Timberframe Build; Insulation Overkill? 100mm Rigid in Ext Studs + Comp. Pboard?

A

apple1

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Folks

Just looking for a quick opinion.....Is it overkill in a TF build to slab
out external walls with 47.5mm composite plasterboard when there
is 100mm rigid Kingspan fitted between studs? House is a bungalow
with potential for attic conversion, so I do plan on slabbing attic with
composite pboard. Thanks in advance, apple1
 
Thanks Syd. Which construction wouldn't in your opinion comply.....the 100mm PU insuation with 12.5mm plasterboard or the 100mm rigid PU with composite board? I presume the former, but just to clarify? Thanks, apple1
 
The composite board is non-breathable, as is PU insulation, therefore you would be surrounding a structural piece of timber on 3 sides with non breathable material, which is definitely inadvisable. The 12.5 plasterboard itself is a lot more breathable.

Personally, i wouldnt incorporate PU insulation into a timber frame build. I would use a woodfibre or thermohemp type insulation completely between the studs and between battens on the internal for extra insulation. This creates a completely breathable construction which is better for both the health of the structure and the health of the occupants.
 
Thanks Syd.....based on your response, I presume your advice is similar for the attic space, i.e. do not slab it out with composite board placed over the trusses which have PU fitted between?
 
that would be my advice if its available to you.

Its a hard one to call because of the prevalence of PU insulation in the industry.

Xtratherms IAB cert allows this construction, im not going to say otherwise, i havent the knowledge to question the IAB process.

There is actually a school of thought that claims its illegal to allow 3 sides of structural timber to be encased in non-breathable material...but thats for another day.

I would simply conclude that breathable construction is a better method of construction and is better for inhabitants, and should be used if the choice is open to a client.
 
Syd...thanks again. That advice suits me fine....from a budget perspective, the 12.5mm is a far more attractive option!
 
apple, remember that your constructions have to meet the regs at least. In the attic consider materials such as woodfibre, cellulous fibres (recycled?), sheeps wool, rockwool etc. These materials do not have the same thermal performance as PU board so extra thickness are required.
 
good point Syd. Does 100mm PU between the rafters not meet current regs?
If not, what do I need to consider & secondly, where might I get a copy of the regs for future reference? Are they available on-line? Thanks, apple1
 
generally, 100mm doesnt meet the current regs.... you need 135mm, thus the insulation backed plasterboard.

PU has a thermal conductivity (TC) of 0.023
Woodfibre would have a TC value of 0.04

what size ceiling joists have you.. .you need about 300mm of woodfibre to meet the 0.16 reg
 
Syd...I'm not sure but I'm opting for attic type versus webbed type. I suspect the joists are 9x2? If I fitted 100mm PU between the rafters, obviously the 2 "long" sides of the rafter wouldn't be completely encased in PU. Would it then be an option to fit the 100mm between & 47.5mm composite under? Similarly, the external studs are 140mm and the panels are factory fitted with 100mm PU, so I guess my only quandry there is whether or not to fit regular 12.5mm pboard or the comp. board? Given the significant difference in price, is there an appreciable (& quantifiable) difference in heat retention if one opts for the latter? Thanks, apple1
 
The 100 PU between the studs will give you a u value of 0.25 currently...

(PS i use xtratherms u valuate calculator for all these calculations)

I will assume your external studs are 140mm, so if you increased this PU to 140mm youd get a u value of 0.22...... the min reg for walls is 0.27..... that would be a significant improvement.

Regarding the ceiling, if its a trussed roof i doubt theyre 9 x 2's. As i stated above, the IAB testing facility seem to have no problem with the encasing issue so im not one to argue otherwise. Seeings as the walls come preinsulated with PU then your construction is not going to be breathable so the 100PU and 47.5 CPB is probably the best option.
 
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