Renovating an old loft conversion, insulation and air gap

[baz08]

Hi, i have bought an old house approx 1907 built, it has had the loft converted on it in the 1970s by the previous owner with limited insuation in place.

I'm in the process of renovating the entire house and have removed all of the old plasterboard and insulation.

The rafters are 75mm x 50mm i want to use Celotex or similar insulation but the manufacturers instructions advise that there needs to be an air gap of 50mm minimum. I only have 75mm to play with so this would leave only 25mm for insulation, i am limited to how much insulation i can place under the rafters as the bedroom door will hit the plasterboard.

Is it possible to leave a minimum air gap of 25mm and insulate with Celotex then insulated plasterboard under the rafters to the depth access will allow or is the 50mm air gap an absolute must have?

 

[freddiemercurystwin]

Is there any felt under the tiles?

Even 50mm Celotex is not that much. If you were to do a loft conversion nowadays you would do something like 90mm between and continuous layer of 40mm under the rafters.

Whatever you cannot fit between the rafters should go underneath, or thereabouts.

BTW separate insulation and then foil backed plasterboard rather than insulated PB may be more economical to buy though more labour intensive to fit.

 

[baz08]

Yeah there is a bituminous felt underthe tiles non breathable. Ithink the most I could get away with on the fron part of house would be about 2.5inches of insulation/plasterboard. It's easier on the back as second bedroom has more space for under rafter insulation/plasterboard

 

[Lucasdemoley]

http://www.celotex.co.uk/applications/pitched-roof-insulation/between-and-under-rafters

Link to celetex specification for insulation.

 

[freddiemercurystwin]

In that case unless you are going to fit continuous ridge ventilation you will make the rafters susceptible to damage from condensation. Your gap (which should be 50mm) should be ventilated at the top and bottom of the pitch.

 

[baz08]

I meant 2.5inches of insulation/plasterboard underneath the rafters, i haven't made any determinations about how much to use between the rafters hence my question here. I've read the manufacturers instructions which state 50mm gap but i wondered if it was possible to make do with less than this.

The roof is not ventilated in any way at the moment, there are no ridge vents and the fascia board is unvented it is the original wooden fascia board.

 

[freddiemercurystwin]

Are you prepared to fit continuous ridge ventilation and also ventilation at the eaves? As previously mentioned, if not then you run a risk of damage to the rafters. A 50mm gap is recommended to allow adequate airflow as there will inevitably be a bit of slack in the membrane. But if you don't bother with the ventilation then you won't get any airflow anyway.

 

[baz08]

Well i can't afford to have new facias and ridge vents as i don't have the money to finish the rest of the house which has been gutted top to toe.

Naive and possibly stupid question but please humour me, if there is no ventilation to start with, does there need to be a gap at all?

 

[freddiemercurystwin]

No it's a reasonable question, it might be ok it might not. Lofts and condensation are a funny old pairing, sometimes where you would not expect problems you do and vice versa. That's not to say you shouldn't do what you can to minimize the risks. What you can do is get some timber wedges and stick them in between the laps of the felt to open up the laps to create some ventilation.

 

[baz08]

Thanks for the advice. This insulation business is far more troublesome than i was anticipating it would be. I'll need to have a think about the best course to go down as last thing i want to do is fill up with insulation all the way then end up with damp patches or worse

 

[baz08]

Just out of interest, has anyone actually had an unventilated roofspace with non breathable felt and filled the entire rafter space with insulation? If so were there or have you had any side affects of this?

 

[freddiemercurystwin]

There are hundreds of threads on the forum with people who have condensation problems due to a lack of ventilation.

 

[tony1851]

Just out of interest, has anyone actually had an unventilated roofspace with non breathable felt and filled the entire rafter space with insulation? If so were there or have you had any side affects of this?

Roofs are fickle; do NOT risk compromising ventilation. Two-inch is what they recommend, and two inches it should be.
Also, do NOT forget the vapour barrier on the warm side.
I say this through personal - and expensive - experience.

 

[Cabbyfan]

I'm doing the same job on my loft soon, BC insist I have ridge tile vents and an airgap of I keep the current old felt. I'm very tempted to take the files off install breathable membrane so I get more insulation etc.

 

[geraldthehamster]

I did what you're doing, in the first floor of a chalet/dormer bungalow.

I fixed 3x2 inch battens to the underside of the 3x2 inch rafters, giving me 6 inches of depth, sufficient for a 50mm air gap, 100mm of Celotex between, and 25mm continuous Celotex underneath, which is what BC asked for.

Cheers
Richard