Loft Condensation - to dehumidify or not?

grumpy_codger

rob7475 said:

You need the stop the heat from your home hitting the underside of your roof. It sounds like you have enough ventilation up there so I'd be checking the insulation is sufficient, especially around the edges and on the loft hatch etc. When the weather is cold outside, it doesn't take much warm air hitting the underside of your roof to create a lot of condensation.

I can only reiterate that heat doesn't cause condensation. Neither does hot air unless it's humid. Air can get hotter because of luck of insulation and heat transfer through the ceiling, but this cannot cause condensation. In fact this makes the loft warmer and less prone to condensation.
It's hot humid air from the house that penetrates to the loft and cools down that causes condensation, not just 'heat'. Leaking roof can be a source of moisture and condensation too.

twopenny

Is there a space between the loft insulation and the eves? 
Should either, in old houses, have a space where air comes up under the eaves or newer places have grill across, under the eaves.
So you should feel a draught.

I'm also suspicious of the bathroom vent. Have you really checked there is no leak. Easy way is to wrap it in paper towels and lightly cover with a waterproof to stop any condensation in the loft affecting it.
Not a fun job but not so bad either.

FreeBear

Cola133 said: The loft hatch is wooden board with no insulation. We have a metal loft ladder which overhangs the door a bit, so I'm not sure about putting insulation on it?

Condensation in the house and loft was obviously a problem for the previous owners. The loft is only part boarded, in front of the gas boiler which we've just had put up there, in front of solar meters and just a little down the centre.

The insulation does look a little old, so maybe I should get that all redone?

I replaced my old wooden loft hatch with a Manthorpe GL260 (they also do a drop down version). Cut out all the cold draughts coming from that area and made a noticeable difference in warmth during the winter. So fitting a decent insulated loft hatch will pay dividends.

Minimum recommended level of loft insulation is 270-300mm. It is a relatively easy job to fit insulation, although if the loft is boarded, that makes it a little more difficult. Raising the boarded area with loft legs would give you room for insulation. A general purpose handyman should be able to do the work and may well be cheaper than a builder. You can get eave trays that allow air to pass up under the felt and stop insulation from blocking the flow. One example -> https://klober.co.uk/roof-ventilation/eaves-roof-ventilation/p/loft-vent-tray

Shame the boiler is up in the loft - There is around 6% heat loss through the casing which could have been captured and used to heat the rest of the house. Got my combi in the hallway where that 180-600W of (otherwise wasted) heat can be put to god use.

Cola133

@FreeBear thank you for all your help and time. That's really useful info. thank you. Sadly the house is very tight on space, so the only place we had for the boiler was in the loft. I'll see if i can get someone in to change the loft hatch and fit those vent trays, they seem a great idea. thank you.

@twopenny thank you for your help and advice. The tip with the bathroom vent is a great idea. I'll see if i can get a handyman to do that. 

many thanks

Ectophile

Cola133 said:

@twopenny thank you for your help and advice. The tip with the bathroom vent is a great idea. I'll see if i can get a handyman to do that. 

many thanks

In that case, while you're checking that the flue pipe from the bathroom fan is actually still connected, do the same for the boiler.  Gas boilers generate a lot of steam, and it should be going out the flue.

FreeBear

Ectophile said: Gas boilers generate a lot of steam, and it should be going out the flue.

Modern gas boilers should be condensing that steam to extract as much energy as possible. So any water produced should be going out of the condensate drain and not up the flue. A boiler running at a decent level of efficiency will only have a few wispy bits of white "smoke" coming out of the flue.

WIAWSNB

If the boiler is contributing to the loft condie, you've got much bigger issues!
Bottom line - if there's enough ventilation in a loft, there will be little or no condensation there. 
That's not so say there isn't enough ventilation in your loft; there could be, but it's just overwhelmed by excessive moisture from your home.
So, step one should be to try and reduce what moisture may be getting up there in the first place. That would usually involve a sealed type of hatch, ideally insulated tho' not essential (being solid plastic, insulation will only reduce heat going up, and not vapour), and then - I guess - see what can be done to reduce normal vapour travel through the p'board ceiling.

For example, I am pretty certain that an insulated plasterboard layer added to the underside of the ceiling would stop this in its tracks, as well as adding a significant level of insulation to the rooms below, but this could be quite costly - tho' a lot easier than re-boarding a ceiling, so perhaps not? This could well be worth it, as it should provide two significant improvements to your home. If this is of interest, perhaps it's worth getting a few quotes from a plasterer? (To clarify - your existing ceiling stays in place - the insulated p'board is simply screwed over it, and skimmed or tape-and-filled. Actually very competent DIYable...)

Or, whether there's a way of adding this vapour barrier to loft side - anyone know? Eg, remove current loft insulation, lay down a vapour barrier snug against the p'board and lapped over each joist (no idea what kind), and then add the full recommended layer of loft insulation.

NB: I don't know if this is a very bad idea, so not to be considered until confirmed. Ie, would the vapour still pass through the p'board, but now be trapped between it and the barrier = damp in the ceiling?
Cola, you seemingly have a very damp house, and we don't know why. Are you planning to spend money on addressing it? Eg, adding wall insulation? 

WIAWSNB

Another thought - the boiler is up there, so that suggests quite a few pipes going to it through your ceiling? Could you check these pipes - what room(s) are they in? Are the holes they pass through nicely sealed?

FreeBear

WIAWSNB said: For example, I am pretty certain that an insulated plasterboard layer added to the underside of the ceiling would stop this in its tracks, as well as adding a significant level of insulation to the rooms below, but this could be quite costly - tho' a lot easier than re-boarding a ceiling, so perhaps not? This could well be worth it, as it should provide two significant improvements to your home. If this is of interest, perhaps it's worth getting a few quotes from a plasterer? (To clarify - your existing ceiling stays in place - the insulated p'board is simply screwed over it, and skimmed or tape-and-filled. Actually very competent DIYable...)

Or, whether there's a way of adding this vapour barrier to loft side - anyone know? Eg, remove current loft insulation, lay down a vapour barrier snug against the p'board and lapped over each joist (no idea what kind), and then add the full recommended layer of loft insulation.

Insulated plasterboard would act like a vapour control membrane except where the sheets butt up and where the screws go through. Having done some of the ceilings here, even with 900x1800 sheets of plasterboard, I struggled. 1.2x2.4m sheets, I wouldn't contemplate using (too heavy and difficult to manhandle up stairs). Overboarding would be a crap shoot as to whether the screw go in the joists first time. To do the work safely requires a stable platform to work off - Step ladders and small hop-ups are not safe (nor are milk crates).

A VCM should not go over the top of any joist - Any damp condensing out would get trapped next to the timber and potentially cause it to rot.

WIAWSNB

FreeBear said:

Insulated plasterboard would act like a vapour control membrane except where the sheets butt up and where the screws go through. Having done some of the ceilings here, even with 900x1800 sheets of plasterboard, I struggled. 1.2x2.4m sheets, I wouldn't contemplate using (too heavy and difficult to manhandle up stairs). Overboarding would be a crap shoot as to whether the screw go in the joists first time. To do the work safely requires a stable platform to work off - Step ladders and small hop-ups are not safe (nor are milk crates).

A VCM should not go over the top of any joist - Any damp condensing out would get trapped next to the timber and potentially cause it to rot.

Jeepers - I'm not suggesting the OP DIYs it!  

But, lawdie, I've done all my own full-size boarding, single-handed, using a 'T' stick... A pro will have all the proper gear.

And it's perfectly possible to accurately mark out on each wall where the joists and noggins are.

It would certainly stop the travel, and help insulate his home.

Yes, I can see that a membrane on top would be a bad call.