Insulating spray

[Anonymous]

We have just bought a former RAF quarter built in 1939 and the loft needs insulating...both for warmth and to eradicate dust and damp. Has anyone out there any experience of the poly spray frequently applied to the underside of tiles between the rafters? Good idea or not?

 

[Anonymous]

Usually not...

 

[Anonymous]

Its not permanent but it is unremovable, so when it fails, you may damage your roof.

Maybe go for something applied in the more traditional style - ie laid between joists or under/inbetween rafters.

 

[Anonymous]

Great way to rot your whole roof structure away.

NT

 

[Anonymous]

Sprayed onto underside of felt, how?

 

[Anonymous]

I think its usually applied onto the underside of tiles, i.e. where there is no felt.

If its applied around/over rafters, then they will not be able to dry out as they will no longer be in the open air.

 

[Anonymous]

So if the rafters are now sandwiched between felt and spray on stuff ,how do they get wet?

 

[Anonymous]

Poly spray not a good idea for the reasons everyone else has stated on innumerable threads here in the past.

I'm interested by your reference to stopping "dust and damp" though. Not sure why you think dust is a problem that's down to the roof and a lack of insulation, but if the roof is at present letting in the damp, you need to get it looked at and find out where it's coming from.

1930s RAF buildings were usually superbly and straightforwardly built (if a bit "of their time" in the warmth and cosiness stakes), and doing repairs properly shouldn't be too costly.

 

[Anonymous]

Well, they aren’t dry now and probably wouldn’t be when they are encapsulated.

My understanding is that they are also likely to continue to suck up moisture through exposed ends in contact with other damp materials such as brick work, tiles, etc. This is normal and the moisture is usually released by evaporation from the large surface are of the rafter that is in a circulation of air.

Without this evaporation, the wood will simply retain a higher level of moisture and will eventually rot.

This happens when wood (or other permeable material) is used in building and is not in a circulation of fresh air – wall plates and joist ends in contact with walls are a great example – if there is no air gap, they rot.

I may be wrong, but this is what I understood.

 

[Anonymous]

I'm not.....a) an apologist for spray on foam, b) a timber exert, c) looking for an argument, but I'm surprised that the moisture which is absorbed by an exposed end of a rafter would evaporate through the sides. I'd have thought it would be much more likely to evaporate back out the way it came in.

 

[Anonymous]

Never mind the details, in our climate with its humid atmosphere, water always gets in to permeable materials somehow. The important point is that if you cover stuff with non-breathable materials, like the spay-on roof foam, it doesn't get a chance to dry out. Fungi, woodworm and elephants move in.
Don't use the spray. Insulate your roof with one of the many vapour permeable alternatives. Sheeps wool is the best but not the cheapest.

 

[Anonymous]

elephants??! I thought I'd encountered most of the pifalls of a period property over the last few years, But I never knew I could expect a plague of pachyderms invading my loft space!

 

[Anonymous]

Many visitors here report having plastered lathes up there as well.

NT

 

[Anonymous]

Tiles and slates break and come loose. Felt has limited life. Lead cracks or falls off. Cement or lime fillets crack, break and fall away. Roofs leak sooner or later.

Foam tends to get done on roofs in poor condition, otherwise why would the owner do anything, so it may well be leaking at the time the foam goes on.

NT

 

[Anonymous]

Not sure how any of the problems you list (bar ageing felt - but I suspect felt has a longer life span than softwood rafters) would be hindered by presence of foam.

 

[Anonymous]

I posted on a thread about this topic a while ago a copy of a letter, in full, which was printed in SPAB News a couple of years ago from an architect. It detailed experience of water ingress into a roof - water can be driven in through small gaps in roof coverings - which had been insulated with this stuff and where the timbers had quietly rotted away, hidden from view, as they couldn't dry out.

It's not in general regarded as good conservation to do something as quick-fix and fairly irreversible as this stuff is. It makes re-use of coverings difficult in future.

If roofs do require insulating, I think there are possibly better methods than this stuff, and which will allow covering materials and timbers to breathe, which can be removed in future.

 

[Anonymous]

Those problem all allow water to reach the rafters. The foam prevents them then drying out. The result is liable to be structural rot.

FWIW felt last a fraction of the time of softwood rafters - unless of course youve got a foamed roof hastening timber rot.

NT

 

[Anonymous]

Your roof must be constructed differently to mine. The rafters and purlins are separated from the slates by the felt on mine so broken slates don't equal water on rafters. The same goes for split lead etc. Providing you repair these kind of things quickly what's the problem? If you're doing any major work you're going to have to replace the felt and probably the rafters anyway and the slates are untouched by the foam.

 

[Anonymous]

If you read the original post, you will see that the question was about application to the tiles - not to any sort of underfelt.

The pics I've seen of this stuff shows it being applied to the timbers also - which I can't see the need for, and it's never a good idea to do something so irreversible.

Personally, I'd run a mile from buying a house treated in this manner. There are better ways to insulate. If a roof needs fixing there are better ways to fix a roof. It's a quick fix bodge.

 

[Anonymous]

I'll leave you to think about it.