insulation advice

selfbuildsoon

We had planned on building our house with a 150mm cavity and 100mm PIR to get a uvalue of 0.18.

I'm just wondering would the u-value be better if we used beads instead.
Also, would you recommend internal lining(are there issues with dampness)?
Thanks:)

sinnerboy

Partial fill of 100 in a 150 cavity will get you to 0.18 W/m2k only if

1. you ignore the affect of wall ties and
2. assume that the boards are perfectly installed i.e. perfectly flat against the inner leaf and all but joints free from gaps and foil taped.

Allowing for the above a more accurate U Value is 0.21 W/m2K

A full fill (150) cavity with grey bead ( lambda 0.033) and using basalt ties will get you to 0.20 W/m2K

sinnerboy

Try to keep all insulation in the same plane or observe the rule of thumb

2/3 in cavity 1/3 internal.

selfbuildsoon

Thanks for that. So does that mean that you can't dryline with the beads?

What insulation would you recommend?

sydthebeat

selfbuildsoon wrote: »

Thanks for that. So does that mean that you can't dryline with the beads?

What insulation would you recommend?

No it doesn't mean you can't use both. It just means you should have 200% more insulation in the cavity than in the dry linging

selfbuildsoon

Thanks for that. Think we're gonna go with the 150mm cavity and 100mm pir. still unsure about drylining!

beyondpassive

There is a 145mm insulation for cavities with 5 mm spacers for 150mm cavities now being made by X. Worth a goo (look). self build soon, alsp consider how you deal with cold bridges from the warm inner leaf sitting on a cold rising wall andthe standard cavity closer block is now a no no. What is your space and primary heat demand per square meter. We find that to work out a specification you need to work back from how much you are prepared to spend annually to keep the house at comfort. Even for houses as big as 3000sq ft we would never let it get over €750 for space heat and hot water, pumps etc, You've one shot at this.

selfbuildsoon

As we're building on a hill in the west, would that 5mm gap in the cavity be suitable or would the wet get through.

Also, does mhrv stop mould growth in general and with internal dry lining?

beyondpassive

Dont dryline, you need the mass of the building to be close to your desired internal temperature. Generally for every watt of heat energy you put in the air, you put seven in the structure, this means that the building is 7 times slower to cool down than if you just heated the air. Also its a pain to fix through. Out all your insulation iin the cavity and be done with it.