Using loft insulation in cavities?

[JesusNinja]

OK, not the intended use, but it seems like such a cheap way to do my garage, with B&Q offing rolls at £3 for 5.5sqm.

I'll be doing the roof of the garage with it, but my quota of 18 rolls would give me enough to do the walls too.

Any obvious problems, aside from it wanting to slump over time?

 

[freddiemercurystwin]

Is this a proper garage conversion?

Its not very good at insulating, that's why you need to use about 3 layers or 100mm thk roof insulation roof when 1 layer of 100mm Celotex or similar would do the same job. If you're doing it to Building Regs it won't be accepted. Its cheap because its not very good.

 

[JesusNinja]

No, it's for a prefab garage I had built a few weeks ago, which I will be using as a workshop for cars. More a bit of something to keep the chill out rather than make it habitable. The alternative would be polystyrene. I'll be panelling over it all with ply.

I did acquire some kingspan via freecycle which has done some of the walls.

 

[freddiemercurystwin]

Fill yer boots then. Insulation on its own will not keep a garage warmer than the outside unless it has a heat source.

 

[JesusNinja]

Insulation on its own will not keep a garage warmer than the outside unless it has a heat source.

Of course. It will have a heat source (other than me!)

 

[^woody^]

Loft insulation does not have the water repellent that wall insulation has. And it may not have the same k value as a similar thickness of proper wall insualtion

 

[JohnD]

you need to use about 3 layers or 100mm thk roof insulation roof when 1 layer of 100mm Celotex or similar would do the same job.

Are you quite sure that the values are 3:1 as you suggest?

 

[freddiemercurystwin]

Are you quite sure that the values are 3:1 as you suggest?

No I've not got my calculator out, that's why I said

about

 

[JohnD]

bit nearer 1.5:1 when aged

R-values

Material m2·K/(W·in)

Fiberglass batts R-3.1–R-4.3

Foil-faced polyisocyanurate rigid panel (pentane expanded ) initial R-6.8

Foil-faced polyisocyanurate rigid panel (pentane expanded) aged 5–10 years R-5.5

 

[JesusNinja]

Thanks everyone for the constructive replies. It seems B&Q are no longer stocking the lagging, so that kind of answers that any way.

The moisture repellent issue is noted however, as well as the relative thermal properties. Some of it has been done with foil faced PIR as I picked some up from FreeCycle, but the budget is very tight, and when all is said and done, its just a garage, so I think that probably the most cost effective thing to do is to use 25mm polystyrene in the hollows in the walls and then sheet over the top with more 25mm polystyrene and panel over the top of that.