Loft Hardflooring & Insulation Sanity check!

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Hello all,

I have spent the last 6 months on and off researching home efficiency improvements. I have read allot and have learnt allot, with a number of headaches along the route lol, but I still consider myself a newbie and I certainly still struggle with the building industry technical terms, so please excuse me if any of my wording confuses matters.

My first job was to clear out the loft, get rid of the old rockwool, re-jig the lighting ciruit wiring to lay below or along the joists so no electrical wiring is crossing over the joists and add a light up there.

Form that point I started thinking insulation......following some research I settled on a compromise and bought Thermafleece ECOGreen loft insulation (recycled plastic fibre wool). I layed 100mm between the joists across the whole loft space and then layed a further 150mm across the joists in the space behind the purlins where the roof pitch makes the space unsuable.

The centre space where the chimney stack is located currently only has 100mm and that space occupies 29m square. I would like to insulate this space but also make it walkable so house items can be stored there long term.

After allot of research on the subject I have again concluded on a compromise. Below I will outline my plan and I would like to hear some opinions on whether I am overlooking any serious detail, as I certainly do not wish conciously to bring the house down :oops:

Joists are measured at 3x2" and I believe they sit across a load bearing wall at the centre of the house which the chimney stack is also attached to.

I have measured the space I am looking to work on at 7M width (across joists) and 1.7M length (along joists) at either side from the centre which add up to the 29M square area.

My intention is to place a 6mm WBP plywood measuring 2440x1220x6 across the joists, and cut to fit. The total weight of each sheet is 11Kg which equates to 11Kg over 2.98m square area. I have calculated to require a total of 6 sheets to cover each side of the 7Mx1.7M area, so this will add a total of 66Kg (including the wasteage) of weight on the joists on each side.

My intention for the 6mm plywood is to form an even load area for some Kinspan TF70 PIR rigid board which I intend to then lay on top. I was planning on using 100mm thick slabs to achieve the same level of insulation applied via the ECOGreen stuff.

To finish, as per Kingspan's reccomendation to sandwich the PIR, I was thinking of adding a 3mm plywood on top and end up with a SIP like panel, but a significantly lower total weight compared with a similar sized SIP panel which I was told weighs in the region of 100-110Kg :eek:

Based on the these materials and roughly summed up, 4Kg per m2 = 6mm plywood. 2Kg per m2 = 3mm plywood and the TF70 around 3.5Kg per m2 based on a confirmed weight per slab of 14Kg. The total weight added per m2 will be roughly around 10.5Kg.

The sanity here is, is this the lightest possible way to achieve a hardboarded and insulated storage area, but more importantly am I going to be asking too much of the 3x2 joists, despite each section backing onto and being separated by a load bearing wall in the centre?

My budget for this is around the 700-800 mark based on using the Ply-PIR-Ply materials.

Hum
 
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Double post.....numpty :LOL: :oops:

P.S. I can provide images of the loft space if required, or I can make an outline drawing of the building plan etc. if necessary.
 
Humour; your post is too long and yet some important info. is missing.
It looks as though your ceiling joists span on to a middle(?) loadbearing wall, but what is the actual total span of each ceiling joist? - that is an important factor in working out how much each joist can support without bending too much.
However, if you are within 1.7m either side of the wall, you might be OK, so long as the span is not too great, you don't store too much, and you keep most of the weight near the wall.
By the way, don't use plywood, use OSB (aka 'oriented strand board') - its just as good as ply for this application and a lot cheaper.
 
Hi Tony,

sorry for the long post, I was trying to putin as much info as possible.

I measured the loft space again and I can confirm its 7M x 6M wall to wall. The 6M span is the length of the joists wall to wall and they are positioned front to back.

I'm not planning to put heavy items up there, just general house stuff.

In terms of the material of choice, Im looking to achieve maximum strength and insulation vs minimum of weight being added, cost whilst important is not my primary driver. How does OSB compare with Ply in terms of rigidity and weight?

Hum
 
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Tony just looked at OSB as you suggested. A source comparing the two materials is here.

Looking at the costs at my local timber's there is a £2 difference against like for like panels, but the OSB has a minimum order quantity attached so I will end up paying significantly more and end up with allot of surplus material, plus if the source is to be believed an extra 15% of weight applies when using OSB whilst having less rigidity vs Ply.

Thanks for the tip but I think I'll stick with Ply on this one :D

I'm more concerned as to wether I need to speak with a structural engineer or building control on this one :confused:
 
Spoke with a structural engineer yesterday who said that the joists are less than average in size :( but also went onto say that on average the standard type of timber roof construction can safely hold up to 22Kg per m2 on the joists (including storage items).

He suggested that provided the load bearing wall splits the joists span in half and each boarded section will be backing onto the load bearing wall, then it should be ok for light storage. :D
 
Spoke with a structural engineer yesterday who said that the joists are less than average in size :( but also went onto say that on average the standard type of timber roof construction can safely hold up to 22Kg per m2 on the joists (including storage items).

He suggested that provided the load bearing wall splits the joists span in half and each boarded section will be backing onto the load bearing wall, then it should be ok for light storage. :D
Just make sure that you place most of the load on or near the load bearing wall :eek:
 
Just make sure that you place most of the load on or near the load bearing wall :eek:

I will. I will also get the SE out to confirm the load bearing wall before I do anything as I also need consultation on removing a wall for a future project. :D

Why the :eek: face?
 
Why not just lay the Kingspan direct on to the joists and then cover with ply?

The top sheet you intended to use is too thin for my liking,
 
Why not just lay the Kingspan direct on to the joists and then cover with ply?

The top sheet you intended to use is too thin for my liking,

I was thinking to do that initially, but 2" every 40cm to hold up the weight sounds like very localised pressure on the PIR material and concerns were raised that the PIR might compress. Also Kingspan themselves reccomend not to lay the material directly onto joists but to spread the load as much as possible.

Fair enough comment on the top layer, I also agree, but Im trying to keep weight to a minimum with this solution.
 

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