should go through the boards to the joists?

2. When screwing the boards to the joists, obviously I want to use countersunk screws. What is the best way to countersink the boards? Is it simply a case of positioning the boards and then drilling through with one of these drill bits that has a countersink included?

3. Similar to question 2 above I need to batton the rafters to get the required depth for my insulation. The rafters are 18cm deep but I need insulation 26cm deep.

Thanks for any advice, all is welcome!

 

[Gobo]

1. I aim to use 22mm thick boards, but how do I go about selecting which screws I need? ie what diameter and length should they be? The joists are 14cm deep so there is no problem regarding selecting a screw that is too long, but what sort of length should go through the boards to the joists?

I would use size 8 length 50mm countersinking screws.

2. When screwing the boards to the joists, obviously I want to use countersunk screws. What is the best way to countersink the boards? Is it simply a case of positioning the boards and then drilling through with one of these drill bits that has a countersink included?

Pre drill 4mm holes through your 22mm board and the screw will sink its self in when tightened.

3. Similar to question 2 above I need to batton the rafters to get the required depth for my insulation. The rafters are 18cm deep but I need insulation 26cm deep.

You would need to baton it out a bit so not to put pressure on your roof tiles/covering. I take it your boarding the rafters too then? Normally the insulation would be around the joists and you would baton up the floor in the loft with 3x2. Otherwise you'll be loosing heat into the loft space from the rooms below.

Remember to sink any wiring etc into joists before placing and fixing boards.

Gob's

 

[RegPrescott1]

You would need to baton it out a bit so not to put pressure on your roof tiles/covering. I take it your boarding the rafters too then? Normally the insulation would be around the joists and you would baton up the floor in the loft with 3x2. Otherwise you'll be loosing heat into the loft space from the rooms below.

Remember to sink any wiring etc into joists before placing and fixing boards.

Gob's

Yes, as we are building an energy efficient house it is normal here in Germany to insulate between the rafters. So I have to do that anyway and have the "option" to do between the joists too, which I plan on doing.

As there is no wiring etc at the moment that isn't a problem

Thanks for your advice!

 

[RegPrescott1]

Well, so far so good. I have boarded out the loft floor and battoned on top of the rafters so that my 22cm insulation fits in. I am now in the stages of stappling the Damp-proof membrane over the first layer of insulation.

Are there any guides availabe to help with this? I have searched the net but can't find anything. I think I am doing it correctly but I was wondfering if there are any tips for instance in checking that the membrane is sealed along the walls and joins? I have been checking by just "eye-balling" it all over but is there any other method?

Reg

 

[freddiemercurystwin]

I am now in the stages of stappling the Damp-proof membrane over the first layer of insulation.

What? and Why?

 

[RegPrescott1]

I am now in the stages of stappling the Damp-proof membrane over the first layer of insulation.

What? and Why?

This stuff

http://www.eviee.co.uk/Isover-Vario-KM-Duplex-Membrane-.htm

 

[freddiemercurystwin]

Ok thats what you've bought but why do you wish to fit a membrane between the insulation layers, I've never seen it done or heard of it being done before and cannot think why you would need or want to do it. Who told you you need to do that and why?

Edit: is this on the roof pitch or the floor, having re-read your 1st post today it mentions the floor and the roof If installing between the roof insulation and the plasterboard on the roof then I understand why now? Any joints should be sealed with tape and against the edges of any structure like gable or party wall with a sealant. The manufacturer will probably sell a suitable tape and sealant although you could get them from any builders merchant.

 

[RegPrescott1]

Ok thats what you've bought but why do you wish to fit a membrane between the insulation layers, I've never seen it done or heard of it being done before and cannot think why you would need or want to do it. Who told you you need to do that and why?

Edit: is this on the roof pitch or the floor, having re-read your 1st post today it mentions the floor and the roof If installing between the roof insulation and the plasterboard on the roof then I understand why now? Any joints should be sealed with tape and against the edges of any structure like gable or party wall with a sealant. The manufacturer will probably sell a suitable tape and sealant although you could get them from any builders merchant.

It is on the roof ie the rafters. Basically, as we are building an energy efficient house I need 26cm of Rockwool or similar insulation. As the rafters are only 18cm "deep" I have battoned on top with an extra 4cm, which brings my available insulation depth up to 22cm. This I have filled with 22cm thick Rockwool. It seems to be standard practice here in Germany to install the membrane between the layers of insulation, although you can have a maximum of 20% of the total thickness of insulation that comes after the membrane (in my case 20% of 26cm = 5.2cm)

I will then batton on top of this with 4cm thick batton and fill that with 4cm thick Rockwool.

This picture shows the procedure quite well

http://www.meinhausordner.de/news/Artikel/images/isover-vario-effekt.jpg

 

[mointainwalker]

I don't know if you considered this, but polyurethane has a 60% higher insulation value than glass-fibre/Rockwool ( PU - U 0.25 for 100 mm, RW U 0.4 for 100 mm ) so you could have used 19 cm of PU to achieve same insulation value. It is also impervious to moisture so if T+G as some is, avoids needs for a membrane

Disadvantage is it would cost you approx three or four times as much.

 

[RegPrescott1]

I don't know if you considered this, but polyurethane has a 60% higher insulation value than glass-fibre/Rockwool ( PU - U 0.25 for 100 mm, RW U 0.4 for 100 mm ) so you could have used 19 cm of PU to achieve same insulation value. It is also impervious to moisture so if T+G as some is, avoids needs for a membrane

Disadvantage is it would cost you approx three or four times as much.

I did consider using polyurethane as I wasn't too keen on working with the dreaded glass/rock wool, but as you point out the main factor against it was cost.

The extra 7cm saved in room space would also have been nice but the rooms are large enough as is.

Regarding the membrane installation I still haven't found any helpful tips on the net. Apart from obviously making sure the membrane is installed the right way round, so that moisture can escape but not enter, I haven't found anything.

 

[freddiemercurystwin]

Regarding the membrane installation I still haven't found any helpful tips on the net. Apart from obviously making sure the membrane is installed the right way round, so that moisture can escape but not enter, I haven't found anything.

I don't quite know what 'tips' you want, its pretty straightforward surely?

 

[RegPrescott1]

Regarding the membrane installation I still haven't found any helpful tips on the net. Apart from obviously making sure the membrane is installed the right way round, so that moisture can escape but not enter, I haven't found anything.

I don't quite know what 'tips' you want, its pretty straightforward surely?

Well I've found a bit now. Stuff like 10cm overlap on joining pieces, 3cm extra between rafters (ie don't pull the membrane tight over them etc.)

Probably just me asking obvious questions but there doesn't seem to be too much to look out for.