Looking for loft Insulation advice....

[Adam182]

Hey Everyone, I'm looking to insulate my loft but wondering what peoples thoughts were on the best way about it. The finish will be plasterboard and skim.

Currently beneath the tiles it has a rough black looking material with about 25mm glasswool stuck onto the back in the most parts (see images) some areas just have the black material.

My original idea was to go with kingspan inbetween the rafters but I understand it needs a 50mm gap behind for breathing, im not sure this is possible with the current glasswool bieng there? would it be ok to have both? Glasswool>Gap>Kingspan.

It would seem kind of wasteful to tear off all the glasswool as it appears to be in good condition.

Another option would be to fill it out with roughly a further 100m of glasswool as I believe an air gap isnt required when using glasswool?

Any feedback is greatly appreciated!

 

[Alastairreid]

The black material is type 1f roofing felt which is tar based and is not breathable.
Dont know what your plans are for the roof space but if you plan on making it habitable theres a lot more than insulation to consider.
Anyway you need to loose the glasswool and increase the depth of the rafters to at least 150mm if you do not already have that.
50mm air gap between the felt underlay and top of insulation board, continuous ventilation high and low level between eave and ridge.
70mm insulated plasterboard to underside of rafters.
To achieve your continuous ventilation you will need either over facia vents, tile or soffit vents for low level and a vented ridge or vent tiles high level.
And a vapour barrier between plasterboard and insulation.

 

[Adam182]

The black material is type 1f roofing felt which is tar based and is not breathable.
Dont know what your plans are for the roof space but if you plan on making it habitable theres a lot more than insulation to consider.

Good job you mentioned this! I was planning on having it as "comfortable storage" so skimmed walls and a couple of downlights. It has a small-ish velux fitted already if that makes any difference.

Anyway you need to loose the glasswool and increase the depth of the rafters to at least 150mm if you do not already have that.

Thats a shame but if thats what it requires that is what I will do.

50mm between the felt underlay and top of insulation board, continuous ventilation high and low level between eave and ridge.

Does this mean a 50mm gap?.

70mm insulated plasterboard to underside of rafters.

What goes between the rafters at the back of the insulated PB and the now 150mm void upto the felt I have created?

To achieve your continuous ventilation you will need either over facia vents, tile or soffit vents for low level and a vented ridge or vent tiles high level.

Will it require a vent tile/vented ridge (high level) and soffit vent (low level) per rafter gap?

And a vapour barrier between plasterboard and insulation.

will vapour check insulated PB suffice for this?

Thankyou for your time, much appreciated.

 

[Alastairreid]

50mm for an airgap adam, this will be continuous between eaves and ridge to dissipate any moisture.

If its just some neat storage space it doesnt have to comply so the insulation between the rafters can be what you want as long as you leave an air gap...and fit a vcl to the underside of rafters before plasterboarding....

if you have a soffit you can fit a continuous vent strip or an over facia vent.

duplex foil backed board will suffice.

The idea is to deal with the moisture which enters the loft from the house and condenses on the first cold spot it meets..in your case the tar felt...does the glasswool feel damp?

 

[Adam182]

50mm for an airgap adam, this will be continuous between eaves and ridge to dissipate any moisture.

If its just some neat storage space it doesnt have to comply so the insulation between the rafters can be what you want as long as you leave an air gap...and fit a vcl to the underside of rafters before plasterboarding....

Yep thats pretty much what it will be, apologies for not stating this from the beginning.

if you have a soffit you can fit a continuous vent strip or an over facia vent.

Its currently just got what I believe are called ''open eaves'' but all the guttering will be getting replaced (rotten cast iron) and if its possible I would like to add soffits as i prefer the look.

duplex foil backed board will suffice.

The idea is to deal with the moisture which enters the loft from the house and condenses on the first cold spot it meets..in your case the tar felt...does the glasswool feel damp?

The glasswool is dry to the touch but the house hasn't been lived in for some time now as I'm doing a full renovation and living elsewhere, there are what I believe to be stains on some areas of the wood however which I believe is a sign.

I've been researching since you mentioned it and I think I have got my head around how it works, I've seen some things called ''lapvents'' that go inbetween the lap of the felt, I'm under the impression these wouldn't be as optimal as the options you suggested, am I right? I can only assume ridge venting would be the optimal method for the top level as it would cover the highest point of the house.

[/QUOTE]

Thanks again Alistair much appreciated.

 

[Alastairreid]

Opening the laps wouldnt do any harm but doubt it would be enough...Air enters at low level (eaves) and exits at high level (ridge) carrying with it any moisture which by passes your vcl. Thats the science and the proven method which works.

 

[Dhjones]

The Rockwool between the rafters has to go. I've no idea why it's been put there in the first place.
Rockwool out.
Fix some 2x2 to underside of all rafters. Treated timbers as a bonus. Foilboard insulation in between rafters, maintaining a 50mm gap between felt and insulation. Arguements sake, you've managed to use 75mm between. Nice tight fits and all gaps foamed up.
Under draw 50mm foilboard to face, try to cross joints with insulation between. 25mm could be used as a minimum.
Foam al gaps again.
Plasterboards fixed through insulation into rafters, again trying to cross the joints with the insulation on face.
The best tool for all this is a foam gun. Get the stuff squirted in every gap visible, twice!! A windy day will show that the foam gun deserves centre spot on the mantle!!

 

[Adam182]

The Rockwool between the rafters has to go. I've no idea why it's been put there in the first place.
Rockwool out.
Fix some 2x2 to underside of all rafters. Treated timbers as a bonus. Foilboard insulation in between rafters, maintaining a 50mm gap between felt and insulation. Arguements sake, you've managed to use 75mm between. Nice tight fits and all gaps foamed up.
Under draw 50mm foilboard to face, try to cross joints with insulation between. 25mm could be used as a minimum.
Foam al gaps again.
Plasterboards fixed through insulation into rafters, again trying to cross the joints with the insulation on face.
The best tool for all this is a foam gun. Get the stuff squirted in every gap visible, twice!! A windy day will show that the foam gun deserves centre spot on the mantle!!

Thanks for the feedback, much appreciated.

Would using foilboard and foaming + taping joints suffice as a vcl?

I've been thinking of purchasing a foam Gun for a while now since I started this full house renovation.

 

[Dhjones]

Thanks for the feedback, much appreciated.

Would using foilboard and foaming + taping joints suffice as a vcl?

I've been thinking of purchasing a foam Gun for a while now since I started this full house renovation.

What type of roofing felt is used? Old black bitumen type stuff or a breathable membrane?
Ventilation is the key.
I've never included a vpl. Let's look at it logically,,,, at what point does it go into the sandwich?? Whichever stage of layer it is, it's going to get peppered with screws holding your plasterboards and insulation in place. So your vpl is now Swiss cheese.
Just make a conscious effort of maintaining that 50mm void between ins. and felt. If breather felt, your jobs done, if not, some vents need installing at eaves and ridge point to create ventilation flow. Various are available to suit different situations.
Have fun. With the foam gun that is

 

[Adam182]

What type of roofing felt is used? Old black bitumen type stuff or a breathable membrane?
Ventilation is the key.
I've never included a vpl. Let's look at it logically,,,, at what point does it go into the sandwich?? Whichever stage of layer it is, it's going to get peppered with screws holding your plasterboards and insulation in place. So your vpl is now Swiss cheese.
Just make a conscious effort of maintaining that 50mm void between ins. and felt. If breather felt, your jobs done, if not, some vents need installing at eaves and ridge point to create ventilation flow. Various are available to suit different situations.
Have fun. With the foam gun that is

Thanks for the feedback you make a great point about the vcl being peppered with screws, and yeah its the bitumen type. Having gathered all the great advice on here I'm pretty confident in what is required now . Having said that this little loft section of the renovation may well be getting put on hold for now, just priced up the ground floor engineered wood/insulation cost for the UFH :/ :/ :/

 

[John D v2.0]

A screw is pretty vapour impermeable so as long as you don't go removing and replacing the screws in many different places you should be OK.

 

[Adam182]

Hi again everyone, drawn up a quick image, just wondering if the celotex between rafters should continue behind the kneewall in the loft and down to the eaves...

 

[Lower]

Why are you insulating between the rafters? Are you planning to try and use the loft and need it to be warm?

If you're not planning to use the loft you're just going to end up paying to heat the loft as well as the rest of the house.

 

[Adam182]

Why are you insulating between the rafters? Are you planning to try and use the loft and need it to be warm?

If you're not planning to use the loft you're just going to end up paying to heat the loft as well as the rest of the house.

Im most likely going to have a few retro consoles up there and a TV on the wall. No furniture just a bean bag to sit on.

I figured insulation needs to go somewhere so why not have it at the very top and have a nice tidy loft space.

 

[Dhjones]

Either insulate the knee wall and meet the insulation between the rafters coming down from the ridge, and then can leave out the ins. After the knee wall towards the eaves, or continuos insulation between the rafters from ridge to eaves and can leave out the knee wall insulation.