Garden room roof insulation

[Jonny]

Hi everyone,

I'm currently building a garden room, it will be site office, come long term office/further accom over time. Built to be non permanent, it is on piers with a 8x2 base, 4x2 walls and 6x2 roof with 10 deg pitch. The room is around 20sqm. 

 

I thought I had cracked this insulation malarkey, but now I'm not sure. I've also already bought the materials without double-checking. For the walls, I was planning 100mm 035 mineral wool with 50mm pir internally. This would have a breather membrane externally and Proclima Intello Plus internally. 

 

For the roof, I was aiming for a warm roof with 100m PIR on the deck with 100mm mineral wool between rafters. I'm not sure, though, after seeing the attached UBAKUS calculation, which suggests a very poor drying time. Have I got the membranes in the wrong places or something else? 

 

I've attached a photo of the current state of the building - still some structural work required on it and also the Ubakus PDF printout. 

u-wert-berechnung (5).pdf

[JohnMo]

20 minutes ago, Jonny said:

For the roof, I was aiming for a warm roof with 100m PIR on the deck with 100mm mineral wool between rafters

Classic bad design hybrid roof - that is the cause of your issues. Either do the insulation above or below.  If doing both a smaller amount below than above.

[Jonny]

Thanks, did have an inkling that may be the case. I’ll try again on the calculator tomorrow with the 100mm PIR only and report back. It’s not going to have a high heating demand. 

[JohnMo]

Or you could do mineral wool between and PIR below

[saveasteading]

Excuse my ignorance. What is this 'drying time'? 

Use dry wood, keep the rain out....

If if isn't dry, then don't cling wrap it until it is?

[IanR]

13 hours ago, Jonny said:

after seeing the attached UBAKUS calculation, which suggests a very poor drying time. Have I got the membranes in the wrong places or something else?

Rule of thumb is to have the most vapour closed layer on the inside and get more vapour open as you work out, so PIR would be better on the inside of the roof. But, you may get away with a ventilation gap under the steel roof so that air will allow the OSB to dry.

[saveasteading]

1 hour ago, IanR said:

allow the OSB to dry.

I'm still confused. Isn't this easy?

Don't get it wet. If it happens then 

close off the outside,  breather membrane inside.

 

I didn't know what UBAKUS  was. It says its for Architects and students.

Implying  that it isn't  for builders, Engineers, Surveyors? 

Is there logic to this?

[ADLIan]

Ubakus is used to calculate U-values and assess condensation risk for building elements. Unless you know the building physics involved rubbish in may give rubbish out. The drying period is that following a period of condensation build up (normally the winter months). Nothing to do with water from rain/snow.

In this instance. Do not use a hybrid roof (insulation above and below the deck). Use a true warm roof with all insulation above the deck. Check MIs for the 'steel' roof as it probably needs a ventilated airspace directly below.

 

[IanR]

1 hour ago, saveasteading said:

I'm still confused. Isn't this easy?

Don't get it wet. If it happens then 

close off the outside,  breather membrane inside.

This is about condensation risk, and if water vapour from the warm humid inside air is likely to condensate within the structure, does it dry out or stay wet for prolonged periods where mildew/fungus/rot with then take hold.

[saveasteading]

Thanks.

There shouldn't be condensation that needs to dry though.

We design so that doesn't occur.

 

[ADLIan]

Occasionally condensation is predicted eg masonry cavity walls & warm flat roofs with built-up felt or single ply membrane. It is then down to the designer to assess the risk - the amount of condensate, does it all dry when conditions allow, adjacent moisture sensitive material etc. BS 5250 and BS 6229 give guidance,