Affixing insulation board without thermal bridges?
 

Hi, I'm renovating a 1927 brick semidetached house in Amsterdam, The Netherlands. I'll be using 80-100mm thick PIR boards on the roof, the walls and the floor to get an Rc of 3.5+ everywhere.

I've been watching a lot of installation videos for these boards lately, and most of them attach the boards using long screws. Some use just one or 2 screws per board, some a lot.

There are also a few videos that use construction adhesive to attach the boards to the wall, but I fear I can't get my brick wall clean and straight enough for that to be a good option.

My question is: do the screws form a significant thermal bridge? Should I contemplate other solutions? I saw for example there are Z shapes you can buy that slide into the board to hold it down, that will reduce the depth of the thermal barrier I guess.

If I'd had to guess the screws would be about 2cm^2 of metal thermal bridge per m^2. On a 15m^2 wall, that's 30cm^2 of metal thermal bridge. Maybe not terrible but would be nice to avoid perhaps?

How are you cladding the polyiso? I'd say you could use two layers where you screw in the inner layer(the screws would need to be inset/flush) and then overlap the seams for the second layer, using the construction adhesive to stick the second layer on. The trouble with this is installing your siding that will protect your precious and fragile foam because I don't know of any protective cladding that can be glued on to polyiso to protect it from the neighbors golf balls, hail, insects, and animals.

Oh I should have been more clear, I'm putting the polyiso on the *inside*. So there'll be plasterboard cladding.

I found this:

Insulation Fixings for wood fibre boards

Which are nylon screws, I guess that would be better than metal for starters.

The screws are a thermal bridge, because steel is such a good conductor. However the cross-sectional area is a very large determining factor in this instance, and the cross sectional area of the screws in your application is very, very small.

If there was a way to avoid screws, it would be better, but the losses from screws will be minuscule.

I think that as you continue to work on your place you will find other heat loss areas that dwarf the effect of screws.

I am working on an old house too (1892) and reducing heat loss is like trying to tame a wild elephant. But bit by bit, I am winning, you can too.

Best,

-AC_Hacker

You are risking serious health issues due to mould caused by interstitial condensation by putting so much insulation between you and the wall.
If you do a condensation risk analysis you will find that the dew point will be inside your house when you finish the work, this is not good.
The only way you can avoid this with this wall buildup is to put a vapour barrier between the insulation and the room, this would need to be taped to the floors, ceilings and around all window and door openings to avoid any possibility of moisture from the room getting in to the insulation.
Even doing this it may still happen. I think you should consult a building professional with experience in the area before you proceed.
As a general rule of thumb no more than 50mm of PUR is used for internal insulation for this reason - it moves the dew point into the wall.

If you are using the foil-faced polyiso board, it makes for a pretty good vapor barrier. If you tape your seams on the inside, that counts as a vapor barrier. Make sure not to inadvertently build a vapor barrier sandwich.

Just to avoid misunderstandings: R=3.5 in the Netherlands has different units than in the USA. So it's not the American R=3.5.
In Holland the units are R=3.5 m2K/W

Thanks Fion, Jeff and Zwerius. I plan on using foil backed board and taping it on the inside for a hopefully close to airtight solution, which would also be the vapor barrier. Also I will be going for balance ventilation, so moisture inside should not be a problem (if I do it correctly).