summer house - internal vapour barrier,insulation,airflow

[n9551217]

Hi

Ill give you some background of the build so far and then hopefully someone will be able to give me some advice as my latest job seems to be trawling the internet!!

SUMMER HOUSE / STORAGE / HOBBY WORKSHOP

BASE
Reinforced concrete slab 100+mm x 7.2 x 3.6m
4x2 treated timber bearers DPC wrapped
9mm OSB3 sub floor

WALLS
4x2 c24 studs with 9mm OSB3 racking & Breather membrane overlapped and taped
19mm treated vertical battens plus airflow top & bottom for waney edge covering

ROOF
222 x 35mm ridge & cross braces 4x2 c24 rafters with 11mm OSB3 & breather membrane
for 19 & 25mm counter-battened cold roof to corrugated steel


As the building is pretty air tight I'm toying with no airflow within the cavities. I plan to seal up the osb gaps at the eaves and within the electrical installation. I then would like to fill the cavities tight with insulation bats, and over board with polystyrene backed plaster boards.

My question is would this system work without the need of an internal vapour barrier to the wall and ceiling; and would it still be 50% substandard to modern foil backed insulation - with air flow, and the inherent insulation gaps, cracks, and service holes?

Many thanks in advance
Tony

ps The boss (mrs) only has a 5k budget and the foil backed stuff may just break the bank!

 

[^woody^]

Any timber frame has an external moisture barrier and an internal vapour barrier - which can be a £10 sheet of polythene if things are tight.

You should not have any air voids within the frame.

 

[n9551217]

Thanks for that Woody. I just have an awkward ceiling to apply a membrane internally, but I guess it has to go on. I was thinking that the air tightness might avoid the need for one without the drafts sucking in air and moisture. What are your thoughts on saving a few quid by avoiding the metal backed insulation sheets and using rockwool with poly backed plasterboards instead?

Tony

 

[^woody^]

Foil faced insulation is roughly twice as efficient as the same thickness in rockwool. But whether the cost difference is worth it for your particular use, is something only you can decide.

75/100mm of rockwool and 25mm of insulated PB may well be good enough for a shed/workshop.

Foil backed plasterboard might be easier for the ceiling

 

[n9551217]

Thanks again for your advice Woody.

I agree. - Shame the poly backed doesn't have a vapour barrier.

Found this site yesterday , its been very useful - especially when costs are concerned:

http://www.vesma.com/tutorial/uvalue01/uvalue01.htm

Ive so far compared 11 different options ranging from £600 to almost £1200 and the U values don't progress the way you'd expect them to!

Tony