Roof Insulation

[Jeff Gaines]

I have a chalet bungalow and temperatures upstairs have been around 33 in the current weather, I either sleep downstairs or have to go through a cooling process which means windows closed all day and opened about 8:30 which gets the temp down to about 26 by bedtime, still uncomfortable.

There is no insulation in the sloping ceiling (does it have a special name?) and I have seen some ads recently for a spray on roof insulation which goes directly on the the roof tiles (actually I suppose the felt unless it's removed beforehand).

I'm not sure that spraying stuff on the underside of the tiles is a good idea but what if this stuff was sprayed on the back of the sloping plasterboard leaving a ventilation gap between it and the roof?

I can get somebody in for a quote but wanted to have a feel for what might work before I do that.

Advice appreciated :-)

[Theo]

Spray foam is popular in the US, but here mortgage companies are difficult about offering mortgages on properties with it in (and so makes your house less saleable). One reason is that it's possible for moisture to get trapped and rot timbers where you can't see them (although that goes for anywhere in the structure that's boarded over too). I'm not sure how well the installers take vapour control into account, and without an appropriate vapour control layer in the right place, the spray foam may make damp problems worse. In your case, you can't spray on the back of the plasterboard because it's expanding foam, so it'll expand until it reaches the tiles, leaving no air gap (there's no way to spray an even sheet on the reverse of the PB to prevent it expanding to fill).

Another is that it's a difficult job to remove and potentially means a re-roof since the tiles can't be reused afterwards, and your roof timbers will be covered in traces of it forever more, and have to be replaced if you want rid of it. So pretty much an irreversible process.

Finally, in the US there are problems where the contractor hasn't mixed the chemicals correctly and your house is full of toxic offgassing, which causes health problems. A decent contractor should know what they're doing, but how do you know until it's too late?

If you can, fitting celotex boards between the rafters and re-boarding with plasterboard is a safer and less irreversible option, but means redecorating afterwards. A less invasive method might be to cut a slot and stuff in rockwool, which means you only need to make good one slot per bay. A thermal camera is good at spotting parts that have been missed.

Theo

[N_Cook]

Mansard roof? giving a sloping ceiling.? Going by the success of flying external "African" sun shades over my windows, perhaps larger such shades flying over at least part of your roof. Would have to be properly engineered with robust fixing points and/or arrangement to decouple and retract in a strong wind gust, also fire hazard if unattended

[Jim Stewart ...]

beats me why you English punters don't use sarking.....

[RJH]

Snip good advice relating to (avoidance of) foam.

From my experience, It'll make a huge difference. In the attic rooms here I have 50mm celotex. Roof temperature is currently 62C. Room temperature 25C, and it won't get much higher. It went to 30C on the 40C day we had. The walls do not feel noticeably warm. TBH it wasn't a benefit I'd thought about.

It'll create a right mess - remove the old PB, fit celotex, seal, board over, skim, paint. But not too expensive (much less than £1k I'd have thought, depending on room sizes) if you can do it yourself and pay for the skim. It's what I did!

[Theo]

Class 3 awnings go up to 30mph, no more. It is difficult to have anything made of fabric that's not going to turn into a sail. Awnings that retract when the wind speed gets too high aren't cheap, and on a roof you can't go outside and do it manually.

However one thing that may help is foil-backed celotex which will reflect some of the radiation, or otherwise lining the voids with foil or other reflective material.

It's also worth thinking about ventilation: is that heat coming in from the sun, or heat from the house that's risen and collected. Insulation will help with the former, ventilation with the latter.

I did some timelapse thermal camera recordings of my east-facing loft conversion over a 24h period, and from those it was clear it was solar gain through the windows, rather than heat coming through the structure (eventually the structure heat soaked and kept the heat into the evening). So addressing the solar gain is top of my list. My conversion has about

80mm of rockwool in most places, so evidently that's preventing solar gain through walls or roof.

Theo

[Theo]

Forgot to mention, I recently came across this:

which is an interesting guide to shading windows. Mostly uses examples from Africa, but the same principles apply (with tweaks for latitude). The exact form of shading depends on the direction of the window as it needs to match the sun angles. In the UK most of those passive shading structures on new buildings tend to focus on south facing aspects (for high sun angles), but it is interesting in that guide that those angled towards the east and west get similar but different structures.

It also explains why a number of buildings have a 'finned' design, because they create shade from the appropriate angle.

Theo

[N_Cook]

Needs a change of outlook/attitude. I went to the pub yesterday evening an hour after sundown. No one in the pub building but dozens outside in the paved over but otherwise teneded by gardener, "green" pub garden. I could only survive out there for 5 minutes before realising the paving had been absorbing 12 hours of sun and was busy releasing it upwards , perhaps for another hour or two more,so I went indoors ,despite protestations from mates who were convinced it was hotter indoors.

[N_Cook]

Interesting seeing the African take on this, unfortunately fully architectural, rather than simpler flying shades. From an earlier thread on this board the contents in that pdf would come under the term "brise soleil". Curiously wherever I see them in this country , they are to counter directly overhead sunshine , which for 50 deg N or higher latitude is, not really appropriate. One student hall or residence near me has no W,S, or even E facing windows, just north facing glas set in sort of angled non-glazed oriels protruding from the facades

[John Rumm]

We do, in both the normal use of the term down here - i.e. under tile felt, and also in the jock sense - tiles under boarded. The latter is less common on new stuff but was quite common on stuff built in the 30s through to the 50s

Neither give much insulation value directly, although do reduce air changes.

[John Rumm]

If you can spare the headroom, you could just underboard the existing PB with insulation backed PB.

A better result would be had by taking down the existing boarding, and insulating between the rafters, and then a second layer under the rafters before reboarding.

If you have 4" deep rafters then you could get in a 2" air gap adjacent to the tiles, 50mm foil faced insulation, and the 30mm full coverage under that. That would only lose you 30mm of space in the room, but give a massive improvement over having none.

I did that in my last place when I converted the loft space, and it made a tremendous difference to the thermal performance of the house in both winter and summer - even thought the loft was reasonably well insulated previously)

Pictures here:

(the current place has rockwool type insulation built into the sloping parts of the ceiling - it is not as effective alas).

[Harry Bloomfield Esq]

Which is a really bad idea and your insurers would insist on it's expensive removal.

[Fredxx]

I've not heard of an insurer insisting on its removal. Can you cite an example?

Yes, it is awful stuff.

[Harry Bloomfield Esq]

No, but I saw plenty of examples of it on TV.

[Andrew]

Older house in the village had this stuff installed at some point. It was sold about 18 months ago and recently had been covered in scaffolding supporting a temporary roof of corrugated iron so that the whole roof could be stripped off and skiploads of manky tile battens, tiles and foam have been removed.

They could have saved the cost of the temporary roof because we haven't had any rain since can't remember. Now the temporary roof has been removed and it has been rebuilt with new goretex membrane and battens but no tiles, which are apparently on a long back order.

[Jeff Gaines]

That's probably why I said "I'm not sure that spraying stuff on the underside of the tiles is a good idea".

I know nothing about insurers requiring its removal.

[Jeff Gaines]

Many thanks for the responses :-)

I like the idea of Celotex boards but I will need to check what is there at the moment if anything. I can see the gap between roof and plasterboard from the storage areas each side of the rooms but it's hotter than the Sun in there at the moment so checking will have to wait! I may be able to slide boards in.

[Andrew]

Your house will be unsaleable if you do.

[Brian Gaff]

Well I have curved ceilings at the eaves and despite loft insulation being put down there and everywhere else, the loft is still backing hot and the top rooms are as well. My guess is that as heat always rises eventually it accumulates, and has nowhere to go due to the insulation in the loft any more. Its a problem. Brian

[Jeff Gaines]

Can I ask a follow up please.

I'm sure somebody mentioned finding hot spots but I can't find the post now. What sort of thermometer would I need to pick up where the hot spots are - walls, ceiling, windows etc?

Many thanks.