Underfloor Insulation

bartandnae, Jul 26, 7:01pm
Im looking at putting some underfloor insulation in my house. What would be the best product out there?

Would it be expol or batts? Which one would be more time consuming? I’m more than capable of doing it myself and have heaps of room under the house.

Thanks
kattagee, Jul 26, 7:10pm
expol is easier and fibreglass bats are nasty if you breathe them in.

i used expol which can be cut to fit where bearers are not standard
tweake, Jul 26, 7:14pm
best insulation imho is polyester.

expol (polystyrene) can have problems due to fitting. its difficult to get it to fit well especially on timber thats not very straight. if it doesn't fit well air will get around it and it become ineffective. plus someone reported finding water on top of it. this can be due to it being low permeability, ie moisture doesn't go through it well so the floor can't dry to the outside. especially if you have sealed floor like polyurethane, lino etc.

batts is horrible if it gets wet, fall to pieces etc. plenty on the net about it.

polyester is easy to install, easy to cut in one direction but not easy on the other.
because you can just staple it in place, its easy to get a good seal on the timber.
also far nicer to install because you often end up under it holding it up etc.

there is also two different ways of doing the insulation.

however first thing is to install ground vapour barrier, ie polythene sheet.
reduce the moisture under the house. that will reduce moisture coming up through the floor and keep in mind insulation does not do well if it gets wet.
pre perforated poly sheet is available so you don't need to muck around with drain holes.
bartandnae, Jul 26, 8:14pm
Thank you. That’s just what I needed to know.
jan2242, Jul 27, 9:40am
The stuff they used in my place looked like sheep's wool. I asked about polystyrene and they said no for 2 reasons. 1, if there is a water leak the water will sit and rot the floor as there is it can't escape. 2./if there's a fire it will go up in an instant.
tweake, Jul 27, 9:58am
sounds like polyester or wool and polyester mix.

polystyrene tends to melt more than burn, but by the time the floor is burning your will either be out of the house or dead.

with water, normally it should dry up through the floor. however if its sealed floor (polyurethane etc) it may get trapped in there and thats when you will have big problems. need to have at least one side for it to dry to.
tweake, Jul 27, 10:06am
i'll just quickly add, the other thing is how its installed.
you can do between the joists which leaves the joists exposed to wick heat through.
or blanket over the bottom of the joists. this puts the joists on the hot side of the insulation. insulates the joists but also adds the mass of the joists to the houses thermal mass.
but it allows air/heat to move between the joist and floor. great if you heat the whole house or have a lot of sun on one side. not so great if you only heat per room. there is some install details as well.

i prefer blanket method but unfortunately i do not have any hard data on which is better, and that it will vary a lot depending on house layout and how you heat.
mansonprincess, Jul 27, 10:14am
greenstuff - also nz made/polyesther
timbo69, Jul 28, 6:24am
Dont bother doing it yourself, I had three little fellas come do mine took them 45mins to 104s/m house cost bugger all
tweake, Jul 28, 9:03am
but did they do a good job or was it just done for looks ?

theres a recent article about insulation on stuff that has pictures of the "good" insulation install and its terrible !

how well insulation works is highly dependent on how its installed. missing small bits makes a big difference. so you do need to be fussy.

however a team of people does make it a lot quicker.
lythande1, Jul 28, 1:07pm
Hot air rises. It doesn't fall out the floor.
wasgonna, Jul 28, 1:33pm
And Snow White was supervising?
tweake, Oct 14, 9:07pm
true, hot air rises. so certainly the ceiling is the most important especially for air sealing.
but heat goes from anything hot to anything cold. whatever is coldest the heat will go out through. up or down doesn't matter.
the other thing is that we are in contact with the floor so floor temp is much more noticeable. walk on cold lino in bare feet you know about it real fast.

there is other benefits to floor insulation, helps reduce drafts through the floor. help keep the floor dry, especially with carpet which is a major holder of moisture.
also it adds thermal mass to the house (as more timber is on the hot side of the house) so it stays warmer for longer when the heat is off.