Roof and Tyvek breathable membrane

[Tim Watts]

Hi,

In the long saga of fixing a mouldy bungalow - I have totally given up on the idea of insulating it from the inside. The dormer conversion means getting celotex between the rafters will be next to impossible - too much criss-crossing of timbers. And the ventilation requirements with old sarking are nearly impossible to achieve.

So, I said "sod it", let's get some quotes for a retile. Been about 40 years since it was done, the pointing is going, gutter boards are shot and the existing sarking is falling apart.

So "they" can slap 3" celotex in from the outside, which will be trivial, leave a 1" air gap (BCO wants this) and cover with Tyvek (which allows the BCO to reduce to 1" airgap).

I've not talked to the BCO yet with actual diagrams, which I want to prepare so I can give the roofing guys detailed specifications. I wante to sound some things out for my understanding here, so I can get it as near to "right" as possible first time. Then I'll ask him for comments and provisional approval, then get the quotes, aiming for work next spring[1]

With Tyvek, does one usually still have soffit vents[2]? And what about up top? In this case "up top" would need to be vent tiles, probably, due to the dormer roof making ridge vents a non starter[3].

I'm rather hoping Tyvek avoids the need for top or bottom vents - would that be correct?

Cheers,

Tim

[1] I have to get quite a bit of stuff done at the same time. I will get the roofrs to fit cedar (or equivalent wood) gutter boards so they can do that as part of their work. I will have to clean and refit the ali gutter (doubt they want to be bothered with wibbling around) as well as drop lots of conduit through for possible soffit lights, power drops to shed etc before it gets sealed tight. And do proper rectangular vent duct for extractor fans instead of the flexy stuff I have now. [2] I'll be replacing the soffits, roofers tear off, I will refit as it is easy access (bungalow) and not time critical as I want to screw it on for easy access. [3] The dormr roof is flat - I will get them to sort out vents in both facias and I will be able to insulate that from inside, 2" between and 2" under.

[Hugo Nebula]

Is this a detached bungalow? If so, then why not consider insulating above the rafters? It would eliminate the need for ventilation.

[Hugo Nebula]

Even if you can't insulate above the rafters (and I can't think why not), then how about counterbattening? With a breather membrane it would allow you to use the full depth of the rafters for insulation.

[John Rumm]

If one looked from the other side - i.e. stripped some tiles of a section, hack off old tile batten and sarking, slapped in full rafter depth of PIR foam, covered with breathable sarking and then re-batten and tile (possibly with same tiles if they are ok), would that be doable/easier?

With breather membrane there is no need for a gap at all IIRC...

Based on info from when I did my loft (so probably not current), you only needed the vents with non breathable sarking.

I kept my sarking and did the airgap and vent thing. I cheated a bit, in the we pulled the ridge off, slapped in a row of weep vents (them things with the "up turn" at one end, mortared them back on. No change in the ridge height at all really, an only a visible difference on one side. We then chopped away a strip of felt from the inside just below the ridge.

Think so. If not you can probably get a dry ridge system now that includes ventilation anyway.

[Tim Watts]

I think it was the effect the change of geometry of adding 3-4" to the height of the roof that worried the bloke. But he wasn't the brighted tool in the box which is why I went off him, so I'll raise the very same idea with the next chap.

Normally, adding to the thickness of the roof would be agreeably no problem, but when you have various flat bits jointed in (that ideally should be left alone because they were re-leaded/refelted only 10 years ago) he may have had a point. This is going to not be cheap job so I'm trying to avoid touching the leadwork and felt if possible until their time comes naturally. Doing the roof is pushing a point, it *could* last another 5-10 years, but I have enough justification for that part.

Here's a picture of the relevant parts:

hope that link works - Google have done something very weird to PicasaWeb!

It shows the major outcrops from the main roof.

That would also be a very inteteresting idea - 1" should not make much difference to the roof geometry but will make a significant difference to the insulation. I (and my BCO) would be *very* happy with 4" celotex all over.

Cheers

Tim

[Tim Watts]

That was exactly my conclusion. As you know, because you've seen it, I was hoping to retrofit celotex from within. But after uncovering various parts internally, the only easy bit to do like that would be the dormer flat roof as it has nice regular joists, end to end and could be easily ventilated from both facias allowing a cross draught over the top of the insulation (the centre if the flat under the ridge is unimpeded).

He did stress he wanted 1" (it would have been 2" without Tyvek and complicated venting arrangements).

It seems reasonable. As the soffits will be planked (my soffits are nearly a foot wide) it would be slightly easier to not need venting. Otherwise I'd leave a 1" gap between the last plank and the wall and put some mesh or a fancy plastic comb in.

I did drill and fit lots of circular vents to the soffits for the bays though - as they are leaded it would make sense to keep these.

OK - guess it's time to invite favoured chap #1 over. He's popular so if I talk to him now, I might just get him for next spring/summer at a time I can be ready to deal with the gutter and soffits (= week off work).

On an aside, the project plods on, very slowly due to new job (inherited a broken-assed VMWare system running 100 linux virtual servers - much firefighting. Once I have the new system in and migrated, it will get more relaxed there.

I am painting the kitchen ceiling and bays over the next 2 weeks while SWMBO

• kids are in China. Then skirting tiles (probably sub that out - I like our floor tiler), paint the walls except for where wall tiles will go and it will be a lot nicer and might inspire me to buy some units - well, wall ones anyway, which will clear the crap up and make me more inspired to do the bottom units and worktops.

Cheers

Tim

[John Rumm]

Yup. Getting at it from the other side is often overlooked, and lifting some tiles off and sticking them back later is not actually that difficult.

Might need to check that.

The other option would of course be to go "warm deck", with the insulation on top of the rafters (hence no titting about cutting in) fix battens through the insulation, and tile that. Flat roof abutments will usually be below the tile level and are hence "easy"...

I was about to say the circular vents are easy to fit if needs be. Failing that you can also get ventilated uPVC (spit) facia boards.

Sounds like fun!

Ah yes inspiration - I find that nearly as hard to find as the tape measure I put down somewhere!

[Peter Johnson]

Some bungalows near me, approx 20 years old, have had what I assume to be grant-aided loft insullation installed by having the tiles removed entirely and reinstated followed by pressure washing. Where only one of a pair of semis was done one half has a bright clean roof and the other a dark and dirty one.

[Jeremy Nicoll - news posts]

So far as I know the only grants available for insulation are either for cavity walls, or laying the fluffy stuff 10" deep on attic floors. I can't see how stripping a roof would be part of that - unless someone scammed the house-owner?

[Tim Watts]

Thanks fo rthe tip off... Might enquire about grants. I assumed grant-aided insulation was "lob a few rolls of glass wool on the loft floor" which is exactly was will not work here.

They weren't council houses were they?

[Jeremy Nicoll - news posts]

I was told recently by the Energy Saving Trust:

even that's not grantable if someone already has some slightly more than minimal amount of insulation... It's madness. You'd think they'd be happy to help people top-up stuff as well as install when there's nothing there at all.

[Dave Liquorice]

'Tis madnness but just lobbing a few rolls out is quick, cheap and a monkey can do it (badly). Same with cavity wall insulation, what about all the housing stock that doesn't have cavities? Of course doing anything with them inside is very disruptive and expensive but why shouldn't a household that is prepared to go through that have to pay the full costs with no financial support? Seems very unfair to me. But then I might be biased as we are going through it ATM. Old drylining strippped out and replaced with 50mm celotex type backed plaster board on the inside of all external walls, solar thermal store system with wood burner and existing oil burner as input as well. What grant can we get? Just the =A3300 RHPP and VAT at 5% on part of the solar system. It will be eligiable for RHI payments if they every get that sorted out for domestic systems. But even at the rumoured 18p/kWHr that isn't a great deal of money from a solar system costing =A38k plus.

[Moonraker]

I am in the process of getting estimates to have my property re-roofed. It has thrown up some interesting informations. Pertinent to you Tyvex needs slightly larger battens than standard, this it seems is a requirement of their guarantee, Worth checking I think.

[John Rumm]

Well you could argue that if you are trying to make as much difference overall as possible with your grants, then you will have most effect adding some insulation to places with none, than you will adding more to places that already have some.

[Jeremy Nicoll - news posts]

Your argument kind of implies that there's no ROI from increasing say 4" of insulation to 10". But if that's the case why do they now recommend a 10" minimum?

They can't have it both ways.

[Jules Richardson]

Surely there *is* - it's just the ROI is much bigger for 10" of insulation on a property that has none, vs. 6" on one that already has 4".

Perhaps what's missing are grants in proportion to the estimated benefit

- e.g. (purely from a economic standpoint) someone adding 6" to an existing 4" should get 60% of what someone adding 10" can claim? (In the real world there are probably other factors to consider, such as comfort of the occupants - someone with no insulation will often be freezing and miserable, but someone with 4" already won't be quite so bad, so the weighting might be non-linear)

I don't think insulation grants even exist this side of the Pond - so, question: what's to stop someone pulling up their existing insulation, claiming they have none, then laying it back down again along with the freebies they just got? Are they subject to random (and mandatory, i.e. they have to let an inspector in) house inspection after installation?

cheers

Jules

[Alan Braggins]

Don't the grants cover the cost of fitting as well as the materials? Saying "if we're going to pay for someone to go in at all, they might as well do it to a decent standard while they are there, put we'll only pay for them to do it on the currently worst houses" sounds reasonable.

It's tough if you struggled to pay for 4" and now don't get an upgrade when your neighbour didn't bother and now gets 10" free, but making that fairer without making the grants more costly to administer (and intrusive to apply for) is tricky.

[Hugo Nebula]

Yes there is, unless there are counterbattens above the membrane. If so...

If there is a gap below the membrane, then correct, provided there is a vapour barrier underneath the insulation. If no gap, then there should be ventilation to the batten space _above_ the membrane.

[Tim Watts]

It makes sense - the wind will blow through between the tiles, but I guess it still needs a little space to flow around to remove any condensation from the tops of the rafters.

Yes - the underside (inside) will be as near as dammit vapourtight for

*most* of the surface - that means: The celotex will be fitted tight between the rafters and will be foamed in where there are difficult bits (criss- crossing wood etc) - that will be in the specification I will give to the contractor.

Further, most of the inside will be sheeted in vapour barrier plasterboard.

There are just a few difficult areas, right down near the top of the wall that are difficult to get to and installing PB right down the last couple of feet (up from the wall plate) might be not possible. Services in the way compound this.

Not sure what to do about these... As I say, the celotex will be foamed in in this region (because of the difficulty of covering, I want this section

100% airtight or draughts will negate the insulation).

So the risk is about 2' of the bottom face of a 2x4" rafter being exposed to air inside times number of rafters. The air will be void air rather than general room air, though one will leak to the other.

Do you think this is a big issue? Would it be sufficient to varnish the inside face of the rafter - something that could be quickly one from the outside while the tiles are off?

Or try to staple some DPM sheet in place - again possible from the outside.

Cheers - and thanks Hugo

Tim