Loft Insulation Questions

[Dean Richard Benson]

Hi all

I recently moved into an old house (approx 250 years old) and have found the old insulation to be fairly thin and broken down (about 1cm mostly thickness mostly).

Since I have 4 inch joist height, my plan is as follows:

1) remove all old dust/rubbish/insulation (bag it all up) 2) staple down polythene sheet strips between the joists (to prevent moisture entering the loft) 3) lay down 4 inches (100mm) of rockwool insulation

Once that has been achieved, I was planning on putting down 6x2's across the existing beams to give me a total of 10 inch of effective joist height.

so phase II was:

1) put down 6x2's across existing joists 2) lay 150mm (6 inch) rockwool inbetween the newly defined joist space 3) board out loft for easy access/storage in the future

however I then stumbled across Polyfoam Supadeck on the knauf insulation site.

.This seems to achieve exactly what I was trying to do in my phase II. (ie, insulate and board)

Now to my questions.

1) does any of the above make sense? 2) does phase I seem sane? 3) does phase II seem sane (either one). 4) am I sane!?

Also, I am having the house fully rewired in a few months, and wondered what the best advice would be on where and how to run the cables in the loft.

Sorry to have wittered on, but I hope someone can help me with my first major diy project.

Thanks

Dean.

[L Reid]

All I'll say is it's gonna be messy. Might be easier just to lay the new stuff on top of the old. Not sure if you'll require a vapour barrier between ceiling and loft, most uses seem to be when insulating between rafters.

Good luck anyway. I have a similar job to do on my old property but havent built up the courage to face the 1" thick pitch.

Leigh

[Peter Taylor]

Dean - the polythene strips will be virtually useless as a vapour barrier. Firstly polythene sheet is only about 30% efficient at preventing vapour movement - you would be better off using aluminium baking foil. Secondly a VB should be a continuous sheet - if you cut it into strips you will have lots of gaps allowing vapour through. It's not vital to have a VB for cold pitched roofs as long as there is adequate ventilation, although I do agree it is a very good idea. If you still feel you definitely need a VB then it might be easier (and much more effective) to fix foil-backed plasterboard over the existing ceilings. Don't forget the loft hatch.

But still only 4" structural depth remember - if you are planning to use the loft for anything other than light storage the joists will not be strong enough.

.>> This seems to achieve exactly what I was trying to do in my phase II.

Good idea! But you really don't need all that insulation thickness (unless you need to achieve a U value to comply with Building Regs). The amount of energy lost through the top 1" of 10" thickness in its lifetime is minimal - probably much less than the energy it took to make, deliver and fix it! 8" would be perfectly adequate IMO, maybe even less if you use rigid board such as you're suggesting.

Don't forget you need to maintain ventilation, so don't block the eaves voids if they allow air movement.

Cables under insulation is a no-no. You should wire above the insulation and use 3" or 4" dia tubes or short bits of pipe to run the cables through down to the ceiling light points and switches. That's what I did (I've got loose fill insulation). Make sure you don't cover any recessed light fittings. I would say go ahead and do your insulation now ready for this winter - the electrician will know what to do when he starts, but make sure he allows for this if he is giving you a quotation.

Hope this is helpful for you - best of luck!

Peter

[Dean Richard Benson]

Yep, understood re the mess. However I have lots of big thick black bags at the ready, and personally would rather remove all of the crud before starting the insulation.

I am having second thoughts anyways about the vapour barrier. The eves do let through a decent amount of air which should mean any moisture that enters the loft won't stick around.

I will let you know how it goes.

Dean

[Dean Richard Benson]

Not entirely sure what to do really. The eves let through a good amount of air, so maybe I should just scrap the polythene/VB idea and put the insulation down.

I have bought a single roll (2.5m x 20m) from Wickes yesterday. The label says "Vapour Barrier - Medium Duty Polythene. For preventing the ingress of moisture."

The reason I was going to lay it in strips was that I didn't think covering the joists was a wise idea, as the moisture would then surely just rot them (not able to escape).

Understood. My other problem that I found out with a tape measure last night in the loft is that the joists are spaced with 400(ish) mm between each joist. However all of the insulation that I can see in the shops seems to be 370mm width. That's about 17 inch centres. Is that completely non-standard?

Yep, that's why the idea of 4" + those insulated rigid boards are looking like a good idea now.

Should most of the ventilation come from the eves, or are there other areas where I should be increasing ventilation once the loft is fully insulated?

If cables under insulation is a no-no, I would have thought it's gonna be a royal pita compared to just throwing the cable down. I understand what you are saying about the tubing. I will certainly remember to mention this to the electrician (he should already know I guess).

Very useful - and thanks.

Dean

[demon news]

In a slightly different vein, has anyone used or heard of RENOTHERM insulation. This insulates the roof allowing you to use the space in the loft. It is a spray on polyurethane foam insulation, claiming twice the insulation value of glassfibre. Just got sent an ad. from them, and as I am thinking about insulating the loft, I thought I'd ask you lot :o) WM

[froggers]

yes - to me it seems extremely expensive for what it gives.........

[Christian McArdle]

It may be better to insulate the rafters instead with celotex. You'll need to check that adequete ventilation can be maintained above the insulation, though.

The advantages are:

1. No extra joists are needed. Just board over the ceiling joists.
2. No need to use choking fibrous materials.
3. Reduces dust in storage area.
4. Storage area is heated and protected against frost.
5. No need for complicated arrangements insulating loft hatch.
6. Vapour barrier easily implemented with foil plasterboard.
7. Looks much better, as you have a smooth paintable ceiling.
8. Will make rewiring simple.
9. Will protect any water pipework against bursting.
10. Can convert loft fully later by adding windows, stairs and strengthening floor joists.

Christian.

[Andy Hall]

Yes. This is even less sensible than buying a Saniflo, although both will land you in the shit.

Whether or not the product works, it is used as a means of doing a quick fix to a roof in poor repair which could even be leaking. Obviously if that were the case then the timbers would deteriorate rapidly.

This type of product has therefore gained a bad reputation as a means for somebody selling a house to cover up the sins underneath. A decent surveyor would spot it and mark it down immediately since it is not possible to inspect behind it.

I would certainly never buy a house where this had been done and am by no means alone in this view. It has a reputation of being a bodge.

If you want to insulate a roof in this way, then the sensible product type is polyisocyanurate foam sheet, sold as Celotex and Kingspan among others. It also has many times the insulation properties for a given thickness than glass fibre and is easy to fit as well. More importantly, it can easily be removed if required.

an application of its use.

.andy

To email, substitute .nospam with .gl

[demon news]

Well thats put me off the foam idea. WM

[Dean Richard Benson]

Ok, how sensible would this be then.

1) insulate up to current joist height (4 inches) 2) insulate rafters using celotex

Do people usually double up like that, or is it usually one or the other?

Also, would the upstairs rooms be warm still or would the warmth simply be trapped inside the loft? (that was my main thinking behind insulating the floor of the loft).

Thanks

Dean

[Christian McArdle]

If you insulated the floor and the roof, then the temperature inside the loft would settle somewhere between that of the house below and the outside. The effect would make the room cooler, but still suitable for storage, and the house marginally more energy efficient. The only disadvantage is the effect on electrical cabling, which can be overcome through design.

The uninsulated ceiling would break any convection currents, so the rooms below would still be very heatable, unlike an open plan lounge with open stairs. Obviously, some heat would escape upwards to warm the loft, but would be trapped there, rather than heating the outside air.

Christian.

[Nicknoxx]

I'll second this. We've just completed a loft conversion and used 50mm foil backed celotex to insulate the roof and it is VERY effective.

N

Andy Hall wrote:

[The Natural Philosopher]

It is a good idea. Otherwise you tend to get condenstation in the cold side of teh insulation.

[The Natural Philosopher]

Thats not teh issue. The issue is percolation of warm moist air from our rooms, into teh insualtion, and condensation in it at the cold side.

In my opinion its both worth doing, and entirely satisfactory to use partial barriers - there will always be a little getting through, but the idea is to prevent a lot, such that the loft ventilaition can dry it out.

Polythene is probably totally acceptable - after all most DPM's are polythene.

[The Natural Philosopher]

No, put vapour barrier between joists, or indeed run it up and over them. The idea is to keep the mositure from teh rooms out of the loft, not the other way around.

Sounds good to me.

It will escape, but downwards into the room as it were. Its a question or relative hnidity and temperature. The beams will be warmed by teh room, so won't be subject to surface moisture. Where that forms typically is on top of insulation - I had a bad case of this after a small roof leak once - the whole top of the insulation was wet to teh tougch, from consdenstrauon in teh roof space, buit teh beams were dry as they conducted heat from the rooms below.

What you are trying to achieve is a loft space that is *NO MORE* humid than the air coming in through the eaves. And a little bit warmer. That way - no condensation anywhere.

Mine was about 1200mm wide. Just cut it and stuff it anyway. All it really does is stop air movement. Thats why plating over it is quite a help - if you get draughts from the eaves into the spaces with the insulation in, it doesn't work half so well IME.

I'd say that 4" is probably as good as you need to go. but transverse

6x2 is of course better. At that level of insulatuon even teh smallets draughts into teh rooms will make big difference, so pay attention to those...

No, eaves are enough. Its also part of bulding regs to put ridge vents in, but IMHO the amount of ventilation from eaves is more than enough.

Its all abour air chnge - not a howling gale. you want to extract any moist air and let new colder air in o get heated and absorbe the tiny residual bits of moisture.

Cables are OK under insulation up to a point. They may need to be a little derated. Remember if they are e.g. running across the top of plasterboard, they have a nice cooling path downward into the rooms.

If you are rewiring after insulation tho, I'd be inclined to string em in the cold space. One of the neatest ways is to lace them to metal slotted trays.

[The Natural Philosopher]

Also depreceated because it doesn't give adequate ventilation to joists.

Best method to insulate at roof level is to use celoezt between rafters, airgap above, or buld layer below rafters

[Mark S.]

Whichever you chose to do get yourself off to Screwfix and buy a mask (the middle one they do, not those crappy white things) and a couple of disposable whole body suits. The suit keeps most of the muck off you and you simply take it off when you are taking a break or finishing for the day. A vac you can get into the loft space is a great help too. :-)

Mark S.