Loft Insulation / boarding - not such a simple science

[oaklands4]

I have a trussed roof space in my 20 year old house which has floor and ceiling joists 70mm deep set at 600mm apart. Currently the floor is filled with a fibreglass blanket to the height of the joists (only 70mm).

My "problem" is to increase the insulation to meet building regulations and also to board the majority of the central loft space with suitable boarding so I can better use it as a storage space. I intend to board the useable space, an area which will measure 5000mm by 7200mm (12 joists). This will leave an area around the boarded space which is not usable for storage anyway because it is too close to the eaves.

Progress so far:

1. I can't simply board over the existing insulation as it is insufficient.

2. From the above measurements, the perimeter is 24.4m and the area is 36m2. Therefore the perimeter/area ratio is 0.677.

3. The building regs mean i am looking to achieve a u value design of 0.16. (I never understand why people keep saying 270mm depth because doesn't it depend what material one is using and their area/perimeter ratio?)

4. Using the calculator on the Celotex website I calculate that I will need around 110mm thickness of Celotex GA3000 which comes in boards that are designed to cut to fit 600mm spaced joists. Does this calculation seem right?

OK, that's about where I have got up to so far. Moving on, the plan is:

1. Take out the existing blanket and replace with 50mm thick Celotex (or equivalent).

2. Board over this.

3. Use the removed fibreglass blanket to top up the surrounding 70mm fibreglass blanket to, say, 210mm (leaving ventilation space at the eaves).

Now, coming to the crux of the matter as I am still 60mm thickness of insulation short.

1. Could I fit further 60mm thick Celotex between the roof joists and would this give me the desired result???
i.e. 50mm between the floor joists plus 60mm between the roof joists.

2. I have based my thinking on Celotex but is there any alternative that would better help me acheived my "simple" objective of a boarded loft that is insulated to building regs?

3. I have not considered the insulative properties of the floor boarding itself. How does this change the above calculations? Not sure what my joiner is going to use?

Would appreciate any advice on any of the above from someone who knows what they are talking about (as I clearly don't!)

Many thanks.

 

[JohnD]

have you considered counterbattens over your existing trusses? You can then lay your extra insulation between them, and board over the top.

It is also possible to put rigid insulation boards on top of the timbers, with chipboard on top. If you were to walk on the insulation without boarding I imagine you would soon put your foot through it.

Counterbattens will spread the load while you are clambering about, but a trussed roof will never be strong. However if the trusses, the battens and the boards are firmly screwed together the deck will be more rigid. It will not be strong enough for heavy objects or as a habitable room.

the foam boards are more effective insulators per thickness than the fibreglass (and more expensive)

 

[oaklands4]

Many thanks for the reply.

I had considered counterbattens laid at 90 degrees to the existing joists but the joiner said it would be too difficult to fit these given the amount of trussing so if we were going to do this we would build up the joists by fitting the batterns exactly over the top of the existing joists (running in the same direction). Not sure i am too keen on this but would appreciate a view. Seems to me that it will add loading and reduce headroom.

Cost of fitting battens seems to be about the same as the cost of purchasing the Celotex so this is why i was going down this route. Would still appreciate a view if it is sensible to do half under the floor boarding and half on the roof - is this a sensible approach or not?

I like your suggestion of rigid insulation boards directly on top of the timbers with chipboard on top. I had not considered this. Can you suggest a specific product or two so I can look into this more? I assume the Celotex would not fulfill this role.

 

[JohnD]

I think Celotex or Kingspan would both do. They are just transmitting the weight from the flooring straight down to the timbers beneath. there is no support except where there are timbers. I have heard of it but never done it myself, someone else may know more. I don't know if you can get a bonded product.

 

[Chris2121]

I have 2 x 4 ceiling joists with 4" mineral wool laid between. I'm adding 100mm Celotex resting on the joists and will eventually lay boards over the Celotex. The only slight problems so far have been needing to pack up some of the joists to get them level, and having to add noggings to support the ends of the Celotex sheets.

When supported by the joists the Celotex is strong enough to bear my weight. It is very light and easy to cut.

 

[stevebamford]

Not sure about this approach.

I can see if you board on top of the celotex then the load is spread over the same surface area which is probably strong enough

But the underneath of the celotex is supported on a 2" wide joist. Just a gut feeling really but would the Celotex get crushed at this point (over time)

As I said nothing to back up the above but maybe others have a view on it

I'd go with JohnD's counterbatten suggestion - with celotex inbetween

 

[big-all]

Not sure about this approach.

I can see if you board on top of the celotex then the load is spread over the same surface area which is probably strong enough

But the underneath of the celotex is supported on a 2" wide joist. Just a gut feeling really but would the Celotex get crushed at this point (over time)

As I said nothing to back up the above but maybe others have a view on it

I'd go with JohnD's counterbatten suggestion - with celotex inbetween

pretty much my concernes except slightly worse than normal because the truss is likly to be 39mm wide and the spacings are 600

now celotect is fine with 100% support underneath and flooring grade on top
now in this case its 7.5% support or less than 1/15th

also with cross battoning because the floor is screwed to it so as the cross battons sag they can transfer load to boards several feet away somthing that cant happen if the floor is floating!!!

 

[Chris2121]

But the underneath of the celotex is supported on a 2" wide joist. Just a gut feeling really but would the Celotex get crushed at this point (over time)

My joists are spaced at 12" centres and there is no sign of this happenning so far. If it does become a problem I'll lift the Celotex (easy) and fit a layer of 6mm ply over the joists.

pretty much my concernes except slightly worse than normal because the truss is likly to be 39mm wide and the spacings are 600

Yes, I don't think laying Celotex without additional support would work in that case.

 

[Chris2121]

The compressive strength of Celotex GA3000 is 120 kPa (17.5 pounds force per square inch).

 

[loftboarders]

there's a number of customer reviews about a new product on the market called s***** - its recently been used at one of our rented houses to create some extra storage space in the loft, but it insulates as well

edited by moderator - various systems which use a rigid foam boar that you can fit loft flooring over, or has boarding attached to it